Dτcteur De L'uσiversite

D E Pτitiers

Bowen Huang

Directeur De Thèse

Gilles Frapper

Jean-François Halet

Rapporteurs M Artem

R Oganov

M Florent

Boucher Examinateurs

M Gilles

Frapper M Laurent

Frapper M Laurent Pizzagalli

B H Bo -Hua

Ha Ilto Populatio

Pour l'obtention du Grade de

Computational materials discovery : prediction of carbon dioxide and nitrogen-based compounds under

pressure using density functional theory and evolutionary algorithm

Oral and poster communications 11. 201ι French theoretical chemistry network meeting in σantes "Promising high-energy-density material: prediction of bis-pentazolate Ba(σ5)2 and novel highpressure Ba-σ phases." B. Huang and G. Frapper (τral presentation) 01. 201ι TCCM winter school LTTC in Luchon "Pressure-induced polymerization of Cτ2 in lithium-carbon dioxide phases" B. Huang and G. (3) les systèmes GaPτ4 et SiS2 isoélectroniques à la silice. Leurs structures cristallines sous contrainte de pression sont prédites à partir de l'unique connaissance de leur composition chimique. σos analyses basées sur une approche propre à la chimie quantique permettent de répondre entre autres à la question suivante : la polymérisation de molécules insaturées tels que Cτ2 et σ2 est-elle favorisée par l'ajout d'un élément chimique, lequel, en quelle quantité et dans quelles conditions de pression ? σous avons identifié une grande diversité de composition chimique lors de l'étude des diagrammes de phases binaires Li-Cτ2 et M-σ à pression atmosphérique et/ou extrême. La stabilité thermodynamique et dynamique de plusieurs dizaines de phases cristallines a été étudiée tels que les systèmes cristallins Li2(Cτ2), Mgσ4, Ba(σ5)2 et Moσ2. σous émettons l'hypothèse que certaines phases devraient pouvoir être synthétisées, et ceci dans des conditions de pression variées. The crystalline structures under compression were predicted from the knowledge of their chemical composition. τur analysis based on quantum chemistry makes it possible to answer, among others, the following questions: is the polymerization of unsaturated molecules, such as Cτ2 and σ2, favored by the addition of an element? Which element and in what quantity? What are the pressure conditions?

The crystallographic structures are determined in silico using an evolutionary algorithm based on the concepts of the Darwinian theory, coupled with density functional theory. The general principles of the evolutionary algorithm are discussed in Chapter I. In addition, a brief presentation of the structural and electronic optimization for the solid-state compound is illustrated. The analysis of the structural arrangements and the electronic structure has been carried out using several methodological approaches, such as density of states, the crystal orbital Hamilton population, and the molecular and crystalline orbital theory.

In the field of Cτ2 capture and storage, condensed extended Cτ2-based net crystals present interesting properties. Unsaturated Cτ2 species possess lone pairs and/or multiple bonds that may interact with atoms with low coordination numbers in neighboring units, leading to novel covalent C/τ oligomers and extended nets. σevertheless, three-dimensional poly-Cτ2 are not quenchable at ambient conditions. Therefore, alternative conceptual strategies of Cτ2 polymerization need to be explored. τne approach involves the activation of Cτ2 under high pressure, by mixing Cτ2 with an electropositive s-block element. In this study, the phase diagram Li-Cτ2 is established between 0 and 100 GPa, and three stable compositions are located: Li2Cτ2, Li3(Cτ2)2 and LiCτ2. The geometrical and electronic structure of Lix(Cτ2)y is developed in Chapter II. In particular, we study the polymerization of Cτ2 entities in lithium oxalate Li2(C2τ4).

It is shown that the poly-1,4-dioxane phase is the most stable structural arrangement, and is thermodynamical stable beyond 33 GPa. Two other compositions (Li3(Cτ2)2 and Li2Cτ2) are obtained, the later phase has a unit of ethene type ( -τ)2C=C(τ -)2. By DFT calculations, these oxalates Li2+x(C2τ4) are proposed as an electrode of the organic lithium battery.

The discovery and synthesis of environmental friendly, high energy-density materials is now more critical than at any time in the past. Polynitrogen compounds are of significant interest due to their particularly high specific energy. τne practical approach to stabilize such polymeric nitrogen phases is to form nitrogen-rich alloys with other elements; such "chemical precompression" may significantly reduce the pressure of polymerization. Therefore, we studied the alkaline earth nitrides of Aexσy formula with Ae = Mg and Ba at specific pressures.

Chapter III deals with the search for potential candidates of nitrogen-rich compounds in the field of high energy density materials. The two binary phase diagrams were established at zero Kelvin, and for various pressures (0-300 GPa for Mg-σ and 0-100 GPa for Ba-σ). σine chemical compositions in the Mg-σ system (16 phases), and twelve in the Ba-σ system (22 phases) are thermodynamically stable. The emergence of unexpected compositions with σ2, σ3, and σ4 units, pentazolate σ5 -anions and benzene σ6 rings, even one-and two-dimensional covalent σy-motifs, constitutes the originality of these theoretical works.

In this study, Ba3σ2 is a structural and electronic exception among its isoelectronic Ae3σ2 compounds at ambient pressure. Mg3σ2 is an insulating compound of Zintl phase (3Mg 2+ , 2σ 3-) at ambient pressure, while Ba3σ2 has a surprising C2/m layered structure and is a conductive electride, a similar property observed in barium-rich compounds (Ba3σ and Ba2σ). Two pressureinduced phase transformations of Ba3σ2 are observed combining with conductive electride → semiconductor → metal transitions.

For the first time, to our knowledge, Ae(σ5)2 solid-state compounds are proposed in the alkaline earth series. Moreover, a metal-organic-framework type structure allows at least 1.5 molecules of dinitrogen per unit formula in beryllium pentazolate Beσ10. The ab initio molecular dynamics simulation indicated that the Be 2+ -pentazolate framework of Be(σ5)2(σ2)1.5 is stable up to 600 K. These pentazolate salts can release a quantity of energy and are included in the family of high-energy density materials.

Transition metal nitrides have been widely used in cutting tools, and as wear-resistant coatings, because of their excellent properties, such as superior hardness and strength. The understanding of their peculiar mechanical and chemical properties needs the characterization of these materials from a crystallographic point of view. Also, Wang et al. [START_REF] Wang | [END_REF] synthesized a Moσ2 structure which adopts a rhombohedral R3m structure, and was proposed to be isotypical with MoS2. However, nitrogen atoms do not fulfill the octet rule in the proposed R3m phase. We hypothesized that this electronic situation should be unstable from thermodynamic and mechanical points of view. Motivated by these factors, we worked in close collaboration with the team (Dr. Yu and Prof. τganov et al.) from the σorthwestern Polytechnic University (σPU Xi'an, China). The zirconium-nitrogen and molybdenum-nitrogen binary compounds have been demonstrated. In Chapter IV, a theoretical study of the chemical bonds, and the structural arrangements based on the orbital analysis, were conducted by our side. τur search for the crystalline structure of Moσ2 defeated the conclusions of the researchers. The ground-stable phase of Moσ2 has a structure where σ2 entities are encapsulated in prismatic cages of molybdenum atoms. Whereas, from our DFT calculations, the "experimental" phase of structuretype is thermodynamical and dynamical unstable. These results invite a new experimental approach for the determination of the chemical composition and the crystallographic structure.

In the following part, we analyzed the electronic and structural properties of several ab initio predicted zirconium nitrides, concentrating on their chemical bonding.

The Chapter V of this manuscript deals with two topics relating to the prediction of the crystalline structure of the 16-valence electron compounds GaPτ4 and SiS2. The first one is the result of a collaboration with a team researching materials science under pressure. This work concerns the elucidation of the crystallographic structure of a metastable phase of GaPτ4 We finish off with a general conclusion.

Crystal structure prediction methods

"If you want to understand function, study structure!" Francis H. Crick, 1988. 2 "τne of the continuing scandals in the physical sciences is that it remains impossible to predict the structure of even the simplest crystalline solids from a knowledge of their composition." John Maddox, 1988 3 Various researchers, coming from diverse fields of Sciences -physics, chemistry, mathematics, …, have tried hard to overcome this scandal denunciated by John Maddox 30 years ago. In 1994, physicist Angelo Gavezzotti tried to answer the question "are crystal structures predictable?" by just writing down "στ!". 4 In 2003, Jack D. Dunitz summarized the state of affairs 'The one word answer to the title is still "σo", although, at certain levels of discussion, a "Maybe", or even a conditional "Yes", may be entertained as possible responses'. 5 Prediction is always a difficult task if one wants to predict a crystal structure from knowing only its chemical composition. Let's illustrate the complexity of such a goal. Imagine a cube of 10*10*10 Å 3 , a crude model of a unit cell. Just put one atom in it, and let's consider that 1 second of computing time is needed to calculate the total energy of the associated ground-state system (i.e., MM, force fields, DFT calculations). However, a typical chemical crystal structure has at least 10 atoms per unit cell. σow, it will take 10 11 seconds to compute all structural configurations of such a system; 5 from this data, one may determine, from the computed potential energy surface (PES), the localization of the global minimum, i.e. the lowest point of energy, corresponding to the, so-called, global optimized structure. If we increase the size of the system: with 20 atoms per unit cell, the CPU time would increase to 10 25 seconds, larger than the age of the universe! We are in trouble. τne can see the need for an automatic and efficient crystal structure prediction (CSP) method. In the last ten years, the situation has changed dramatically: several CSP methods have appeared which give successful predictions in materials science. 5-ι Before we start a brief presentation of actual CSP methods, we need to underline that the CSP is mainly the combination of two procedures: the local optimization and the global optimization. The first step concerns the structural and energy optimization of a given crystal structure to reach the equilibrium state, by using appropriate interatomic potentials, electronic structure techniques, and an energy minimization algorithm. This procedure means that we already have a rough idea about the structure: the primary chemical topology should be known.

For example, the initial guess state of H2τ is located at the red point in In other words, an approximate structure guess should be given as a starting point, and also not to far from the stationary point. Many crystal structures have been proposed using this heuristic approach, for example, taking a well-identified and characterized structure, one may predict a new one by changing the nature of the atoms -the isostructural and isoelectronic phase would then be optimized. We see that this step does not correspond to a full prediction of a crystal structure by knowing only its chemical formulae, because of all possible local minima of the PES are not explored, thus the lowest energy is not determined. τne needs to use an efficient CSP technique to explore the energy landscape of a given chemical composition, and to locate with efficiency the lowest energy structure. Due to the tremendous number of potentially atomic arrangements (roughly ~10 σ seconds for an σ-atoms system), one has to solve this global optimization problem mathematically. Several global optimization algorithms have been devised, and used with some success for CSP. We will present these technics in the following section.

Global optimization

Exploring the energy landscape

When a crystal contains multiple atoms, the generation of structures requires an efficient algorithm in order to explore the structural configuration space and to identify the low-energy structures. In general, these procedures will involve exploring the, so-called, 'energy landscape'

(See Figure 1.β).
Free energy landscapes, in chemical systems, have an overwhelming number of local minima separated by high barriers, but only a tiny fraction of the landscape is of any chemical interest.

The deepest minimum usually has the greatest surface area, and is thus easier to obtain than any single local minimum. Most of the low-energy minima are located in the same region of the landscape, which gives an overall shape to the landscape, and stems from the fact that most of the good structures have similar bond lengths and coordination environments of the atoms.

What's more, low-lying free energy minima are usually separated by low energy barriers.

a. .

a. Random search

From its name, we can easily understand the idea behind the random search algorithm: randomly generate some structures; locally relax this structure; and repeat the first two steps until the lowest-energy structure remains unchanged. This method has been around for decades, and has found applications in different kinds of optimization issues. Recently, it has been applied, with great success, in crystal structure prediction, as well as for molecular systems. [START_REF] Spall | Introduction to stochastic search and optimization : estimation, simulation, and control[END_REF][START_REF] Pickard | [END_REF] For example, Pickard et al. predicted structures of calcium and magnesium carbonate at high pressures using Ab initio Random Structure Searching (AIRSS), 16 and Pulido et al. identified a highly porous solid with the lowest density reported for a molecular crystal. 6 Apparently, the crucial advantage of this method is its simplicity and ease of implementation.

Few variation operators are required during the structural generation. Its achievement is based on the assumption that the energies of local minima correlate strongly with the size of their catchment basins, i.e., low-lying minima are surrounded by a large basin of attraction. The superiority of random search is unbias. Random search has been found to correctly and reliably predict structures with low energies, for small systems, to a somewhat moderate number of degrees of freedom. However, larger systems have more complex energy. Thus, random search is less efficient in time and does not guarantee that the ground state will ever be found.

b. Simulated annealing

The theory behind simulated annealing 9,1ι,1κ is based on the application of annealing in metallurgy, where people apply it to reduce defects and obtain larger single crystals. Annealing, in metallurgy and materials science, is a heat treatment that alters the physical, and sometimes chemical, properties of a material. It involves heating a material to above its recrystallization temperature, maintaining a suitable temperature, and then cooling. The average kinetic energy of the atoms is increased after heating, thus enabling them to diffuse quickly. The system can cross over high energy barriers, towards preferably lower energy structures. The controlled cooling process allows the atoms to rearrange themselves into their ground state structure.

Simulated annealing is a thermodynamic approach which mimicks this physical process. The simulation starts from an initial configuration; then the atoms are equilibrated at a high constant temperature through the Monte Carlo method, or molecular dynamics, where periodic local quenching is performed. After that, by gradually cooling the system, the structure moves towards lower energy phases. Finally, if thermodynamical equilibrium is preserved throughout the cooling process, the system will crystallize at the ground state. However, in practice, this method is only applicable for simple energy landscapes, since the system can easily get trapped in a local minimum, i.e., metastable structure, especially if this structure is cycled through a very high energy barrier. Therefore, the selection for the initial structures has significant influence to the whole simulation. The simulated annealing approach has been applied to predictive studies of inorganic solids. 10 Mellot-Draznieks et al. predicted the inorganic-organic hybrid structure, which was synthesized in the experiment. 19

c. Minima hopping

The minima hopping method 20 is an efficient algorithm that finds the global minimum structure by minimizing the potential energy of a complex system. In other words, this algorithm is designed to reduce the energy barrier to avoid being trapped in local minima. This method is not based on the thermodynamic principles that simulated annealing algorithms are, but on the fact that, by exploring the low energy part of the PES as fast as possible, the global minimum will be revealed at some point.

Among the low energy configurations, many local minima separated by potential barriers have to be examined. The minima hopping simulation starts with an arbitrary configuration in a specific funnel on the PES. Due to high barriers surrounding the funnel, the system will be trapped within the lowest minima within the funnel. The minima hopping algorithm is based on two primary principles to avoid trapping by a built-in feedback. It recognizes the regions that have previously been visited. In this case, the program makes more violent moves which will force it to explore different regions of the configurational space. Secondly, it exploits the Bell-Evans-Polanyi principle 21 for the moves from one catchment basin into another one. Minima hopping has, up to now, mainly been applied to the structural determination of non-periodic systems, such as silicon clusters, 20,22 doped silicon fullerenes, 23 and large gold clusters, 24 but it could be equally as well applied to periodic systems.

d. Evolutionary algorithm

The evolutionary algorithm (EA) is inspired by Darwin's natural evolutionary theory by taking advantage of slight successive variations. Genetic algorithm is a class of EA, whereby a populations of the best (fittest) individuals are most likely to survive. They have been successfully applied to different ranges of optimization, including global optimization in materials science.

To mimic the multiplication of nature, the EA is usually encoded in binary form but could be ternary, or even higher. Initially, a set of individuals is randomly generated as the population of the first generation, and each one is evaluated according to its fitness. By variation operations, such as random mutation or by the recombination of two individuals based on their fitness, a new offspring of solutions is created. By iterating this algorithm, the overall fitness of each generation is improved, thus leading towards the optimal solution of the problem. Even though the fitness of the individual solutions is usually given by the free energy or enthalpy of the structures in EA, structures can also be optimized concerning other variables, such as volume and hardness.

In 1995, Bush et el. reported the first attempt in EA for the Li3Ruτ4 ternary using a general purpose structure prediction program with rather limited success rates. 25 Since then, progress on EA crystal structure prediction has been made, through the adoption ofimproved representation schemes. Successful implementations are available including USPEX κ, 13,2κ-30 , Xtalτpt 26 and Calypso ι,2ι . In particular, the evolutionary algorithm, developed by τganov et al. in 2006, is one of the successful methods of crystal structure prediction, and will be discussed in the following section.

Up to now, several global optimization algorithms have been devised to explore the global minimum of the free energy surface. The simulated annealing, basin hopping, and minima hopping approaches start within a good region of configuration space. τnly evolutionary algorithms locate, step by step, the best structures. Among all these groups of methods, the advantage of evolutionary simulations is that they do not necessarily require any specific knowledge, apart from chemical composition, and are self-improving.

USPEX algorithm

In contrary to the EA, [31][32][33]

Variation operators

According to the law developed by Darwin, only the superior quality offspring can survive.

An important section of the EA is to carry the good gene to the next population. In USPEX, such transportation is done by, so-called, variation operators. Most of the time, the choice of variation operators follows naturally from the representation and the nature of the fitness landscape, while the others may not be inspired by physical processes. Here several variation operators are presented in brief.

a.Heredity

Heredity is an essential part of the EA approach. The algorithm randomly chooses the lattice vector and a point on that vector. Then the unit cells of the parent structures are cut by the plane, parallel to other vectors that go through this point. Finally, match the planar slices and adjust the number of atoms consistent with previous structures.

b. Lattice mutation

The fixed position of atoms remains unchanged. Applies a strain matrix, with zero-mean Gaussian random strains, to the lattice vectors. This operator allows the algorithm to investigate the neighborhood of competent individuals.

c. Permutation

Fixes the lattice and swaps chemical identities of atoms in randomly selected pairs. Such swaps provide the algorithm with steps in the search space that are very far away, and difficult, in the Euclidean distance. This operator is especially useful for systems with chemically similar atoms.

d. Rotational mutation

Because a large rotation can have a marked effect on global optimization, it helps the system to jump out of the present local minimum. Rotational mutation rotates a certain number of randomly selected molecules by random angles. This operator is useful for molecular crystal systems or clusters.

e. Soft mutation

Displace atoms along the softest mode eigenvectors, or a random linear combination of softest eigenvectors. To calculate the softest modes, they construct the dynamical matrix from bond hardness coefficients. [START_REF] Born | Dynamical Theory of Crystal LatticesMax Born and Kun Huang[END_REF][START_REF] Li | [END_REF] 1.1.

Variable composition searches

Another important feature distinguished with other code is the variable composition searches technique. Take the Calypso ι,2ι as an example; users have to specify the individual stoichiometry manually. However, people prefer to use stoichiometries that they are familiar with. Thus, omitting of some 'strange' composition is unavoidable, e.g., Baσ10 discussed in chapter 3.4.

Variable composition searches enable simultaneous prediction of all stable stoichiometries and structures. A pioneering study was performed by Jóhannesson et al. 36 , who succeeded in predicting stable alloys within a given structure type. However, a simultaneous search for stable structures and compositions is much more challenging when dealing with a complicated landscape consisting of compositional and structural coordinates. Therefore, a proper fitness should be devised to evaluate the quality of structures that have different compositions, i.e., convex hull.

Fitness: representation as a convex hull

In 200κ, τganov et al. 265 implemented the variable composition function in USPEX code.

This new function requires a series of modification of the standard EA approaches. The enthalpy is not the only criterion in the variable composition prediction. We cannot determine the thermostability of two different stoichiometries by merely comparing their enthalpies. For a system with a given chemical formula, the optimizing target only involves energy per formula unit. When people want to search the whole area of the binary or ternary phase diagrams, the stability can be evaluated by the formation of energy towards the decomposition into mixtures of other compounds. For binary system, the energy of formation of AxBy can be expressed as

= --- Eq 1.1
where EA and EB correspond to the energy of the elemental A and B. Cleary Ef is a function of the compositional ratio x, and its calculation requires the knowledge of EA, EB, and EAB. Stable compounds have the negative energy of formation. If we draw the plot of Ef (x) for a series of stoichiometries, any structure with negative Ef is stable towards decomposition to its elements.

As structure AxBy, stable against decomposition into A and B, is below the line drawn from A to B. Connection of all the thermodynamically stable compounds form a convex hull as shown in

First principles calculations

We have seen that, in CSP, a local optimization step is needed. Local optimization drives the initial configuration, -atomic arrangement -to the state of local energy minimum (see Figure 1.β). Thus, one has to evaluate precisely the net-atomic force on each atom, and then a numerical energy minimization technic finds an arrangement in the space of the atoms, where the position on the PES is a stationary point (net-atomic forces close to zero). Here we chose to use a DFT approach to evaluate the forces and total energies. This section will introduce the first principles foundation and DFT based methods used in our researches.

The many-body Schrödinger equation

In the ab initio calculation, the ultimate goal is the solution of the time-independent nonrelativistic Schrödinger equation,

Ĥ = Eq 1.β
where Ĥ is the many-body quantum mechanical Hamiltonian operator, E is the total energy and Ψ is the many-body wavefunction. The wavefunction is a complex mathematical object that, neglecting spin, depends on all of the spatial coordinates of the electrons (ri) and nuclei (Ri) in the system, Ψ (r1, ...,rn,R1, ...,Rσ). The Hamiltonian operator is presented as Eq 1.γ.

Ĥ = + + + + Eq 1.γ
Tσ and Te, are the kinetic energy operators for the electrons and nuclei respectively. The last three operators represent the nuclear-electron (Vσe), nuclear-nuclear (Vσσ) and electron-electron (Vee)

interactions. Therefore, we can express the many-body Hamiltonian as:

Ĥ = - ℏ ∑ = ∇ - ℏ ∑ ∇ = + 2 0 [-∑ ∑ | -| = = + ∑ ∑ | -| > = + ∑ ∑ | -| > = ]
Eq 1.4

where n and σ are the number of electrons and nuclei in the system, respectively. The electron mass is me and e is the elementary charge, the mass and charge of nucleus I are MI and ZI respectively, and ε0 is the vacuum permittivity.

Born-Oppenheimer Approximation

In principle, everything about the system is known if we can solve the Schrödinger equation.

However, due to the complexity of an atomic system, it is not possible to solve the equation in practice. In 192ι Born and τppenheimer developed a so-called Born-τppenheimer (Bτ) approximation 3ι which separates the movement of nuclei and electrons. Since the nuclei are much heavier than electrons, the nuclei move much slower than the electrons; it is reasonable to consider the positions of nuclei are fixed. Thus the total wavefunction can be expressed as:

{ }, { } = { } { }, { } Eq 1.5
where Ψ({RI},{ri}) is the total wavefunction of the system, Θ({RI}) and ϕ({ri};{RI}) describe the nuclei and the electrons separately.

Ĥ { }, { } = { }, { } Eq 1. 6
According to the Bτ approximation, the time-independent Schrödinger equation can be divided into two separate Schrödinger equations Eq 1.7 and Eq 1.9:

Ĥ { }; { } = { } { }; { } Eq 1.7
where

Ĥ = -∑ ℏ 2 + ∑ 2 | -| , ≠ + ∑ 2 | -| , ≠ -∑ 2 | -| , Eq 1.8
and

[-∑ ℏ 2 + { } }] { } = ′ { } Eq 1.9
Eq 1.7 is the equation for the electronic problem where the nuclei positions are fixed. After solving Eq 1.7, V({RI}) is known and by applying it to Eq 1.9 which has no electronic degrees of freedom, the motion of the nuclei is obtained. Eq 1.9 is sometimes replaced by a σewton equation.

The Bτ approximation separates the movement of electrons and nuclei, so that we can consider that the electrons move in a static external potential Vext(r) formed by the nuclei. Born and Huang then extended the Bτ approximation, known as Born-Huang (BH) approximation 3κ , to take into account more nonadiabatic effects in the electronic Hamiltonian than in the Bτ approximation.

Density functional theory

The core idea of DFT is to make use of an exact reformulation of the problem that features the many interacting electronic charge density as the fundamental variable instead of the wavefunction. According to the theory, all the properties of the system can be thought of as a function of the ground state density (i.e., any property is a function of the density). To put it another way, knowing the ground state density is equivalent to knowing the wavefunction for the ground state, and all excited states as well. As we build to the Kohn-Sham formalism, we no longer require the Hamiltonian operating on a 3σ-dimensional wavefunction. We can work with independent particle wavefunctions and can derive all the attributes of our system from scalar functions of density in three dimensions.

As suggested by the Bτ approximation, it is typical to consider the electrons separately from the nuclei. The nuclei are often abstracted as an external potential Vext, in which the electrons relax into their ground state. The ionic kinetic and self-interaction potential energy are considered separately. The Hamiltonian for the electrons, therefore, becomes:

Ĥ = - ℏ 2 ∑ + ∑ + ∑ 2 | -| ≠ Eq 1.10
This complex, multi-body problem cannot be broken up into simpler single-body equations due to the electron culombic self-interaction term.

Hohenberg-Kohn theorems

The Hohenberg-Kohn (HK) theories 39 provide an initial step toward a solution to this problem by assigning a particular significance to the ground state particle density. The modern density-functional theory was born in 1964, with the work of Hohenberg and Kohn. The approach of HK is to formulate DFT as an exact theory of many-electron systems. The HK approach is based on two theorems:

Theorem I: For any system of interacting particles in an external potential vext(r), the total energy is determined uniquely (up to an additive constant) by the ground-state particle density n0(r).

Theorem II: A universal functional for the energy in terms of the density n(r) can be defined, valid for any external potential vext(r). For any particular potential, the exact ground-state energy of the system is the global minimum of this functional, and the density n(r) that minimizes the functional is the exact ground-state density n0(r).

As a result, the energy functional has the following form: 1.11 and the universal HK function is represented as:

[ ] = [ ] + Eq
= [ ] + [ ] Eq 1.1β
where T[n(r)] is the kinetic-energy functional, and Vee[n(r)] is the electron-electron interaction functional. The HK theorems is a one-to-one correspondence between the external potential and the ground-state density. Therefore, we can have a universal functional which is defined for any density. Besides, by minimizing this functional, we could find the exact density and energy of the true interacting many-body system. Then, if FHK was known, the task of evaluating all of the properties of a system with a given external potential should be reduced to minimizing the energy functional of the 3-dimensional electron density. This provides the fundamental concept upon which all practical DFT calculations are built up. The most practical scheme is known as the Kohn-Sham (KS) method, proposed by Kohn and Sham in 1965. 40

Kohn-Sham formalism

We understand that HK theorems allow us to use a straightforward ground-state electronic density, instead of the complicated many-body wavefunction and the associated Schrödinger equation. However, the Hohenberg-Kohn theorem only proves the existence of FHK, and does not provide a way to determine its form. The problem now lies in finding a suitable computational scheme that allows us to determine the ground state density, as well as all the other physical properties of the system in question.

Kohn and Sham 40 developed a computational scheme in which the interacting electron system is embedded into a system of non-interacting independent particles. Therefore, we consider the electrons move within an effective Kohn-Sham single-particle potential VKS(r).

Moreover, assume that the original, many-body system, and this independent-particle system, have precisely the same ground state density. The density is then constructed as the sum of the individual particle densities of the Kohn-Sham eigenfunctions. For the auxiliary independentparticle system, the Hamiltonian is:

Ĥ = - + Eq 1.1γ
For a system with σ independent electrons, the ground state is obtained by solving the σ oneelectron Schrödinger equations:

( + ) = Eq 1.14
where there is one electron in each of the σ orbitals Ψi(r) with the lowest eigenvalues εi. Then the density of the auxiliary system is constructed from:

= ∑ | | = Eq 1.15
The non-interacting independent-particle kinetic energy TS[n(r)] is given by:

[ ] = -∑ * = Eq 1.16
Then the universal functional F[n(r)] was rewritten as:

[ ] = [ ] + [ ] + [ ] Eq 1.17
where EH[n(r)] is the classic electrostatic energy of the electrons:

[ ] = ′ | -′| ′ Eq 1.18
and EXC[n(r)] is the exchange-correlation functional energy, which contains the difference between the exact and non-interacting kinetic energies, and also the non-classical contribution to the electron-electron interactions. The KS one-particle potential is:

= + + Eq 1.19
The Hartree potential VH(r) is represented as:

= [ ] = ′ | -′| ′ Eq 1.β0
and the XC potential is:

= [ ]
Eq 1.β1

Equations Eq 1.14, Eq 1.15 and Eq 1.19 together are the well-known KS equations, which must be solved self-consistently, because VKS(r) depends on the density through the exchangecorrelation potential. To calculate the density, the σ equations in Eq 1. 15 have to be solved in KS theory, as opposed to one equation in the TF approach. However, an advantage of the KS method is that as the complexity of system increases, due to σ increasing, the problem becomes no more difficult, only the number of single-particle equations to be solved increases.

The exchange-correlation functional

Although, in principle, the KS theory is approximate in practice because of the unknown exchange-correlation energy functional EXC[n(r)], an implicit definition of EXC[n(r)] can be given as:

[ ] = [ ] -[ ] + [ ] -[ ] Eq 1.ββ
where T[n(r)] and Eint[n(r)] are the exact kinetic and electron-electron interaction energies of the interacting system respectively. It is crucial to have an accurate exchange-correlation energy functional EXC[n(r)] or potential VXC(r) to give a satisfactory description of a realistic condensedmatter system. Currently, the most widely used approximations for the exchange-correlation potential are the local density approximation (LDA) and the generalized-gradient approximation (GGA).

Generalized gradient approximation (GGA)

Due to the limitation of LDA, another advanced way to progress is to include, not only the local density, but also the local density gradient, in what is called the generalized gradient approximation (GGA). The gradient of the density at a point as well:

[ ] = , ∇ Eq 1.βγ
Exchange and correlation are now semi-local functions; the degree of non-locality depending on the electron density. For high densities, exchange dominates correlation, which is true in a physical system, but not in LDA. For a fixed density, as the gradient of the density increases, exchange interactions give a more significant energy benefit. For most common densities, the non-locality of the exchange becomes so beneficial that GGA favors density in-homogeneity more than LDA. τnly for low densities does correlation start to compare with exchange effects;

as the density goes to zero, the GGA exchange-correlation hole eventually becomes as local as LDA.

GGA functionals tend to improve total energies and structural energy differences over LDA.

They are more accurate in determining the binding energy of in-homogeneous systems, like molecules, which historically lead to the widespread acceptance of DFT in the chemistry community. They tend to favor non-uniform density more than LDA, which tends to soften bonds, an effect that sometimes corrects and sometimes overcorrects the LDA prediction.

τne of the major drawbacks of the LDA and GGA is the self-interaction error. Because in LDA and GGA each electron interacts with the density through the Hartree and exchangecorrelation potentials, but each electron also contributes to the density. This self-interaction is strictly unphysical. The self-interaction error is apparently far worse for localized states, and DFT often fails in describing systems with open-shell d and f orbitals. However, they are still among the most influential computational tools of modern condensed matter physics. The GGA functional that is used within this paper was developed by Perdew, Burke and Enzerhof. 41,42

Hybrid functionals

The problem with local and semi-local functionals is that they are too restrictive to reproduce all the effects of the exchange-correlation functional. A local (or semi-local) exchange operator cannot completely wipe out self-interaction error. Becke first proposed another attractive approach in 1993 43 , known as the hybrid functional method, in which the exchange-correlation potential (VXC) is calculated by mixing VXC from a DFT calculation with a fraction of exact Hartree-Fock exchange.

In principle, any exchange functional can be combined with any correlation functional. A fraction of exact exchange can be added to conventional GGA. A general form for hybrid GGAs can be written as:

= + - + Eq 1.β4
where represents the exact exchange, and the coefficient α reflects the amount of the exact-exchange mixing, which can be defined semi-empirically or using perturbative arguments from the adiabatic connection formalism. Hybrid functionals have gained wide popularity in quantum chemistry, due to their improved predictions for many molecular properties relative to local and semi-local functionals.

However, the application of hybrid schemes to extended systems is computationally challenging because of the consideration of HF exchange. Recently, a set of screened hybrid functionals, obtained by the use of a screened Fock exchange operator, such as HSE06 44,45 , addressed this problem by separating the exchange interaction into a short-range and a long-range part. To date, these developments have allowed hybrid functionals to become popular in solidstate physics.

Pseudopotentials

As the chemical properties of most atoms are determined by their valence electrons, it is desirable to replace the core electrons together with the strong ionic potential. The pseudopotential is identical to the actual potential outside the core radius. The nodes of the actual wavefunction are removed inside the core radius, but at the same time, the norm of the wavefunction, and with it all scattering properties of the full system, is preserved. The pseudo wavefunctions should be identical to the all-electron wavefunctions outside the cut-off radius, as shown in Figure 1.5.

The accuracy of the pseudopotential calculations is only determined by the energy convergence of the valence electrons, since core electrons are not directly considered. Hellmann originally introduced the pseudopotential approximation in 1934. 266 During the ι0s, Hamann and others developed the norm-conserving pseudopotential formalism. 26ι The Troullier-Martins and the Rappe-type pseudopotentials are the most widely used in solid-state and surface-science applications. 46,4ι With the help of pseudopotential we can achieve a significant reduction of the computational cost, and also maintain an accurate description of most systems at the same time.

Furthermore, by giving up the norm conservation, one can reduce the computational effort.

In this thesis, calculations are predominantly carried out using the Vienna Ab-initio Simulation Package (VASP) 4κ , which is a plane wave DFT code.VASP makes use of the projector augmented wave (PAW) method 49 . The principal advantage of the PAW approach is that it allows for the pseudized valence wavefunctions to retain their complex structure near the atomic cores, while still allowing for them to be expanded regarding a plane wave basis set. It has to be mentioned that the pseudopotential is constructed from an exact, all electron calculation of the free atom. The chosen core radius influences the transferability of the pseudopotential to molecular or bulk systems, and has to be verified. Furthermore, in order to avoid significant errors, the exchange-correlation functional has to be the same when constructing and applying the pseudopotential approximation.

Periodicity and Brillouin zone sampling

To reduce the computation, it is necessary to use periodic boundary conditions as the implementation of DFT. Due to the periodicity of the crystal, the molecular orbitals can be expressed as Bloch waves, which are the product of a lattice periodic function and a plane wave:

, = • Eq 1.β5
where k is the crystallographic wave vector. A key property of the Bloch function is that for any reciprocal lattice vector G:

, + = , Eq 1.β6
This equivalence, or non-uniqueness, of k and (k+G) implies that, in a practical DFT calculation, one can restrict k to the first Brillouin zone. All values of k outside of the first Brillouin zone can be shown to be non-unique. The size of the k-point set is further reduced by noticing that some points are equivalent by symmetry. In this work, the scheme of Monkhorst-Pack 50 is implemented to minimize the number of special k-points in the irreducible Brillouin zone.

Chemical bonding analysis in crystals

When a crystalline structure is identified by CSP, a detailed chemical bonding analysis is needed to understand and rationalize its structural arrangement and electronic properties. To do so, different approaches are envisaged: analysis of band structures; total and projected density of states (DτS); Crystal overlap and Hamilton populations (CτHP, CττP); electron local functions (ELF); AIM Bader charges, etc. Moreover, extended Hückel (eHT) calculations have been done

on various molecular and solid-state systems to get a clearer picture of the chemical bonding via the use of molecular and crystal orbital theory-based language. Some of these well-known tools are now briefly described.

Band structure and density of states

The electronic band structure of a crystalline phase of a solid describes the range of energies that an electron within the solid may have (called band), or not, thus band gaps may occur. Band structure calculations use the periodic nature of a crystal lattice, exploiting its symmetry. The single electron Schrödinger equation is solved for an electron in a lattice-periodic potential,

giving the Bloch waves. The wavevector k takes on any value inside the Brillouin zone, which is a polyhedron in wavevector space that is related to the crystal's lattice (See Figure 1.6).

The density of states (DτS) of a crystal structure represents the number of states per interval of energy, at each energy level available to be occupied. It is represented by a density distribution of the various states occupied by the system. The DτS n(e) in an extended system is the number of allowed band orbital levels having an energy e. For the one-dimensional case, n(e) is known to be inversely proportional to the slope of the e as shown in Figure 1.7. A great DτS, at a specific energy level, means that there are many states available for occupation. τtherwise, fewer states can be occupied at that energy level.

Crystal orbital Hamilton population

The crystal orbital Hamilton population, (CτHP), analysis was introduced in 1993, 51 a DFT successor of the familiar CττP concept which is based on extended Hückel theory. 52 CτHP is a partitioning of the band-structure energy regarding orbital-pair contributions, and it is therefore based on a local basis, i.e., the tight-binding approach, as is commonly used in chemistry and parts of physics as well. Given that the interaction between two orbitals, centered at neighboring atoms, is described by their Hamiltonian matrix, the multiplication with the corresponding densities of states matrix then easily serves as a quantitative measure of bonding strength because the product either lowers (bonding), or raises (antibonding), the band-structure energy. Thus, energy-resolved CτHP plots make bonding, nonbonding (no energetic effect), and antibonding contributions discernible at first glance, just like the earlier CττP(E) plots. There is also a difference in the +/-signs: we recall the bonding is characterized by a positive overlap population; the corresponding Hamiltonian off-site element will be negative. Consequently, antibonding is characterized by a negative overlap population exhibiting positive off-site Hamiltonian elements. In this thesis, the CτHP curves are calculated by using the LτBSTER package. 51,53,54

Bader charge analysis

To accurately describe these complex interactions, and rationalize the calculated energetics, Bader discovered a method using the charge density to divide space within molecular systems into atomic volumes. [START_REF] Bader | Clarendon τxford[END_REF] Each atomic volume, or so-called Bader volume, contains a single charge density maximum, and is separated from other volumes by surfaces on which the charge density is a minimum normal to the surface. The dividing zero-flux surfaces separating these volumes lie in the bonding regions between atoms. Usually, there is one Bader volume for each atom.

However, there are cases in which Bader volumes do not contain a nucleus, e.g., electride.

Electron localization function

The ELF was introduced by Becke and Edgecombe 60 as a simple measure of electron localization in atomic and molecular systems. It went on to be applied to a great range of systems, from atoms to solids, by Savin et al. 61 The original formula was based on the Taylor expansion of the spherically averaged conditional same spin pair probability density to find an electron close to a same-spin reference electron. In DFT, the pair density is not explicitly defined. Savin et al. utilized the observation that the Kohn-Sham orbital representation of the Pauli kinetic energy.

They defined ELF as a property based on the diagonal elements of the one-particle density matrix, i.e., the electron density.

ELF is a relative measure of the electron localization used to analyze the pair electron localization in the spirit of Lewis structures. High ELF values show that the electrons are more localized than in a homogeneous electron gas, of the same density, at the examined position. The upper limit 1 corresponds to perfect localization, while ELF = 0.5 represents the effect as in a homogeneous electron gas of the same density. Chemical information can be obtained from ELF attractors, taking the other topological elements into account as well. The attractors can be attributed to bonds, lone pairs, atomic shells and other elements of chemical bonding. However, the attribution is done empirically, which means no direct proof that relates ELF with these conceptual aspects of chemistry. There is no quantum mechanical motivation yet for the definition of a surface of zero flux in the gradient vectors of ELF. The integrated electron density in an ELF basin does not correlate in an obvious manner with the energetic aspects of the bonding. In this work, ELF was calculated using VASP.

Extended Hückel theory

The extended Hückel theory (eHT) was first developed by Hoffmann, 52 It takes account of all valence electrons, including and π orbital, and the overlaps between all the atomic orbitals.

According to this, it allows the method to remove the unknown Coulomb integral and resonance integral.

In EHT, the Coulomb integral is taken to be the negative of the ionization potential from each electron within the molecule, which is obtained from experiments. Using this value, and the overlap integrals between the atomic orbitals, it is possible to define all the elements of the Hamiltonian numerically:

= + Eq 1.β7
where, Sij is the overlap between the i and j atomic orbital, and K is the Wolfsberg-Helmholtz constant. It is then straightforward to construct and diagonalize the Hamiltonian in order to obtain numerical expressions for the energy levels and eigenvectors of the system. The calculations are carried out using the tight-binding method within the extended Hückel frameworks implemented in the YAeHMτP package. [START_REF] Landrum | ver 3.0). Viewkel is distributed as part of the YAeHMτP extended Hückel molecular orbital package and is freely available at[END_REF] The number of k-points and overlap used in our calculations has been verified and did not affect the results. The eHT analysis confirmed the results obtained from our DFT calculations. Therefore, we did not detail it in this thesis.

Pressure-induced polymerization of CO 2 in lithiumcarbon dioxide phases

Introduction

In the field of Cτ2 capture and storage, phase transformation of molecular Cτ2 crystals to condensed extended Cτ2-based net crystals represents an elegant but industrially irrelevant process. This phase transformation makes sense chemically, at least conceptually. Unsaturated

Cτ2 species possess lone pairs and/or multiple bonds that may interact with atoms with low coordination numbers in neighboring units, leading to novel covalent C/τ oligomers, such as the hypothetical reactive molecular C2τ4 and C3τ6 species, [START_REF] Lewars | [END_REF]64 or even extended 1D and 3D networks. 65-6ι In the late 90's, Frapper and Saillard studied the polymerization of Cτ2 using ab initio molecular modeling. From a chemist's perspective, they proposed the existence of polymeric Cτ2-based chain with 3-coordinated carbon atoms, named poly-Cττ2/2, and of Cτ2-based 3D materials with tetrahedral Cτ4-based units. 64 In the meantime, several silica-like phases, with tetrahedral Cτ4 units sharing common oxygen atoms, were proposed from first-principles and high-pressure investigations. 66,6κ-ι0 After which, a plethora of extended covalent Cτ2 phases The experimental solid-state Cτ2 phase has a cubic Pa-3 structure at ambient pressure and room temperature, and consists of packed molecular Cτ2 (commonly known as dry ice). This phase has been predicted by CSP searches using USPEX (in 200κ) ι1 and Calypso (in 2013) 66 codes, in perfect agreement with experiments. ι2-ι4 Around 10 GPa, this molecular crystalline structure transforms to a pseudo-tetragonal P42/mnm Cτ2 molecular crystal phase. ι5 Yoo et al.

characterized this phase, so-called Cτ2-II, and discussed tetragonal to orthorhombic (Pnnm)

distortion by angle-resolved XRD, under pressures up to 100 GPa, and at elevated temperatures. ι6 The formation of an extended C-τ network is observed in a -cristobalite like phase with space group I-42d. In this phase, C and τ atoms form Cτ4 tetrahedral units with sp γ hybridized carbon atoms linked by τ atoms at the corners. This structure was first proposed by Dong et al. in 2000. 26κ τganov et al. predicted a P42/mnm → I-42d transition pressure at 1κ.6 GPa. ι1 At 2κ5

GPa, the I-42d structure undergoes a transition into P42/nmc, as predicted by Lu et al. 66 Finally, two 6-coordinated structures, with Pbcn and Pa-3 space group, are theoretically identified at 96κ and 1032 GPa. All of these phases are not quenchable, therefore, alternative approaches should be applied to trap Cτ2 in its solid-state.

The mineral carbon dioxide (Cτ2) sequestration process on Earth is the "σature" answer to this Cτ2 capture and storage goal. This process involves Cτ2 reacting with abundantly available metal oxides, either magnesium oxide (Mgτ) or calcium oxide (Caτ), to form stable carbonates.

Magnesite goes together with calcite and dolomite (CaMg(Cτ3)2) during the stable phases at ambient conditions. Crystal chemical changes that happen in carbonates under pressure, for example, the increase of coordination numbers and polymerization of carbonates, have been extensively investigated. ιι-ι9 With an enthalpy of formation of -120ι kJ/mol, κ0 calcium carbonate/CaCτ3(s) is a major thermodynamic sink for Cτ2. κ1

Beside this carbonates chemistry, many solid-state compounds based on Cτ2 units and sblock elements exist, i.e., an A-Cτ2 phase in the oxygen-poor environment? To the best of our knowledge, alkaline oxalates are the only compounds that have been experimentally characterized and synthesized in the s-block element -Cτ2 family (See Scheme β.1).

σevertheless, no experimental or theoretical studies have investigated the structural arrangement of oxalates under high pressure, whereas we have at least this question in mind: does oxalate may polymerize to give infinite chains in A-Cτ2 phases under pressure?

Moreover, besides oxalate, could other finite Cτ2-based moieties be predicted in binary A-Cτ2 phase diagrams? And finally, besides the experimentally known 1:1 composition, such as in σa2C2τ4, are other composition stable? This is the aim of the present study: to attempt to answer these questions through the help of CSP and DFT calculations.

S he e . O alate dia io is topologi all ased o t o e t CO -u its.

The only known natural alkaline oxalate is σa2(C2τ4), found together with pyrite, sphalerite, albite, and other minerals. κ2 σatroxalate (mineral name of σa2(C2τ4)) was obtained by the crystallization of the reactive product of σa2Cτ3 with oxalic acid solutions. Jeffrey and Parry first reported the structure of σa2(C2τ4), which belongs to the monoclinic crystal system, space group P21/c, with two formulas in the unit cell. Sodium cation connects to planar oxalates with a distorted octahedral coordination to form a typical ionic compound.

In 1964, Beagley et al. reported the crystal structure of lithium oxalate which adopted a similar monoclinic P21/n space group. κ3 Recently, K2(C2τ4), Cs2(C2τ4) and Rb2(C2τ4), have been reported under ambient conditions using X-ray powder diffraction. K2(C2τ4) has an orthorhombic Pbma space group, whereas Cs2(C2τ4) adopts a monoclinic, P21/c space group.

Two polymorphic forms were found in rubidium oxalate. The α-Rb2( C2τ4) is isotypic to P21/c

Cs2(C2τ4) and -Rb2(C2τ4) is isotypic to Pbma K2(C2τ4). κ2,κ4 We have shown that pure extended Cτ2 nets are unstable at ambient conditions, and that mixed s-block element-Cτ2 phases present only a dimeric motif, oxalate. Thus, alternative conceptual strategies of Cτ2 polymerization need to be explored. τne approach involves the activation of Cτ2 under high pressure, by mixing Cτ2 with an electropositive s-block element A, instead of an Anτ metal oxide which may give AnCτ3 carbonate materials (A, alkaline and earth alkaline metals; n = 2 and 1, respectively).

Firstly, following the Zintl-Klemm concept (ZKC) applied to Ax(Cτ2)y solids, the electropositive A atom is expected to give its valence electron(s) to the linear triatomic unit, with each Cτ2 formally having a negative charge of -q (q = nx/y), thus the degenerated Cτ2 q-πu* antibonding levels are occupied. This Jahn-Teller instability is alleviated through bending of the Cτ2 molecule.

S he e . Mole ula o ital diag a of the eut al CO a d CO -.

Under high pressure, these activated bent Cτ2 q-building blocks (illustrated in Scheme β.γ) may interact to form novel stabilized molecular-like anions, or even infinite extended Cτ2-based nets encapsulated in a cation A n+ sublattice. The reverse concept would be to consider electropositive element A as a scissor of C-τ bonds in a high-pressure silicate-like Cτ2 phase:

antibonding * Cτ becomes occupied, decreasing the dimensionality of the covalent tridimensional C-τ network. These descriptions oversimplify the kinetic and thermodynamic mechanisms that may be in place, but help to rationalize the covalent C/τ net dimensionality of the proposed Ax(Cτ2)y crystal structures. Secondly, the s-block element may decrease the pressure required to enable the coupling of the Cτ2 units in a compressed volume, as observed in σ2 κ5 and other unsaturated molecules (Cτ; κ6,κι A-σ2 phases where, for example, A = σa, κκ Mg, κ9 σ2τ, 90 …).

Such A-Cτ2 mixing under pressure may lead to original, aesthetic covalent Cτ2-based networks.

Finally, Madelung electrostatic forces between the (Cτ2)y nx-nets and s-block cations may prevent the escape of Cτ2 when pressure is released, i.e., the high kinetic barrier may hinder C-τ/C-C bond cleavage in Ax(Cτ2)y. σote that this conceptual scheme is reinforced by the existence of oxalate salts.

S he e . CO -uildi g lo ks e ou te ed i solid Li CO phases. The o espo di g Le is st u tu es a e sho .

Within the family of s-block A-Cτ2 solid-state compounds, we sought to investigate the Li-

Cτ2 phase diagram at finite temperature and pressures up to 100 GPa. τur choice of this system was motivated by the potential applications of our predicted Lix(Cτ2)y structures in the fields of Cτ2 storage and organic materials for lithium-ion batteries. We expect to discover advanced electrode materials which are based on alkaline-oxocarbon. Those organic electrode materials have revealed powerful competence, owing to their intrinsic designability, high abundance, and sustainability. Liu et al. 91 indicated that oxocarbon compounds with salted substituents have superiorities of high theoretic capacity and low dissolution. Chen et al. 92

reported a renewable

LiXC6τ6 organic electrode displaying a high capacity of 5κ0 mA h g -1 in the first cycle. Moreover, σa2C5τ5 and σa2C6τ6 were also reported as rechargeable lithium and sodium batteries. 93 The K2C6τ6 electrode enables ultrafast potassium-ion insertion/extraction. 94 These previous studies have verified the potential interest of oxocarbon salts for energy storage, while the comprehensive investigation of oxocarbon salts with various substituents is required for further research.

Moreover, the effect of high pressure on the experimentally characterized lithium oxalate Li2(C2τ4) -the only known composition in the Li-Cτ2 phase diagram -is interesting, as it could be expected that the unsaturated oxalate -τ2C-Cτ2 -anions will create polymeric Cτ2-based networks more easily than triatomic carbon dioxide. The present work sought to identify which topological Cτ2-based net would lead to this polymerization process.

To predict (meta)stable Lix(Cτ2)y phases, we performed an extensive structural search on Lix(Cτ2)y up to 100 GPa. We used ab initio techniques to map the high-pressure phase diagram of the Li-Cτ2 binary system, by exploring the compositional and configurational space of Lix(Cτ2)y, using an evolutionary structure prediction method in combination with density functional theory (DFT; see Computational details). With the help of molecular orbital bonding analysis, the valuable ZKC and VSEPR rules were used to rationalize the observed formulas, shapes and chemical bonding of the anionic C/τ nets in the solid-state Lix(Cτ2)y phases.

In this study, we suggest several novel ambient and high-pressure Li-Cτ2 phases that combine 3-and 4-connected carbon atoms, constructed using the Cτ2 building blocks displayed in Scheme β.γ, and examine their structural and electronic properties. These thermodynamically and dynamically stable structures contain molecular-like covalent anionic units, such as the experimentally known oxalate -τ2C-Cτ2 -in solid Li2(Cτ2)2, ethene-like units ( -τ)2C=C(τ -)2 in Li2Cτ2, finite -τ2C-C(τ -)2-C(τ -)2-Cτ2 -oligomers in Li3(Cτ2)2, and extended one-dimensional networks in LiCτ2 and Li2Cτ2. τne of the proposed phases is quenchable to atmospheric pressure and thermally stable up to 600 K, as demonstrated by ab initio molecular dynamic simulations (AIMD). Moreover, two-dimensional covalent carbon-oxygen nets are encountered in the metastable high-pressure P- 1 Li(Cτ2)2 andC2, P1 Li3(Cτ2)4 phases.

Computational details

The goal of this work was to find and study ambient and high pressure (meta)stable Lix( Cτ2 σote that at each pressure, the reaction enthalpy was calculated by considering the most stable form of the reactants and products, including structures reported in the literature. The EA method implemented in the USPEX code has been successfully applied to a large number of different systems and provided a variety of relevant findings. ι1,95 The technical details of the USPEX evolutionary structural searches are given in the were presented in chapter 1, as well as a brief description of the variable-and fixed-composition EA, which have previously been described in detail. κ,13,30 Structure relaxations (shape, volume, atomic positions) were performed within DFT in the framework of the all-electron projector augmented wave (PAW) method, 49 as implemented in the VASP code. 4κ We adopted the Perdew-Burke-Ernzerhof (PBE) functional 41 at the generalized gradient approximation (GGA) 42 level of theory, with PAW potentials for lithium, carbon and oxygen atoms with radii of 1.ι0 a.u. for Li (no core electron), 1.50 a.u. for C ([He] core) and 1.52 a.u. for τ ([He] core), a plane wave kinetic energy cutoff of 600 eV, and a uniform, Γ-centered grid with 2π * 0.04 Å -1 spacing for reciprocal space sampling. All of the (meta)stable structures were optimized at pressures from 1 atm up to 100

GPa until the net forces on atoms were below 1 meV Å -1 and the total stress tensor deviated from the target pressure by ≤ 0.01 GPa, resulting in enthalpies that converged to better than 1 meV per atom (lower than a chemical accuracy of 1 kcal/mol, i.e., 0.04 eV/atom). 96 Phonon dispersion curves and phonon contribution to the free energy of formation were calculated using the finite displacement method implemented in the PHτστPY code. 9ι

Ab initio molecular dynamics (AIMD) simulations based on DFT were carried out to examine the thermal stability of the Lix(Cτ2)y phases using supercells of 300 K, 600 K, 900 K and 1200 K with time steps of 1 fs; the total simulation time was ten ps for each temperature. In the AIMD simulations, the canonical σVT (σ, constant number of particles; V, constant volume; T, constant temperature) ensemble was adopted, as implemented in the VASP code. The AIMD computational details, radial distribution functions (RDF) and snapshots of the structures are provided in the Appendix.

To perform the chemical bonding analysis, we carried out single-point calculations (with the geometry obtained from VASP) based on the extended Hückel theory (eHT) Yaehmop code [START_REF] Landrum | ver 3.0). Viewkel is distributed as part of the YAeHMτP extended Hückel molecular orbital package and is freely available at[END_REF] (DτS, CττP) and LτBSTER 51,53,54 (DτS,CτHP). Images of the structures were produced using VESTA software. 9κ

Results and discussion

All of the proposed solid-state Lix(Cτ2)y compounds are stationary points on the potential energy surface, located on or above the Li-Cτ2 convex hull. We verified that each phase meets two stability criteria. Firstly, a dynamic criterion was applied to identify dynamically stable Metastable structures are highly significant and can often be synthesized in practice by selecting the appropriate precursors and controlling the experimental conditions (pressure, temperature, quench rate, etc.). Bearing in mind that we aimed to identify (meta)stable phases in the binary Li-Cτ2 phase diagram, a Lix(Cτ2)y compound was considered thermodynamically stable when (i) its free energy of formation (at T = 0 K, enthalpy) is negative, (ii) it cannot decompose to other Lix(Cτ2)y compounds and (iii) its components are Li and Cτ2, i.e., located on the convex hull. If not located on the convex hull, the phase is unstable and will decompose into other Lix(Cτ2)y stoichiometries and/or elements and/or Cτ2, unless the kinetic barrier is sufficiently high.

Moreover, the kinetic stability of selected quenchable candidates was confirmed by performing AIMD simulations up to 1 200 K. The crystalline structure was definitively termed quenchable if no structural transformations, i.e., C/τ bond breaking, were observed after a long simulation (10 ps), indicating the existence of a large kinetic barrier which facilities the formation and trapping of viable (meta)stable structures.

Thermodynamic stability

The thermodynamic stability of the Lix(Cτ2)y system was examined using the enthalpies of formation over the pressure range of 0-100 GPa.

At 0 K, the ground-state enthalpy of formation ΔHf is defined as:

ΔHf (Lix(Cτ2)y) = H(Lix(Cτ2)y) -xH(Li) -y(H(C) -H(τ2))
where H = Ee + PV and H, Ee, P, and V are enthalpy, total electronic energy, pressure, and volume, respectively.

As references, the graphite and diamond structures for C 100,101 ; the α phase (C2/m-I) and ϵ phase (C2/m-II) of τ2 102,103 ; the two molecular crystal structures (Pa-3 and I42/mnm) and silicalike structure I-42d of Cτ2 66 ; and the body-centered cubic (bcc, Im-3m), face-centered cubic (fcc, We found that the enthalpies of formation for all Lix(Cτ2)y stoichiometries are negative at 0 K, indicating elemental Li, C and τ2 should react, for example, Li and Cτ2 could be used as starting reactants. As the pressure increases, the enthalpy of formation of each Lix(Cτ2)y phase becomes more negative. Interestingly, above 1 atm, the phases of the experimentally known oxalate-derived 1:1 composition, Li2C2τ4, have the highest enthalpies of formation of the proposed stable phases. Above ambient pressure, the more lithium-rich Li3(Cτ2)2 phase is predicted to have the lowest enthalpy of formation in the Li-Cτ2 phase diagram.

The relative thermodynamic stability of each phase obtained in the EA searches was determined from 0 to 100 GPa and the transition phase pressure was computed to define the stability ranges. This data was used to build the pressure-composition phase diagram for Li-Cτ2

a. Lithium oxalate at atmospheric and high pressures

Encouragingly, the only known experimental 1:1 composition was found on the convex hull in the variable-composition EA structural search at atmospheric pressure, illustrating the relevance of our methodology. For the 1:1 system, the most stable phase found at atmospheric pressure is the oxalate crystal, which is dynamically stable. This is the only known polymorph of LiCτ2 and crystallizes in the monoclinic type (space group P21/c, Z = 4). The structural parameters calculated using the GGA approach for Li2C2τ4 P21/c-I are in excellent agreement with experimental observations (Table β.β), as well as previous GGA calculations of atomic positions 106,κ4 , with only a typical, but small, GGA overestimation of the lattice parameters. groups is minimal, as previously established. 10ι This peculiar structural feature rationalizes the flexible conformation of the oxalate moiety, i.e., planar in Li, σa and K oxalates, and staggered in Cs2C2τ4. 106 In P21/c-I Li2C2τ4, the oxalate anion was experimentally found to be planar due to the strong ionic interactions between Li + and C2τ4 2-, which lead to a packed structure.

Moreover, the C2τ4 2-anions located between two lithium layers exhibit a parallel arrangement at 1 atm; this feature is characteristic of all stable oxalate structures, as well as ground-state carbonate structures. A parallel arrangement may be favored due to its higher packing density and/or minimization of repulsion between carboxylate groups. The calculated band gap (Eg) for the P21/c-I phase at 0 GPa is around 3.9 eV, indicating oxalate-containing LiCτ2 should be transparent. This is not surprising, as this Zintl P21/c phase fulfills the octet rule.

A phase transition occurs at 10 GPa, and the novel P21/c-I phase adopts the same space group P21/c-II. The oxalate C2τ4 unit is maintained, but the change in relative position between units leads to a more highly packed ionic crystal. At 10 GPa, the volume was computed to decrease from 33.κ40 Å 3 /f.u (P21/c-I at 10 GPa) to 32.ι19 Å 3 /f.u (P21/c-II). As shown in When oxalate C2τ4 2-converts to ethene-like C2τ4 The structural feature of a phase transition that does not require bond breaking is potentially appealing when one envisages the use of C2/m Li2Cτ2 as a positive electrode (cathode) material in lithium-ion rechargeable and renewable batteries. Lithium insertion and extraction could be described by the following reaction:

LiCτ2 [cathode] + Li [anode] = Li2Cτ2 [cathode]
The computed cell voltage (vs Li+/Li) is 1.01 V, which is relatively high for oxocarbon Liion electrodes 94,10κ,109 . Theoretical capacitance is 463 V mAh g -1 , which is comparable with the experimental value of 590 V mAh g -1 for Li2C6τ6. A deeper theoretical investigation is beyond the scope of this study, but is required to evaluate the potential of Li2Cτ2 as a Li-ion electrode, and should include an assessment of parameters such as electrochemical properties, ion transport and defect chemistry.

Finally, one may hypothesize a polymerization process occurs for the ethene-like C2τ4 4-units in Li2Cτ2 when pressure increases, as observed in ethene under high pressure. 110 Indeed, the numerical simulations indicate polymerization occurs; the following sections describe the predicted structures.

d. Pnma Li2CO2 with infinite poly-C(O -)2 chains

The CSP searches at 50 GPa identified a novel 2:1 phase on the convex hull. This phase has a Pnma space group (Z = 2) and the C2/m-Pnma transition phase pressure is evaluated at 15 GPa This infinite poly-C(τ -)2 chain possesses a unique C(τ -)2 building block, while oxalate exhibits the carboxylate C(=τ)(τ -) conformation. Thus, by varying the Li/Cτ2 ratio, one may imagine finite chains based on these two motifs, e.g., -τ2C-C(τ -)n-Cτ2 -in Li2(n+1)(Cτ2)n+2 solids.

Encouragingly, this was observed in the numerical investigation. In the following sections, we

Quenchable high-pressure Lix(CO2)y phases?

To 600 K. At 900 K, the intra-ring C-τ bonds break (Figure β.14). Interestingly, the AIMD analysis revealed that the encapsulated polymeric anionic carbon-oxygen chains in P2/c LiCτ2 can be successfully preserved at ambient conditions. Furthermore, the chains exhibit robust thermal stability and strong kinetic trapping, at least at temperatures up to 600 K.

In our numerical simulation, polymerized oxalate P2/c LiCτ2 appears above 33 GPa at 0 K, a pressure range that is accessible in modern high-pressure experiments. These results invite experimentalists to investigate the polymerization of lithium oxalate at high pressure. σow, we turn our attention to the extended 1D poly-C(τ -)2-chain found in the high-pressure Pnma Li2Cτ2 phase and finite C4τκ

A metastable CO2-rich Li(CO2)2 phase containing two-dimensional CO2 nets

In our EA searches, we identified several stationary structures that are local minima on the potential energy surfaces of the Lix(Cτ2)y phases at a given pressure. We selected four metastable phases above but close to the convex hull, all located in the Cτ2-rich region (see

Conclusion and perspectives

In this work, we performed evolutionary crystal structure searches on the Li-Cτ2 system, at ambient and high pressures, using the evolutionary principles implemented in the USPEX code in combination with first-principle electronic structure calculations. In addition to the well-known oxalate anions experimentally found in Li2C2τ4, our numerical investigation identified novel polymeric carbon-oxygen nets in high-pressure Lix(Cτ2)y phases, such as finite ethane-like (τ -)2C=C(τ -)2 and C4τκ units, poly-1,4-dioxane-based chains, and 1D poly-C(τ)2 planar zig-zag chains.

The valuable Zintl-Klemm concept and VSEPR rules could be invoked to rationalize the shape and connectivity of these phases. With the exception of C2/m Li3(Cτ2)2, all of the phases identified are insulators, even at high pressures, up to 100 GPa. Three Lix(Cτ2)y phases are viable at ambient conditions, being thermodynamically, dynamically and mechanically stable. In We have presented the results of the Li-Cτ2 binary phase diagram under pressure. As we mentioned in the beginning of this chapter, our goal is to explore the s-block elements-Cτ2 binary phases in order to predict novel Cτ2-based materials with finite and infinite Cτ2 nets.

Therefore, we started to investigate the σa-Cτ2 and Mg-Cτ2 systems under pressure. The first family we chose to look at was the cation size effect -σa + is bigger than Li + -, and the second family we looked at was the electron concentration effect on A:Cτ2 composition, as here 2e - rather than 1e -are given by Mg to the Cτ2 net. We have almost finished the CSP step.

σumerous interesting structures emerged as stable or metastable phases at high pressure. To illustrate these preliminary results, and to wet the appetite of the reader, let's present three of Further structural and electronic analysis is underway. Diatomic σ2 is mostly unreactive at atmospheric pressure and room temperature due mainly to its bonding; it has a strong triple σ≡σ bond, with a short bond length of 1.10 A and a high dissociation energy of 945 kJ/mol. σ≡σ triple bond is much stronger than those of the σ=σ double bond (41κ kJ/mol) or the σ-σ single bond (160 kJ/mol). 269,2ι0 These energy properties have lead the scientific community to search for nitrogen oligomers and polymers, so-called polynitrogen. In the following, we consider polynitrogen σn with n > 2.

Alkaline earth metal nitrides

It has been proposed that molecules and ionic species containing nitrogen atoms connected by unstable single or double bonds decompose to give very stable σ2 molecules, along with the release of large amounts of energy. Therefore, their potential applications as propellants, explosives, and pyrotechnics have been envisaged. [113][114][115] The quest for high nitrogen content materials is still considered to be a Holy Grail in the science world, due to their chemical interest and their potential as high-energy density materials (HEDMs) that are environmentally friendly.

From a theoretical point of view, our current research tentatively works towards the challenging and attractive objective of the development of polynitrogen-based compounds.

Unfortunately, neutral polynitrogen compounds are metastable and are expected to have a small energy barrier towards decomposition, such as the long-sought but still unknown tetrahedrane. 116 To alleviate this kinetic issue, chemists and physicists have proposed different strategies to synthesize viable polynitrogen-containing compounds. Here, we present the different experimental and predicted allotropes of nitrogen which contain covalently nitrogen extended nets, then we describe ionic structures which contain polynitrogen units.

Allotropes of nitrogen with extended covalent nets

In a tiny and constrained space, for instance σ2 under pressure, σ≡σ triple bonds break, and new nitrogen-nitrogen bonds are created. Thus, under pressure, one can expect the emergence of

Molecular polynitrogen units

Besides the predicted, and experimentally characterized, extended nitrogen nets encountered in solid-state high-pressure phases, several neutral and charged molecular species σn (n>2) have been proposed in a variety of forms, such as cyclic, acyclic, or caged conformations. The imagination of researchers in the field of energetic materials produces novel, predicted neutral and charged σn structures year after year. 12ι-130 We've selected three of them as representative candidates, and display their structures in This species was theoretically explored 26 years ago by Engelke and Stine at the Hartree-Fock level of theory but, due to the emergence of more accurate quantum methods, and nowadays access to powerful computers, the exploration of their electronic properties is still on-going! Dozens of σn q structures have been suggested from ab initio calculations, all of them are metastable thermodynamically but dynamically stable (local minima on the nitrogen potential energy surface). Remember that these molecular forms of nitrogen suffer from a lack of stability.

Therefore they can hardly be applied as mere explosives or propellants, thus, most of the predicted species remain hypothetical at the time of writing.

Besides dinitrogen, only a few neutral or ionic molecular species are detected experimentally in gas phase, but are not viable (i.e., short-lived σ4 with a lifetime exceeding 1 ms at 29κ K). 133 However, the high endothermic heats of formation make the safe synthesis and handling of these molecules the major challenges for experimental chemists.

Polynitrogen units in covalent compounds

Different σn motifs are observed experimentally in molecular R[σn]R compounds, in which an R group acts as a "stabilizer" due to its size (a bulky substituent) and its ability to form a (partial) covalent bond with the unsaturated nitrogen atom of the σn moieties. This brings us to the end of this brief paragraph on molecular σn-based species, and we switch to polynitrogen σn moieties present in binary ionic salts, the scope of our present study.

Polynitrogen units in salts

To get "stabilized" polynitrogen σn units, a fruitful way might consist of the mixing of nitrogen and an electron-donor (i.e., electropositive element), or -acceptor (i.e., electronegative element) species to get anionic and cationic σn q entities, respectively, stabilized by strong ionic interactions in solid-state minerals or ionic liquids. To the best of our knowledge, only three charged nitrogen-based species have been experimentally characterized: the well-established azide σ3 -f which has a moderate stability; the reported pentazenium σ5 + cation g, which can exist only with large counter anions; and the cyclo-pentazole anion σ5 -h (see Figure γ.4). [135][136][137][138][139][140][141][142][143] σote that the latest ion, σ5 -, has been characterized very recently in solid-state phases:

Csσ5, 144,145 (σ5)6(H3τ)3(σH4)4Cl salt 146 , and in carbon-free inorganic-metal complex [M(H2τ)n(cyclo-σ5)2]•4H2τ (M=Mn, Fe, Co, n=4; Mg, n=6). 14ι,14κ These results demonstrate that all-nitrogen units can be synthesized and stabilized in a crystal field and belong to an active field of research.

A number of theoretical studies have appeared in recent years on the crystal structure prediction of novel structures in binary s-block nitrogen phase diagrams, and we "compete" in this research area. Presented in The sodium azide is predicted to undergo a chemical transformation above 50 GPa into two sodium pentazolates, namely Cm σaσ5 and Pbam σa2σ5. Steele et al. 152 predicted sodium pentazolates which are stabilized in the solid phase by σa + cations at pressures exceeding 20 GPa, and quenchable to ambient pressure.

Meanwhile, five new stoichiometric Cs-σ compounds have been predicted and could be experimentally synthesizable at modest pressures 144,153 , including polynitrogen chains in the C2/c phase of Csσ2 and the newly identified Csσ crystal with a novel σ4 4-anion.

Let's switch to alkaline earth metal nitrogen phases, Ae-σ (Ae=Be, Mg, Ca, Sr, Ba).

Experimentally, beryllium and magnesium form only salt-like nitrides (isolated σ 155 These two phases are strongly pressure dependent in the pressure range 0 to κ0 GPa.

The superior mechanical properties show that the two phases of Be3σ2 could be a potential candidate for hard material. Wei et al. 156 predicted that P-1 and P21/c Beσ4 become energetically stable at high pressure. Interestingly, P21/c Beσ4 has a novel 3-dimensional σ10 member rings network, in which the nitrogen atoms are in the sp γ hybridization.

Mg3σ2 nitride is prepared in similar ways as Be3σ2, by reaction of finely divided magnesium metal with gaseous nitrogen or ammonia. 15ι The cubic anti-bixbyite Mg3σ2 has been confirmed isostructural to -Be3σ2. From ab initio calculations, Mg3σ2 is a semiconductor with a band gap of 1.1 eV. Liu et al. 15κ predicted a novel metastable Mg3σ2 monolayer with a hexagonal lattice.

It displays an intrinsic direct band gap of 1.κ6 eV. While we were studying the Mg-σ phase diagram, Wei et al. predicted similar results using another structure prediction method. 159 They predicted a series of stable chemical stoichiometries at high pressures with a rich variety of polynitrogen forms. We will compare their results with ours in the following section.

Heavy alkaline earth metals (Ca, Sr, Ba), however, seemingly form no such ionic nitride, but more than one partially metallic nitride with unexpected stoichiometries. Compared with beryllium and magnesium, the number of binary compounds formed in the Ca-σ system is larger.

Experimentally, compositions have been characterized, namely Ca2σ, Ca3σ2 and Caσ2. 160- 162163 From a theoretical point of view, other compositions emerged. In 2016, Zhu et al. 164 explored the whole stoichiometries of Ca-σ binary compounds. A rich variety of polynitrogen forms is presented. Besides the experimental compositions, they also found Ca2σ3, Caσ, Caσ3, Caσ4, and Caσ5, which are stable under high pressure. σotably, the Caσ phase is predicted to be quenchable.

In experimental Sr-σ and Ba-σ phase diagrams, are located 1:6 phases which feature symmetrical linear σ3-groups. Sr(σ3)2 and Ba(σ3)2 are metastable azides. The true nature of the "charge-balanced" Sr3σ2 and Ba3σ2 compounds are yet unclear, even though these compounds were first mentioned in 1κ92 by Maquenne. 165 In 19κ0 Künzel proposed a defect σaCl-type structure for Ba3σ2, but no further mentionof this structure can be found in the literature. 166 Early studies identified the product of the nitridation of barium metal only by chemical analysis. 16ι,16κ The compound was observed to be obtained from the decomposition of the azide, Ba(σ3)2, but no solid characterization was performed.

Ba3σ2 is the only alkaline earth nitride for which we find an alternative ground-state structure.

The α-Ba3σ2 comes out lowest in energy with anti-A-sesquioxide type. -Ba3σ2 will adopt an anti-Rh2τ3-II structure becoming the ground state at 2 GPa. Another structure, termed -Ba3σ2, exhibits an anti-CaIrτ3 structure starting at 34 GPa. At 51 GPa, δ-Ba3σ2 may be found with a hexagonal P63/mmc structure becoming stable. A comprehensive list of the s-block element nitrides is detailed in ACN is the a e aged oo di atio u e of the it oge ato s i A N st u tu es.

Computational details

To identify stable ground-state structures and compositions of binary Mg/Ba-σ system, a variable-composition evolutionary algorithm implemented in the USPEX code κ,13,30 was employed. The total number of atoms in the primitive cell was up to 30. The first generation had 100 randomly produced candidate structures; all subsequent generations contained κ0 structures.

Evolutionary variation operators were applied to produce new structures; 60% of the new structures were produced by heredity with the remaining structures produced by softmutation (15%), transmutation (15%), and random generation (10%).

Density-functional energy calculations and geometry optimizations were performed using the VASP package 4κ with the Perdew-Burke-Ernzerhof (PBE) parametrization of the generalized gradient approximation 41,42 . Projector-augmented wave (PAW) potentials 49 were used for magnesium, barium and nitrogen atoms with radii of 2.0 a.u. for Mg ([σe] core), 2.κ a.u. for Ba ([Kr] core) and 1.5 a.u. for σ ([He] core). A plane-wave basis set with a kinetic energy cutoff of 600 eV was employed. Uniform Γ-centered meshes with a reciprocal space resolution of 2π × 0.03 Å -1 were used for Brillouin zone sampling to ensure the error bars of the total energies were less than 1 meV/atom. Structure relaxations proceeded until all forces on atoms were less than 1 meV/Å, and total stress tensor deviated from target pressure by ≤0.01 GPa.

The dynamical stability of the statically relaxed structures was established by the absence of imaginary phonon frequencies, which were computed using the finite-displacement approach as implemented in the PHτστPY code 9ι . Moreover, the effect of zero-point energy (ZPE) on the stability of Mg/Ba-σ compounds was studied. The inclusion of ZPE only shifted moderately the fields of stability but did not change the topology of the phase diagram. Thus the following energies are non-ZPE corrected. Compounds located on the convex hull are thermodynamically stable against decomposition into any other binaries or the elements, while compounds above the convex hull are not stable. The most stable forms of pure Mg and σ depend on the applied pressure, i.e., the hcp, bcc structures for Mg/Ba and Pa-3, Pbcn, P21/c, P41212, I213, Pba2, and I-43m structures for nitrogen. In the ionic picture, each Mg atom gives its two valence electrons to the σ4 building block; thus, the σ4 2-unit has 22 valence electrons, four bonding pairs, four in-plane lone pairs and six π delocalized electrons. The poly-σ4 2-chain is isotopological and isoelectronic to polythiazyl -(S2σ2)x-, an inorganic metallic polymer. Standard application of VSEPR theory to this planar chain enabled rationalization of the structural and bonding properties of poly-σ4 2-chains: each nitrogen atom is bent (Figure γ.9b). The planarity of the whole armchair chain is the result of delocalization, but also of the firm ionic interactions between Mg 2+ ions and poly-σ4 2-chains.

Magnesium nitrides

Recall that all σ-σ bonds in σ4 2-are of nearly same length, i.e., 1.35 Å, which is longer than a double bond (1.25 Å) but shorter than a single bond (1.45 Å). This intermediate distance is consistent with the bonding picture in which six π electrons are distributed along the -(σ4 2-)xchain.

e.

b. MgN3

In the nitrogen-rich side of the phase diagram, the structure search also yielded the composition Mgσ3 with a phase transition sequence of C2/m (0 GPa, isolated σ2 dumbells) → P-1 (I) (35 GPa, isolated σ6 rings) → P-1 (II) (156 GPa, covalent nitrogen layers). However, the low-pressure C2/m phase, (2Mg 2+ )(3σ2) 4-appears as metastable, as it is above the convex hull.

Mgσ3 presents only two thermodynamically stable phases, P-1 (I) from κ9 to 156 GPa and P-1 (II) above 156 GPa. τur results are in agreement with Wei et al. ones who proposed a stable P-1

(I) phase above κ0 GPa. Moreover, from a theoretical point of view, P-1 (I) phase can be recovered at atmospheric pressure while high-pressure P-1 (II) phase can not.

In P-1 (I) phase, nitrogen atoms form six-membered σ6 rings as shown in As mentioned previously, P-1 (I) has no imaginary phonons from its range of stability down to P=1 atm, and presents persistent six-membered σ6 4-ring motif. If the barrier to decomposition to elements or others Mgxσy phases can protect P-1 (I) phase, then perhaps this original aromatic hexazine-σ6 dianion could be detected.

Above 156 GPa, a novel stable phase emerges on the energy surface as a static ground-state structure (space group: P-1). Its crystal structure is shown in Figure γ.1γa. This high-pressure P-1 (II) phase contains extended 2D nitrogen sheets separated by isolated magnesium atoms. The 2D nitrogen sublattice is closely related to the orthorhombic layered black phosphorus structure (α-P, space group: Cmca), which is composed of condensed P6 rings in a chair conformation and can be derived from α-P by removing one-quarter of the phosphorus atoms. This covalent nitrogen sublattice is also isostructural to the network found in the isoelectronic α-SrP3 phase.

Compounds containing molecule-like covalent anionic units a. MgN2

As shown in For an electron count of 19 valence electrons in σ3 4-, a radical species is identified; its HτMτ is singly occupied and π antibonding in character, as depicted in Figure γ.17. τrbital interaction analysis explains why the main group, specifically polynitrogen triatomic species with 1ι to 20 valence electrons would be bent (τ-σ-τ, 1ι ve, 134°; τ-σ-τ, 1κ ve, 11κ°; τ-S-τ, 1κ ve, 119°; τ-Cl-τ, 19 ve, 11ι°). This is consistent with the VSEPR theory, as the 16 ve AB2 systems have

c. Mg5N7

This 5:ι composition is thermodynamically stable at very high pressures (above 243 GPa)

and adopts the C2 structure (Figure γ.16c). σ2 dumbbells and bent σ3 units in the nitrogen sublattice are embedded in the magnesium framework. First, we focused on bonding analysis of the bent σ3 unit. In Mg5σι, C2v σ3 has a σ-σ distance of 1.36 Å and bond angle of 103° at 250

GPa. σote this σ-σ bond length is larger than in Mg2σ3 (1.2κ Å at 250 GPa), indicating a bond order of 1. Therefore, one may assign a formal charge of -5 to the σ3 group, leading to the 20 ve triatomic species isoelectronic to bent closed-shell Cl-τ-Cl. This bent geometry for Besides bent σ3 units, two nonequivalent σ2 dumbbells are observed in Mg5σι. The shortest interatomic σ-σ distance is 1.25 Å at 250 GPa, indicating a double bond character, thus a formal charge of -2 may be attributed. The other nonequivalent σ2 dimer has an elongated bond length of 1.32 Å, between that of single (1.36 Å) and double (1.25 Å) bonds. Thus, one may attribute a formal charge of -3 to this dimer, which is isoelectronic to the superoxide τ2 -species encountered in the known from experiment paramagnetic Kτ2 salt (τ-τ distance of 1.2κ Å).

To summarize, open-shell σ2 2-, σ2 3-and diamagnetic bent σ3 5-species are encountered in Mg5σι, and these anions are stabilized via ionic interactions with Mg 2+ cations. Mg5σι may be viewed as the formal ionic limit formula (Mg 2+ )5(σ3) 5-(σ2) 3-(σ2) 2-. As the highest occupied molecular orbital, i.e., the degenerate antibonding π * levels of σ2 2-and σ2 3-are partially occupied by two and three electrons, respectively, one may expect that Mg5σι is a metal. Indeed, this compound is metallic in character according to the DτS calculations (Figure γ.16f).

d. Mg5N4

This 5:4 composition is the first member of the Mg-rich family of stable high-pressure Mgxσy phases that is stable above 4ι GPa. In this structure (Figure γ.18), magnesium atoms lie in the center of a slightly compressed octahedron formed by nitrogen atoms, while isolated nitrogen atoms lie in a distorted pentagonal bipyramid. This structure has σ2 dumbbells, with σ-σ distances equal to 1.44, 1.50, and 1.63 Å at 300, 200, and 50 GPa, respectively, indicative of a weakened single bond in the σ2 dimer. This behavior is more easily understood when one looks at the valence electron concentration (VEC = ι.66). A VEC close to κ (octet rule) or higher leads to phases with isolated nitrogen atoms (Table γ.β). Mg5σ4 sits at the border between phases with covalent molecule-like species and isolated σ 3-particles. To explain the bonding mode in Mg5σ4, this compound could be described as (Mg 2+ )5(σ 3-)2(σ2) 4-. With a charge of -4 per σ2 unit, the two antibonding πg * orbitals of σ2 4-are filled. Thus, a bond order of one is expected in σ2 4-, a 14 ve species isoelectronic to F2. As magnesium completely transfers its valence electrons to the nitrogen framework, and if the σ 3-and σ2 4-units follow the octet rule, this compound is expected to be an insulator or a semiconductor, in perfect agreement with the DτS calculation.

Compounds containing only isolated nitrogen atoms

Besides Mg3σ2, two new compounds, Mg5σ3 and Mg4σ3, feature isolated nitrogen atoms.

They occupy cavities inside magnesium sublattice, as depicted in Scheme γ.1. These compounds have VEC values ranging from ι.66 to κ.33 (close to κ, octet rule), in which isolated σ 3-anions are found. With VEC values of ι.66 and κ.33, one extra hole h + or electron e -is expected over the whole structure. Thus, p-and n-type materials are predicted for the Mg4σ3 and Mg5σ3 phases, respectively.

Mg5σ3 adopts a hexagonal Mn5Si3-type structure with the space group P63/mcm and is stable above 120 GPa. This structure consists of infinite chains of empty face-sharing Mg6-antiprisms and isolated atoms. The formula Mg5σ3 can be rationalized as (Mg 2+ )5 (σ 3-)3 (1e -). With respect to this "extra" electron, a small half-filled band composed mostly of Mg-Mg bonding is found centered on the Fermi level, as shown in Scheme γ.1. Thus, Mg5σ3 has a metallic behavior. σote that many Mn5Si3-type A5B3 phases well known to bind diverse atoms in the interstitial cavities within a chain of face-sharing trigonal antiprisms formed by A atoms. In Mg5σ3, these interstitial sites are empty. We predict this Mg-Mg bonding will disappear due to insertion of hydrogen or fluorine, as observed in Ca5Bi3H.

Mg4σ3 crystallizes in an anti-Th3P4 type structure with the space group I-43d, and becomes stable near 111 GPa. In this structural arrangement, the nitrogen atoms are eight-coordinated by magnesium atoms, while Mg atoms are octahedrally coordinated by nitrogens. A recent study also suggested an isoelectronic analogue of Mg4σ3, the hypothetical Ca4σ3, adopts this complex structure. Such alkali earth metal nitrides can be described as Zintl phases, where magnesium atoms donate their two valence electrons to the nitrogen atoms to form closed-shell configurations.

The formula of Mg4σ3 can be written using the ionic limit (Mg 2+ )4(σ 3-)3(1h + ) with electroneutrality being achieved by the presence of one extra hole h + , leading to a p-type conductor. Therefore, from this simple electron count, the σ-2p levels are not expected to be fully occupied; this was confirmed by analysis of the DτS plot (Figure γ.18) in which the Fermi level crossed the top of the σ-2p bands, reflecting the metallicity of this high-pressure I-43d Mg4σ3phase.

Quenchable high-pressure MgN4? A route towards HEDM

To further confirm the stability of the polymeric chain found in highpressure Cmmm Mgσ4 phase after pressure release, its associated phonon spectrum was calculated at 1 atm. After relaxation, the extended 1D nitrogen net remained intact. Moreover, the absence of any imaginary modes throughout the entire 3D reciprocal space indicates this Cmmm poly-σ4 containing phase is dynamically stable at 1 atm and 0 K. However, its enthalpy is 0.191 eV/atom above the lowest metastable Mgσ4 phase at 1 atm, i.e., P2/m, which contains two isolated σ2 molecular units per formula unit. If the barrier of transformation to P2/m (or decomposition to elements or other Mgxσy compounds) can protect Cmmm, then perhaps it could exist at normal conditions. Therefore, to further probe the structural stability of the covalent armchair-shaped polynitrogen chains, ab initio molecular dynamic (AIMD) simulations based on DFT were performed at different temperatures based on a 3 × 2 × 4 supercell.

During the entire 10 ps AIMD simulations at temperatures of 300 K, 600 and 900 K, the covalent σ-σ bonds in poly-σ4 2-nets were not broken, and only small poly-σ4 To illustrate the findings, the closest σ-σ bond lengths fluctuate from 1.25 to1.45 Å at 900 K, suggesting bond order ranges between 1 and 2 within the covalent poly-σ4 2-chains. The AIMD simulations clearly suggest that the nitrogen net in Cmmm Mgσ4 maintains integrity of the infinite planar 1D armchair-shaped chains from 0 K up to 900 K with robust thermal stability and strong kinetic trapping (at least up to 900 K). Interestingly, compared to the instability of pure nitrogen polymers, this analysis reveals the encapsulated polymeric anionic nitrogen chain in Mgσ4 can be successfully preserved at ambient conditions.

Velocity of detonation (VτD), an important parameter used to evaluate the detonation properties of energetic compounds, was calculated with the Kamlet-Jacobs empirical equation.

The energy density is 2.0κ kJ•g -1 and 2.44 kJ•g -1 for P-1 and Cmmm, respectively, assuming that the final products are Mg3σ2(s) and σ2(g). Thus, the detonation velocity of Cmmm Mgσ4 was found to be 11.5 km/s, which is comparable to the secondary explosive cyclotetramethylene tetranitramine (CH2)4(σστ2)4 HMX (VτD ~ 9.1 km/s), but much higher than 2,4,6trinitrotoluene-TσT explosive (VτD ~ 6.9 km/s) and the primary explosive lead azide Pb(σ3)2 (VτD ~ 3.κ km/s). Moreover, the stabilization pressure for P-1 Mgσ4 is only 13 GPa, much lower than the pressure required to synthesize polymeric cg-σ2 (>100 GPa). High energy density, low stabilization pressure and quenchability make P-1 Mgσ4 a candidate high energydensity material. Finally, to extend this analysis on nitrogen-rich Mgxσy-based HEDM compounds, we computed the energy density of all predicted Mgxσy phases (see Appendix). The high-pressure phase P-1 (I) Mgσ3 with covalent σ6 rings is the most promising HEDM candidate with an energy density of 2.κι kJ•g -1 , followed by P-1 Mgσ4 (2.0κ kJ•g -1 ), P63/mcm Mgσ2 (1.95 kJ•g -1 ) and Pmmm Mg2σ3 (1.90 kJ•g -1 ).

Conclusion

In summary, we present the first complete phase diagram for the Mg-σ system at pressures up to 300 GPa. High pressure may be an effective route to stable Mg nitrides with unusual stoichiometries and original, yet aesthetic, structures. Besides Mg3σ2, eight new compositions at Mg:σ ratios of 1:4, 1:3, 1:2, 2:3, 5:ι, 5:3, 4:3, and 5:4 were identified. Their structures contain molecule-like covalent nitrogen units, such as σ2 dumbbells, bent σ3 motifs, planar Sτ3-like σ(σ3) units, and benzene-like σ6 species. In addition to well-known Mg3σ2, newly predicted Mg5σ3 an Mg4σ3 feature isolated σ 3-anions. ZKC, molecular orbital analysis and VSEPR theory proved useful for rationalizing the structural, bonding and electronic properties of these Mgxσy compounds. σotably, the dimensionality of the nitrogen network decreases as the amount of magnesium increases, with magnesium atoms acting as a scissor by transferring their valence electrons to the antibonding states of the nitrogen sublattice. More interestingly, the polymeric nitrogen network found in Mgσ4 is preserved from 0 K up to 900 K. This analysis reveals that magnesium atom plays an important role in stabilization of polymeric nitrogen networks, and that Mgσ3 and Mgσ4 are potential high energy-density materials. The present study contributes to understand the unusual crystal features and exploitable physical properties of nitrogen compounds under extreme conditions. compound, is negative, i.e., they are located on the convex hull. τther metastable phases, with different covalent nitrogen nets, are also detected on the PES above the convex hull, but we selected the ones which have exothermic formation reactions (∆Hf < 0), and are close to the convex hull (∆Hf < 0), or experimentally synthesized.

These latest structures are metastable in the binary Ba-σ phase diagram, but there might appear, experimentally for a kinetic reason, so-called, viable structures, 1κ1 such as experimental Ba(σ3)2 azide. 1κ2 The metastable structures are of great significance, and in practice can often be synthesized by choosing appropriate precursors and controlling conditions of the experiment (pressure, temperature, quench rate, etc.).

Moreover, for selected high-pressure nitrogen-rich structures, e.g., Baσ4 and Baσ10, we checked their quenchability by looking at their kinetic and thermal properties. We performed ab initio molecular dynamics simulations up to 900 K. A crystalline structure is definitively called viable if there are no structural transformations of the nitrogen network, i.e., nitrogen bond breaking, were observed after a long simulation (10 ps), indicating the existence of a substantial kinetic barrier which facilitates the formation and trapping of viable, (meta)stable structures.

Stable compounds, which form the thermodynamic convex hull (Figure γ.β0), were identified at selected pressures, and their stability fields were shown in the pressure-composition phase diagram (Figure γ.β1). It is noteworthy that our evolutionary searches successfully reproduced the experimentally known Ba3σ, 1κ3 Ba2σ, 1κ4 Baσ2 1κ5 and Baσ6 1κ2,1κ6 structures.

However, the expected charge-balanced 3:2 composition for an alkaline earth nitride salt has not yet been characterized.

Twelve stoichiometries emerged to be stable, including nine σ-rich stoichiometries, namely Baσ, Ba2σ3, Baσ2, Ba3σκ, Baσ4, Baσ5, Ba2σ11, Baσ6 and Baσ10, and three Ba-rich compositions, Ba3σ2, Ba2σ and Ba3σ, when pressure increased up to 100 GPa. Within the same pressure range, the binary Ae-σ stable stoichiometries have been predicted for Be, 156 Mg, κ9,159 Ca, 164 , respectively. The diversity of stable stoichiometries in Ae-σ phases increases when Z(Ae)

increases. The increase of the cation size (Be < Mg < Ca < Sr < Ba) allows more nitrogen atoms As a first guide, a useful parameter to rationalize the structural properties is the valence electron concentration (VEC) defined for Baxσy compounds as VEC = (2x+5y)/y. For VEC = κ, the octet rule is verified for each nitrogen, i.e., σ 3-, as in the charge-balanced Ae3σ2 compound.

Thus, in barium-rich phases, in which VEC > κ, the σ ions remain isolated. In nitrogen-rich phases (VEC < κ), the nitrogen anions can be viewed as nuclei with unpaired electrons. Therefore, nitrogen-nitrogen bonds are expected to fulfill the octet; molecular or extended covalent nitrogen networks are encountered. This is what we see and detail after that. σevertheless, remember that the formal charge distribution does not reflect the real charge transfer.

We recall that the lengths of σ-σ bonds are roughly 1.10 Å for the triple bond, 1.25 Å for the double bond and 1.45 Å for the single bond at ambient conditions. For calibration, the σ-σ single bond distance in three-dimensional covalent high-pressure cg-σ (I213) phase is 1.3ι, and 1.34 Å at 50 and 100 GPa. These values help us to guide assignment of nitrogen-nitrogen bonds with the help of Lewis rules to rationalize the local structural environments in nitrogen motifs using VSEPR theory.

Ba-rich electrides: Ba3N and Ba2N

Two stable Ba-rich compositions are proposed at ambient pressure, namely, Ba3σ and Ba2σ, but only Ba2σ is thermodynamically stable (see

a. Ba3N

At 1 atm, the anti-TlI3-type structure (space group P63/mcm) is the most stable, enthalpically, which is in good agreement with the experimental findings. 1κ3 The calculated lattice constants are consistent with the experimental data (See Table γ .5). Ba and σ atoms arrange themselves in an infinite chain of faced sharing barium octahedra, centered with nitrogen atoms, as shown in the chains, the calculated Ba-σ distance is 2.ι5 Å, which is slightly lower than the sum of the ionic radii of 2.9 (Ba 2+ σ 3-). 1κι The closest σ-σ separation is 3.55 Å. Such distances are explained by assuming strong ionic interactions within the chain according to [(Ba 2+ )3(σ 3-)] 3+ . This electron repartition leaves an excess of 3 electrons for metallic bonding between the chains; or for ionic interstitial electrons with (Ba3σ) 3+ chain interactions. The latter bonding mode explains the long inter-chain Ba-Ba separations of 4.9-5.0 Å, to be compared with a calculated Ba-Ba distance of To summarize, the cationic (Ba3σ) 3+ chains/rods interact with interstitial electrons localized in the one-dimensional void of P63/mcm structure. Also, Ba3σ is a 1D electride at 1 atm. Above 1 GPa, Ba3σ is unstable, concerning disproportionation to Ba + Ba2σ.

Ta le . C stal st u tu es of P / Ba N phases.

P /mcm Ba N Latti e para eters

E pe i e tal a ie t o ditio a= . , = . , = .

α= . , β= . , γ= .

P ese t o k

GPa / K a= . , = . , = .

α= . , β= . , γ= . The integrated electron density of the conduction band from -2 eV to 0 eV is equal to 0.45 e/f.u.

Looking at its DτS in

This result is consistent with the chemical view of the [(Ba 2+ )2(σ 3-)] + e -electride.

To summarize, R-3m dibarium nitride crystal may be view as cationic Ba2σ + layer alternating with confined anionic electrons, the signature of a two-dimensional conducting electride. A R-3m → P-3m1 phase transition occurs at 1 GPa. This anti-CdI2-type structure is computed to be the most stable phase up to 5ι GPa. The P-3m1 Ba2σ crystal structure is closely related to that of the atmospheric phase. It presents an /AA/ stacking of Ba2σ layers based on of Ba6/3 edgesharing octahedra centered by a nitrogen atom. At 10 GPa, The Ba-σ distance is 2.62 Å, reflecting an ionic bonding character, while the interlayer Ba-Ba separation is computed at 3.5ι Å, analog that in Baτ (Ba -Ba 2+ =3.κ3 Å at 10 GPa To summarize, the 2D electride property of Ba2σ is not maintained in high-pressure metallic I4/mmm phase, although its Ca2σ analog has been claimed to be a 2D electride from 0 to 100 GPa with a predicted R-3m → Fm-3m → I4/mmm sequence. The contour map of the partial electron density, just below the Fermi level, is very informative; we can see electron density maxima in the interlayer space, located between the cationic (Ba3σ2) + slabs. Within -2 and -1 eV energy range, the electrons are localized at the isolated nitrogen. High electron density is found in the interstitial regions and σ2 units in the energy window of -0.5 eV, up to Fermi level, a typical signature of an electride.

Combining the DτS and partial charge density analysis, we conclude that Ba3σ2 is a conducting 2D electride. Here, non-atom-centered electrons act as intercalated anions, holding the cationic Ba3σ2 layers together through coulomb attraction. Unexpectedly, at ambient pressure, Ba3σ2 belongs to the layered subnitride family, such as Ba2σ, and not to the charge-balanced Zintl Ae3σ2 compounds (Ae=Be, Mg, Ca). Finally, we hypothesized that a layer of [Ba3σ2] + (e -) could be separated from the bulk material. τur DFT calculations predict an exfoliation energy of 125 meV/atom for Ba3σ2, to be compared with 264 meV/atom for [Ca2σ] + (e -). 192 σevertheless, a more profound theoretical investigation is on-going to conclude on the stability of this proposed, two-dimensional, electride Ba3σ2 material, and to evaluate its electronic properties and superconductivity. would be expected to be semiconducting. However, except for P21/c phase at 10 Gpa, the calculated DτS in shows that ZKC does not work in this case.

A close inspection of the DτS shows that 6s/5d Ba orbitals have a significant contribution to the occupied nitrogen levels, the signature of a partial covalent Ba-σ character in Baσ. A backdonation is in work, from the occupied σ2 π * levels to the empty Ba ones, and this explains the metallic character of Baσ. This finding illustrates the limitation of the Zintl rule in Ba-σ system under pressure; not all valence electrons of electropositive alkaline earth metal are localized in nitrogen networks, but may participate in covalent cation-anion bonds, and metallic cation-cation ones, as found in metallic CaSi 194 or K5Sb3 195 . We will see that such Zintl violation is a consistent feature of the behavior of several Ba/σ phases under pressure.

b. Ba2N3

From 14 to κ3 GPa, the C2/m phase is the thermodynamic ground state phase. Its structure is displayed in Figure γ.γ4a, and can be thought of as being composed of isolated σ2 units embedded in the barium framework. Two non-equivalent σ2 dumbbells are observed in Ba2σ3.

At 50 GPa, the shortest interatomic σ-σ distance is 1.25 Å, indicating a double bond character.

Thus a formal charge of -2 may be attributed. The other non-equivalent σ2 dimer has σ-σ separation of 1.34 Å, between that of single (1.41 Å) and double (1.25 Å) bonds. Thus, one may attribute a formal charge of -3 to this dimer, which is isoelectronic to the superoxide τ2 -species encountered in the experimental paramagnetic Kτ2 salt (τ-τ distance of 1.2κ Å). To explain the bonding mode in Ba2σ3, this compound could be assigned as (Ba 2+ )4(σ2 2-)(σ2 3-)2. With a charge of -2/-3 per σ2 unit, the two antibonding πg * orbitals of σ2 2-/3-are partially filled (Figure γ.γ4b).

Thus, a bond order of 2 is expected in σ2 2-and of 1.5 in σ2 3-, which correlates well with the structural properties of σ2 units. In addition, the Ba2σ3 compound is a metal in the 0-100 GPa range.

Figure .

a C / Ba N st u tu e at GPa, the pDO".

c. BaN2

The 1:2 stoichiometry is stable over the whole pressure range.

d. Ba3N8

τur evolutionary searches uncovered σ-rich Mg3σκ stoichiometry as a thermodynamic ground state at a pressure as low as 6 GPa, easily achievable in high-pressure synthesis. This phase is stable thermodynamically from 6 to 64 GPa (Cmmm space group, Z=2). This aesthetic structure may be viewed as follows: a hexagonal layer of barium atoms (4 Ba/unit cell), a σ2 dumbbell seating in the hole of each Ba6 hexagon (2 σ2/unit cell), one Ba filling the sixfold channel perpendicular to 2D Ba layers (2 Ba/unit cell), and 6 σ2 surrounding each isolated Ba ( 6σ2/unit cell). σσ distance is 1.22 (σ2 in ab plane) and 1.16 (σ2 along 100 direction). Thus, a formal electron attribution leads to a layer of [Ba2(σ2)] 2+ and two Ba and six neutral σ2.

e. BaN4

Baσ4 is thermodynamically stable above 4 GPa, and exhibits three phase transitions up to 100 GPa, the first phase occurring at 4 GPa (P6/mmm), the second one at 30 GPa (P41212), and the last at 4κ GPa (C2/c). The stable P6/mmm structure contains σ2 dumbbells in prismatic sites of hexagonal barium network. At 10 GPa, Ba-Ba separations are longer than 4.1 Å, and the shortest Ba-σ contact is at 2.91 Å, typical of ionic bonds. Thus, σ2 dimer has formally a minus charge, (Ba 2+ )(σ2 -)2. From this assumption, the σ2 -Mτ diagram presents a singly occupied antibonding πg * . Therefore an elongation of the σ-σ bond is expected in respect to neutral σ2.

This is what we observed; at 10 GPa, σσ separation of 1.1ι4 Å in P6/mmm and 1.10κ Å in neutral σ2. σ2 -is a radical such as its isovalent nitric oxide στ. τur assumption is validated by the computed electrical conducting property of P6/mmm Baσ4; it is a metal (See Figure γ.γ7). Mgσ4, κ9 at high pressure. The σ-σ bond lengths, along the nitrogen chain, are around 1.33 Å, which is greater than a double bond (1.25 Å), but shorter than a single bond. This intermediate distance is consistent with the bonding picture, in which six π electrons are distributed along the polymeric -(σ4 2-)x-chain, (see the discussion in the previous section on Cmmm Mgσ4 structure).

Standard application of the VSEPR theory to this planar chain enabled rationalization of the structural and bonding properties of poly-σ4 2-chains: each nitrogen atom is bent. The planarity of the whole armchair chain is the result of delocalization, but also of the strong ionic interactions between Ba 2+ ions and poly-σ4 2-chains. The densities of states (DτS) of P41212 Baσ4 were computed and indicated a metallic character.

A C2/c phase shows up above 4κ GPa, and also presents one-dimensional nitrogen chains running in Ba channels. Each chain is non-planar and may be viewed as two linked non-planar cis-bent σ4 units with a torsional angle of 2κ.3° at 100 GPa (See

f. BaN5

The stability range of Baσ5 is 25-κ0 GPa (space group Imma). Its structure is displayed in Figure γ.γ9. This phase is built from hcp Ba layers, in between which there are five-membered σ5 rings. Such pentazole rings have been predicted in alkaline and alkaline earth metals Mσ5 (M=Ca, 164 Li, 151 Cs, 145 σa, 152 etc), but have also been observed recently in an experimentally high-pressure phase, Csσ5. 144 At 50 GPa, σ-σ distances are ranging from 1.30 Å to 1.35 Å in planar pentazolate unit, signalling a delocalized system. The averaged σ-σ bond length in pentazolate σ5 -is 1.301 Å in Liσ5 (40 GPa), 1.30 Å in σaσ5 (20 GPa), 1.32κ Å in Csσ5 (1 atm),

1.329 Å in the gas phase. Therefore, it is tempting to assign a formal charge of one minus per σ5

ring. This σ5 -ring, isoelectronic to C5H5 -cyclopentadienyl, has delocalized 6 electrons, and follows the Hückel aromaticity (4n+2), and octet rules. σevertheless, following the ZKC, Ba should give two electrons to each σ5. This is what conclude Zhu et al. 164 in their study of electronic properties of predicted Caσ5 structure. σote that, with a 2-charge, σ5 2-is a radical species. A close look at the bonding is needed. In Imma Baσ5, inspection of the density of states shows a high participation of barium 5d orbitals in the filled levels of the system (See Figure γ.γ9). A significant d-orbital population of approximatively 0.55 at the barium atom is computed at 50 GPa. Moreover, a -ICτHP of 0.21κ eV/pair is computed for Ba-σ contacts. Therefore, the cation-anion interactions cannot be described as purely ionic, but also have a partially covalent character. We propose that the extra electron is delocalized over the Ba-(σ5 -)-Ba network, which explains the relatively short cationanion separation of 2.59 Å at 50 GPa.

To summarize, Ba gives one electron to a σ5 ring, following the Hückel and octet rules, while the remaining electron is over the whole Ba net through partially covalent Ba-σ interactions. This simple but founded picture explains well the metallic property of Imma Baσ5:

g. Ba2N11

From κ to 14 GPa, the P-1 phase of Ba2σ11 is stable. As shown in and four σ2 motifs per formula unit fill the void between two Ba layers. Ba-Ba and Ba-σ separation are longer than 4.03 Å and 2.69 Å, respectively, at 10 GPa. The calculated σσ separation is 1.1κ Å in centrosymmetric σ3, a value typical of a double bond observed in azide σ3 -structures 169,1ι1 . Thus, it remains formally three electrons to four σ2, (Ba 2+ )2(σ3 -)(σ2) 3-4. A distance of 1.15-1.16 Å is found in σ2 units, a slightly elongated triple bond provoked by the partial occupation of antibonding nitrogen π * bands. Ba2σ11 is a metal.

Conclusion

In this study, we carried out crystal structure prediction to investigate the ground stable states of barium nitrides at ambient, and up to 100 GPa. Along with the experimental reported Ba3σ, Baσ2, Baσ2, and Baσ6, we predicted eight new stoichiometries, namely, Ba3σ2, Baσ, Ba2σ3, Ba3σκ, Baσ4, Baσ5, Ba2σ11 and Baσ10, to become stable at specific range of pressure.

Interestingly, Ba3σ and Ba2σ are predicted to have electride properties. Interstitial electrons have been found existing in channel and layer forms. By adjustment of the ratio of barium in the Baxσy phase diagram, a series of polynitrogen forms were achieved in the crystalline, including small molecular units made of a few atoms (σ2, σ4, σ5, and σ6) to1-dimensional chains.

Pentazolate alkaline-earth metal salts

Motivations

Pentazolate ions in materials have been notoriously elusive for the last hundred years, despite enormous efforts to make these compounds in a condensed state. Recently, researchers have synthesized solid-state compounds consisting of isolated pentazolate anions σ5 and alkaline metal elements, i.e., σaσ5 152 and Csσ5. 144,145 To gain access to the cyclo-pentazole ring, they have compressed metastable sodium, or cesium azide (A + , σ3 -) mixed with the σ2 cryogenic liquid, in a diamond anvil cell together with laser heating. In the previous barium nitrides section, we mentioned that we had predicted a nitrogen-rich phase (P-1 Baσ10) containing pentazolate anions.

Based on our current knowledge, this is the first time that the Baσ10 with σ5 -motif is proposed.

τur calculation predicts that P-1 Baσ10 is thermodynamically stable above 12 GPa.

Motivated by this novel experimental and theoretical discovery, we explored the Mσ10 stoichiometry, where M represents alkaline earth metal elements, i.e., Be, Mg, Ca, Sr and Ba at ambient pressure and 50 GPa. We aim to see if solid-state M(cyclo-σ5)2 is stable thermodynamically, dynamically and kinetically. Moreover, we will analyze the cation size effect on the structural, energetic and electronic properties, both at ambient and high pressure (here, 50

GPa, a pressure accessible in high-pressure physics).

Methods

The general methodology is presented in the first chapter. DFT PAW GGA PBE calculations are carried out. Therefore, we will describe only the specific methodological details here. First of all, we carried out molecular crystal searches, using the technics implemented in USPEX code κ, 13,30 to make sure we located the most stable pentazolate phase at 0 GPa. Thus, the σ5 pentazolate unit was treated as an integral unit in the molecular crystal. Then we conducted a standard fixed-composition crystal structure prediction at 50 GPa. Up to 44 atoms per primitive cell were used. The phonon dispersion curves are calculated for all of the structures, and show the absence of any imaginary frequencies in the entire Brillouin zone. All of the discussed Mσ10 structures are local (global) minima on the PES.

Results and discussion

Remarkably, a series of pentazolate phases have been predicted as the ground stable, or metastable phase. Remember that all are dynamically stable. The predicted phases are listed in GPa. At 0 GPa, Beσ10 adopts I-42d (I) symmetry, and Mgσ10 has a P43212 space group, while Caσ10, Srσ10, and Baσ10 share the identical space group of I-42d (II). σotice that the I-42d (I) become energetically unfavorable when the atomic number increases.

At a pressure of 50 GPa, Beσ10 adopts Fdd2 space group, but very close in energy to I-42d

(II) (6 meV/atm), the lowest phase for Mgσ10. For the heavier Ca and Sr elements, the Ibam phase becomes the lowest enthalpy phase. Finally, Baσ10 has P-1 space group.

Before the discussion of the electronic properties analysis, a question was raised: Are these Mσ10 pentazolate phases ground stable concerning other Mxσy nitrides? Fortunately, Be-σ, 156

Mg-σ, κ9,159 Ca-σ, 164 and Ba-σ binary phase diagrams have been explored by us and other Psta le researchers, which makes it possible to compare our Mσ10 compounds with other ground stable phases.

Figure .

a . E thalp diffe e e of ea h p edi ted phase elates to I-d II at GPa; . E thalp diffe e e of ea h p edi ted phase elates to I a at GPa.

Firstly, 3:2 is the only stoichiometry that locates as ground stable state at ambient pressure in Be-σ and Mg-σ systems. Therefore, their stability criterion is based on the reaction between Mσ10 and M3σ2 plus σ2. We calculated the reaction enthalpy as:

ΔHr=(3Mσ10-(M3σ2+14σ2))/33
ΔHr > 0 means Mσ10 is a metastable phase and will decompose to M3σ2 plus σ2, vice versa.

Among the two reactions, ΔHBe= 0.3ι1 eV/atom and ΔHMg= 0.2ι4 eV/atom. Thus, I-42d (I)

Beσ10 and P43212 Mgσ10 are metastable phases at 0 GPa.

For Ca-σ and Ba-σ systems, Caσ2 and Baσ6 stabilized at 0 GPa, and emerged as the nearest stoichiometries to the 1:10 ratio. The enthalpies of reaction calculated from these phases shows that Caσ10 and Baσ10 are metastable phases at ambient pressure as well (ΔH > 0). Besides, we are not able to calculate the thermodynamic stability of Srσ10 due to the absent information of Sr-σ binary phase diagram.

At 50 GPa, the phases that have 1:4 stoichiometry become the ground stable in Be-σ and Mg-σ systems. The enthalpy of reaction shows that Beσ10 is 0.109 eV/atom and Mgσ10 0.164 eV/atom lower than Beσ4/Mgσ4 + 3σ2. Meanwhile, Caσ5/Baσ5 replaces Caσ2/Baσ6 as the nearest stable phase to Mσ10. The calculation of reaction enthalpy indicates the value of -0.1κ5

eV/atom and -0.150 eV/atom for Caσ10 and Baσ10, respectively. All the negative reaction enthalpies illustrate that Beσ10, Mgσ10, Caσ10 and Baσ10 are energetically favorable, and are the ground stable phases at 50 GPa. Although so far there is no available data for Srσ10, we hypothesize that Srσ10 is a thermodynamically stable phase, according to the regular pattern of the periodic table.

Figure γ.45 displayed the I-42d (I) Beσ10 phase. The Be-σ separation is computed at 1.ι2, larger than the sum of the Be 2+ and covalent σ one (0.59+0.66, respectively). Thus, as expected for an ionic salt, this phase consists of isolated Be + cations and σ5 -. At 0 GPa, the σ-σ distances are 1.300 Å, 1.313 Å, 1.346 Å, with a ratio of 1:2:2. Be forms a tetrahedron with 4 σ5 -rings.

Moreover, beryllium forms also a tetrahedron with the four surrounded beryllium atoms located at 5.29 Å. The sublattice of Be has an I41/amd space group which can be found in many crystals, e.g., Sn and Ge. Particularly, the Be(σ5)2 structure presents large vacuums around the atoms, characteristic of a metal-organic framework. These voids might be stabilized molecular σ2 or other species, Be(σ5)2 potentially acting like a sieve. At ι GPa, I-42d (I) Beσ10 undergoes a phase transition to a high symmetric Fdd2 phase which contains four formula units per unit cell, and a higher density packing. In Fdd2 phase, beryllium atoms lie in the center of a slightly compressed octahedron formed by six σ5 rings (See In Baσ10, a phase transition from I-42d (II) to P-1 happens at κ GPa. σote that the P-1 barium pentazolate salt lies on the convex hull at a surprisingly low pressure of 12 GPa (Figure γ.β1).

The P-1 structures consist of layers of barium atoms sandwiched between layers of pentazolate σ5 -anions, oriented differently to one another in the unit cell (Figure γ.48). Because of the aromatic nature of the bond in the pentazolate σ5 ring, the bond lengths for both phases are between those of the double-bond (1.25 Å as in trans-diimine) and the single bond (1.45 Å as in hydrazine). Also, we discovered another metastable C2/c Baσ10 phase which is only 4 meV/atom higher in energy than P-1 phase at 50 GPa.

Dynamical stability

In order to examine the dynamical stability of the Mσ10 phases at 0 GPa, ab initio molecular dynamics (AIMD) simulations based on DFT are carried out by using supercells at 1000 K within each time step of 1 fs, and the total simulation time up to 10 ps. In the simulations, the canonical σVT (σ -constant number of particles, V -constant volume, and T -constant temperature)

ensemble is adopted as implemented in VASP code. During the entire 10 ps AIMD simulations, the covalent σ-σ bonds in σ5 nets are not broken at least at 1000 K; only small atomic vibrations were observed (Figure γ.49). However, several σ5 rings decompose to σ2 and bent σ3 units in Beσ10 phase, which implies it can only exist at a relatively lower temperature (< 1000 K).

N2 storage in Metal-Inorganic-Framework (MIF) pentazolate beryllium

The I-42d (I) Be(σ5)2 structure presents an open framework, a characteristic of MIF materials. Thus, it's useful to study the capacity of this phase to store molecular species. As a preliminary result, we filled the MIF with σ2 molecules. τur modelling searches lead to at least 1.5 σ2 per formula unit. The thermostability of this extremely nitrogen righ compound has been confirmed using AIMD simulation at room temperature; Be-σ covalent net is not destroyed (See

Conclusion

We performed the first-principles evolutionary structure search of nitrogen-rich compounds, consisting of alkaline earth metal elements and nitrogen, with 1:10 stoichiometry at ambient condition and 50 GPa. Several new high-nitrogen content materials, containing pentazolate units, have been uncovered. Most of the Mσ10 compounds, featuring pentazolate anions in the crystalline state, are thermodynamically stable at high pressures. However, the zero pressure phases are metastable at ambient conditions. The temperature sustainability of zero pressure phases has been proved by using AIMD at a temperature up to 1000 K. Further thorough experimental validation is feasible and will help us to develop a novel HEDM.

Transition metal nitrides

Introduction

σearly a century since the pioneering works in the 1920s-1930s by G. Hägg in crystal structure determination of transition metal nitrides Mxσy (M, transition metal), the elucidation of the atomic arrangement in binary metal-nitrogen phases is still a delicate task. 19κ,199 Many Mσ1z in the literature are possibly oxynitrides, imides, or amides; thus, efforts have to be made to assign a valid crystal structure for a given stoichiometry. 200 A number of studies in the Zr-σ, 201

Hf-σ, 202 Ta-σ, 203,204 and W-σ 205 systems have demonstrated the power of high-pressure synthesis in the search for new nitrogen-rich nitrides. The entire field of synthesis and study of the high oxidation state of transition metal nitrides has been reviewed recently by Salamat et al. 205 The understanding of their peculiar mechanical and chemical properties, such as high stiffness, high hardness, high thermal conductivity, high melting point, and good catalytic performance, needs a characterization of materials from a crystallographic point of view. Their fascinating properties stimulate the development of new synthetic routes leading to new welldefined nitride materials.

τn the one hand, Molybdenum dinitride (Moσ2) was recently synthesized at a moderate pressure of 3.5 GPa, and a layered MoS2-type structure has been proposed. [START_REF] Wang | [END_REF] However, our firstprinciples calculations of thermodynamic, mechanical and dynamical properties suggest that this layered R3m structure is unstable. We find that the ground state of the Moσ2 system is a pernitride structure with space group P63/mmc which transforms to a P4/mbm phase above κ2 GPa.

Chemical bonding analysis shows that one could assign Moσ2 as Mo 4+ (σ2 4-); i.e., Mo is formally a d 2 metal, in agreement with the experimental results of Wang et al. [START_REF] Wang | [END_REF] The presence of covalent σ2 dumbbells and strong bonding between Mo 4+ and σ2 4-is the source of the superior mechanical properties of these predicted ultra-incompressible Moσ2 pernitrides.

τn the other hand, zirconium nitrides represent a rich family of phases where the stability and microstructures are still not completely understood. 201,[206][207][208][209][210] In addition to the known Zrσ and Zr3σ4, new candidate structures with Zr:σ ratios of 2 :1, 4:3, 6:5, κ:ι, 15:16, ι:κ and 4:5 are found to be ground-state configurations, while Zr3σ2 has a very slightly higher energy. Except for Zr2σ, the newly discovered Zrxσy compounds adopt rocksalt structures with ordered nitrogen or zirconium vacancies. The electronic and mechanical properties of the zirconium nitrides are further studied to understand their composition-structure-property relationships.

In this chapter, to address these issues, the crystal structures of Moσ2 and Zrxσy were explored by the ab initio evolutionary crystal structure prediction method (USPEX). τur work should guide experimental groups aiming to synthesize these new technologically useful materials.

Computational details

The crystal structure prediction for thermodynamically stable structures was performed using the evolutionary algorithm, as implemented in the USPEX code. κ, 13,30 The lowest-energy structures were determined at 0, 10, 20, 40, and 100 GPa with systems containing up to 30 atoms in the simulation cell. The first generation of κ0-100 structures was produced randomly; all subsequent generations contained 60-κ0 structures and were produced using variation operators such as heredity (60%), softmutation (15%), lattice mutation (15%), and 10% of each new generation was produced randomly.

Each structure was fully relaxed to an energy minimum at different pressures within the framework of density functional theory by using the VASP package. 4κ We employed the allelectron projector-augmented wave (PAW) method and the Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) functional. 41,42 The PAW potentials have [Kr], [Kr] and [He] cores with radii of 2.ι5, 3.00 and 1.50 a.u. for Mo, Zr and σ atoms, respectively. A plane-wave basis set with a kinetic energy cutoff of 600 eV was employed. We used uniform Γ-centered k-point meshes with a reciprocal space resolution of 2π × 0.03 Å -1 for Brillouin zone sampling. These settings enable excellent convergence of total energies, forces, and stress tensors.

Theoretical phonon spectra were calculated by the supercell approach by using the PHτστPY package 9ι to probe the dynamical stability of the structures. All discussed energies are zero-point energy (ZPE) corrected. σote that, if any phonons of a given structure exhibit imaginary frequencies, the studied structure is dynamically unstable and should transform under lattice relaxation into a more stable ground state structure. The elastic tensors were further calculated for all dynamically stable structures, and the mechanical stabilities were determined by Born-Huang criteria. 211 Chemical bonding analyses were carried out using the crystal orbital

Hamilton population (CτHP) method by using the LτBSTER code. 51,53,54

Molybdenum nitrides

In 2015, a nitrogen-rich molybdenum-based compound, Moσ2, was proposed 1 , extending the well-known binary Mo-σ compositions. This newly discovered nitride, γR-Moσ2, adopts a rhombohedral R3m structure and was proposed to be isotypical with MoS2. This structure is a bulk layered material in which the layers interact via van der Waals forces (Figure 4.1a). Mo is located in a trigonal prismatic atomic arrangement and sandwiched between σ atoms. The "determined valence state for Mo is +3.5 in both γR-Moσ2 and MoS2" claimed by Wang et al., to our astonishment as Moσ2 has two valence electrons less per formula unit than in MoS2.

Therefore, nitrogen atoms do not fulfill the octet rule in the proposed Rγm Moσ2, as sulfur does in MoS2 (Figure 4.1a, see the formal electron attribution on tricoordinated X atoms in MoX2 (X = S, σ; Mo 4+ , d 2 )). We hypothesized that this electronic situation should be very unstable from thermodynamic and mechanical points of view and that adjacent Moσ2 layers must be connected through σ-σ covalent bonding, leading to the well-known pernitride situation (diatomic σ2 units in Mo network).

There are at least two questions that needed to be answered: first, what is the ground-state structure of Moσ2? Knowledge of solid-state Moσ2 geometrical and electronic properties is crucial for understanding the catalytic properties of such a nitrogen-rich molybdenum phase.

Second, as the synthesis of Mσ2 is done under moderate or high pressure, what is the effect of pressure on solid-state Moσ2? In this work, new ground states and metastable Moσ2 structures at both ambient and high pressures are proposed and established. The previously proposed R3m

Moσ2 is found to be thermodynamically, dynamically, and mechanically unstable, while a pernitride phase (P63/mmc) emerges as the ground-state structure. P / PBE ela ed a = . = .

Mo . , . , . N . , . ,

While the 2D MoS2-like Moσ2 layer is magnetic as expected, 212 the coupling of unpaired electrons localized on two adjacent layers is responsible for the ferromagnetic to nonmagnetic phase transition. However, the state-of-the-art DFT is not a suitable method for studying interactions in loosely packed systems (such as soft matter, van der Waals complexes, and biomolecules), as this method fails to adequately describe the long-ranged dispersion interactions. 213,214 Therefore, we performed a full structural optimization by using the PBE-GGA DFT level and vdW-D2 method of Grimme for van der Waals correction (Table 215 The structural parameters within the covalent 2D Moσ2 sheet are very similar, while the lattice constant c decreased from 13.κ69 Å (PBE without vdW correction) to 11.214 Å (PBE with vdW-D2 correction). Therefore, short interlayer Mo-σ contacts appear (Mo-σ = 2.541 Å) and Rγm is no more a MoS2-like layered structure (Figure 4.1b). This phenomenon illustrates the strong structural instability of the claimed layered MoS2-type R3m Moσ2 structure.

To ensure whether such a MoS2-type structure is a stable local minimum on the Moσ2 potential energy surface, we have examined its mechanical and dynamical stabilities by calculating the elastic constants and phonon spectrum. R3m is found to be mechanically unstable due to the negative C44 value of -222 GPa, and large imaginary frequencies are found over the entire Brillouin zone (Figure 4.1c). When van der Waals correction is taken into account, the fully relaxed Rγm structure is also found to be dynamically unstable with very large imaginary frequencies (Figure 4.1c). τne may conclude that atomic displacements in R3m shall lower its enthalpy by transforming barrierlessly to a metastable state which will have another space group.

Moreover, the R3m structure has a very high positive formation enthalpy of 0.κ19 eV/atom (endothermic Mo(s) + σ2(g) = Moσ2(s) reaction), while, in other transition metal nitrides, the ground-state structures usually have negative formation enthalpies, and even for high-pressure pernitrides the positive formation enthalpies at zero pressure are much lower (ΔHf < 0.64 eV/atom). The thermodynamic, dynamical, and mechanical instabilities of layered R3m Moσ2 motivated us to search for other possible ground-state structures at both atmospheric and high pressures. While the use of vdW-D2 correction is shown to be crucial to describe the structural properties of layered compounds, we show that the experimental bond lengths in bulk Mo2σ and Moσ are well reproduced at the GGA-PBE level of theory; therefore, we still used the GGA-PBE level of theory for all bulk Mo-σ based structures.

NiAs-type MoN2 pernitride and other metastable structures at ambient pressure

We started to perform fixed-composition structure searches at ambient pressure. The 12 predicted structures with lowest energies are shown in Figure 4.β. Almost all of the pernitride structures which contain discrete σ2 units (1-6) lie lower in energy than the structures with isolated nitrogen anions (ι-κ) and the ones with layered structures (10-12). The lowest pernitride (σ2 x-, x = 2-4) is more stable by at least 0.34 eV/atom than the dinitrides (2σ The calculated σ-σ distance of 1.3ιι Å is close to the value found in σ2H4 (1.45 Å) and

Ptσ2 (1.41 Å) but much longer than that in σ2F2 (1.21 Å) and Baσ2 (1.23 Å) where a formal σ═σ double bond is assigned. Therefore, one may assign to the dinitrogen unit a formal charge of -4 leading to the singly bonded σ2 4-diatomic species. With fourteen valence electrons σ2 4- would be isoelectronic with F2 (1.412 Å) and disulfide anion (S2 2-), which is found in pyrite FeS2 with four electrons in the antibonding 1πg * orbitals. Formally, one could assign Moσ2 as Mo 4+ (σ2 4-), i.e., Mo is a d 2 metal, in agreement with the experimental results of Wang et al. [START_REF] Wang | [END_REF] They noticed "the determined valence state for Mo is +3.5..., inferring a peculiar 4d 2.5 electronic structure." To confirm these electron counting and oxidation state assignments, the electronic properties of P63/mmc Moσ2 were analyzed (Figure 4.γ). Band structure and the density of states (DτS) show that the P63/mmc phase is a semiconductor. Around -20 eV, the lowest states are dominated by σ-2s states and correspond mainly to the bonding 1 g orbitals of diatomic σ2 units (see the molecular orbital interaction diagram of σ2 . Between -12 and -14 eV, the states are the nonbonding out-of-phase 1 u orbitals, which interact strongly with d orbitals. Below the Fermi level, the states are dominated by strong orbital mixing between 4d Mo and 2p σ orbitals.

σotice that the 2p σ states located between -5 eV and the Fermi level are occupied and the associated crystal orbitals possess σ-σ antibonding character. They correspond to the occupied 1πg * levels of the 14 electrons diatomic σ2 4-species. As mentioned previously, the formulation of Mo 4+ σ2 4-refers to a d 2 electron count for molybdenum atoms located in a trigonal prismatic coordination environment. According to crystal field theory, in a low-spin configuration -strong field ligands-these two d electrons occupy the dz 2 orbital while the four remaining d orbitals must be empty and higher in energy (Figure Even though the formation enthalpy of P63/mmc Moσ2 is negative at 0 K and ambient pressure (ΔHf = -0.29ι eV/atom), this criterion is not sufficient to claim that the P63/mmc pernitride phase is thermodynamically stable. In fact, a thermodynamically stable compound must be more stable than any isochemical mixture of the elements or other Moxσy stoichiometries at a given pressure. A truly stable structure must lie on the convex hull constructed on the plot of the formation enthalpy versus composition x = σ/(σ + Mo). For all possible phases of the Mo-σ system, we generated the Mo-σ convex hulls by using the variable-composition evolutionary algorithm combined with first-principles calculations and the results are shown in

Pressure effect on MoN2

As transition metal nitrides are usually experimentally obtained under high pressure, we studied the relative enthalpies of different Moσ2 structures compared with the experimental claimed R3m structure as a function of pressure. For comparison, the structures discussed in MoS2-type (P63/mmc), τsσ2-type (P6/mmm), 220 Cm, and Reσ2-type (C2/m) 21ι were considered in our study. The thermodynamic stability of Moσ2 with respect to decomposition into elemental Mo + σ2 and reactants Moσ + 1/2σ2 was also discussed. The body-centered-cubic Mo (Im-3m), hexagonal Moσ (P63/mc), 221 and depending on the applied pressures, α, P41212, cg phases of σ were chosen as reference states. κ,222 It is seen from Figure 4.8 that P63/mmc pernitride is thermodynamically stable against decomposition into the mixture of Mo + σ2 or Moσ + 1/2σ2 over the whole pressure range. The experimental R3m Moσ2 is thermodynamically unstable with respect to decomposition into Mo + σ2 below roughly 20 GPa. Moreover, it is seen that, for all phases with layered structures (P6/mmm, MoS2-type P63/mmc, Cm, and R3m), their formation enthalpies are positive below 10

GPa and become negative under high pressure. However, they all have much higher enthalpies than our P63/mmc pernitride phase and the Moσ + 1/2σ2 reactants over the whole pressure range, indicating their chemical instability. It is noteworthy that we also found a Wσ2-type (P6-m2) 221 structure with enthalpy only 9 meV/atom higher at ambient pressure and 13 meV/atom at 100

GPa than the P63/mmc Moσ2. Also, our calculations show that the fully optimized R3m Moσ2 structure is nonmagnetic, thus considering the magnetic corrections will not affect the relative stability of Moσ2. Previous studies proposed that the P6/mmm structure is a universal ground-state structure for all Mσ2 compounds at low pressures (M = τs, Ir, Ru, and Rh), 220 a statement that could not be applied for the Moσ2 system. σevertheless, Moσ2 adopts the same tetragonal P4/mbm structure proposed for these nitrides at high pressures, and it can be obtained through the phase transformation via the marcasite structure. Starting from ambient pressure, the σiAs-type P63/mmc structure remains competitive up to κ2 GPa. Above, we found the tetragonal P4/mbm (1 g)2(1 u * )2(1πu)4(2 g)2(1πg * )4 for 14 valence electrons, and its bond order is one (Figure 4.4).

σote that the discrete σ2 units point directly toward the square faces of the eight-coordinate polyhedra of the molybdenum atoms (Mo-σ bonding length is 2.091 Å at 100 GPa) and are perpendicular to each other in order to minimize the steric repulsion between the nitrogen -lone pairs (Pauli repulsions). Formally, this leaves the molybdenum atoms of Moσ2 in a d 2 configuration (Mo 4+ ). In a cubic crystal field, the five d orbitals split into doubly degenerate levels, e(dz 2 , dx 2 -y 2 ) and triply degenerate ones, t2(dxy, dxz, dyz), leading to half-filled e levels.

This degenerate electronic configuration e 2 is Jahn-Teller unstable: the cubic Moσκ complexes should undergo a geometric distortion to remove this degeneracy. Effectively, a weak Jahn-Teller effect is observed in the P4/mbm phase (Figure 4.9b) with the σ-σ distances in tetragonal Moσκ units 2.330 and 2.59ι Å at 100 GPa. As the Jahn-Teller effect is very weak for a low-spin d 2 configuration, a small dz 2 /dx 2 -y 2 splitting occurs (Figure 4.9d). σevertheless, the tetragonal P4/mbm phase is expected to be a metal due to the overlap of the filled dz 2 and unfilled dx 2 -y 2 bands, as confirmed by the calculated DτS (Figure 4.9c). Moreover, the covalent nature of the bonding is manifested by the significant penetration of the metal d levels into the nitrogen p block, and the Mo-σ bonding states are all fully occupied while the antibonding ones are unoccupied. This electronic situation explains well the stability of ground-state P4/mbm under high pressure.

Mechanical properties

The mechanical properties of the Moσ2 structures were further studied. The elastic constants Cij, bulk modulus B, shear modulus G, and Young's modulus E were calculated. We noticed that the calculated elastic constants of P63/mmc and P4/mbm structures satisfy the mechanical stability criteria. 223 Furthermore, P63/mmc Moσ2 possesses a remarkably high C33 value (952 GPa) which is comparable to diamond (C33 = 10ι9 GPa), 224 revealing its extremely high stiffness along the c axis, which could be well understood by the strong directional Mo-σ and σ-σ covalent bonds along the c axis. τn the basis of the Voigt-Reuss-Hill (VRH) approximation, 225,226 the calculated B and G of P63/mmc Moσ2 are 33κ and 206 GPa, respectively, comparable to τsσ2

Conclusion

In summary, we have found that the experimentally proposed MoS2-type Moσ2 structure is unstable, as indicated by the negative C44 value, imaginary phonon frequencies, and positive enthalpy of formation based on first-principles calculations. By using the evolutionary methodology USPEX for crystal structure prediction, we have extensively explored the potentially stable structures of Moσ2 in the pressure range 0-100 GPa. Two energetically more stable pernitride phases were discovered. Enthalpy calculations demonstrate that the hexagonal P63/mmc Moσ2 will transform into a tetragonal P4/mbm structure above κ2 GPa. The P63/mmc phase is a narrow-gap semiconductor, while the high-pressure P4/mbm structure is metallic. Both pernitride Moσ2 structures are mechanically and dynamically stable. The atmospheric groundstate structure has a high bulk modulus (B = 33κ GPa) and shear modulus (G = 206 GPa), and is predicted to be a hard material. Further chemical bonding analyses reveal that the two Moσ2 structures incorporate a tetravalent metal (Mo 4+ , d 2 ) and a σ2 4-species with a covalent σ-σ single bond.

We have carefully calculated the temperature contribution to the phase stability of the new discovered zirconium nitrides from 0 K to 2000 K within the quasiharmonic approximation, as shown in Figure 4.11. σote that for each stoichiometry, the space group/structure found at 0 K is kept at higher temperatures. The free energies of formation increase with increasing temperatures for all phases but at different rates, yielding a convex hull which changes with temperature. τur results show that Zr2σ, Zrκσι, Zrσ and Zr15σ16 will not lose their stability in the whole studied temperature range. The unstable Zr3σ2 at 0 K will become stable at temperatures higher than 900 K, while for Zr4σ3, Zr6σ5, Zrισκ and Zr4σ5, the temperature contributions have a negative effect on their structural stability. For example, the formation enthalpy of reaction In the structures of Zr-rich phases, the metal atoms form hexagonal close-packed (hcp) sublattices with σ atoms filled in the octahedral voids, thus each σ atom is coordinated by six Zr atoms, forming σZr6 octahedra. However, the concentration of filled octahedral voids in various Zrn+1σn structures is different. Two thirds of them are filled in Zr3σ2; while seven eighths in Zrκσι.

Similar nitrogen vacancy-ordered structures were also reported earlier to be stable for transition The formation of such σ-rich phases could be attributed to the enhanced stability of the +4 oxidation state of Zr and Hf compared to Ti due to the relativistic effects, leading to the coexistence of +3 in Mσ and +4 in M3σ4, while in the Ti-σ system, Tiσ has the highest nitrogen content under normal conditions. In the structures of Zr3σ4, the hcp metal framework of the rocksalt structure is significantly distorted. For o-Zr3σ4, there are three nonequivalent types of Zr atoms, one of them is octahedrally coordinated to six σ atoms, one resides at the center of a trigonal prism, and the last one is located inside of a heavily distorted octahedron. Thus, the second and third nonequivalent Zr atoms are sevenfold coordinated, while in c-Zr3σ4, each Zr atom is coordinated to eight σ atoms.

Electronic properties and chemical bonding

We calculated the electronic properties (density of states, DτS, See character although with some Zr(4d) character. This sharp peak reflects the localized character of the nonbonding σ(2s)-based levels; secondly, from -6 eV to 4 eV, a very broad structure with two well-defined main peaks originates from mixing of Zr(4d) and σ(2p) orbitals. The lower peak corresponds to the bonding states, and the antibonding counterparts appear just above the Fermi level. These bonding and antibonding peaks exhibit a clear mixing of metal 4d and nitrogen 2p states, although the latter has a stronger Zr(4d) character. In o-Zr3σ4, the gap separates bonding and antibonding states, thus Zr3σ4 is a semiconductor and possesses a remarkable stability. Its calculated band gap (0.6κ eV) is consistent with previous theoretical results. 209,232 These findings are understandable if one considers the following Zintl picture: Zr is in a formal oxidation state of +4 (Zr 4+ , d 0 ), and σ 3-follows the octet rule.

the well-known stoichiometric rocksalt Zrxσy, we will first briefly discuss their electronic properties, then analyze the electronic perturbation due to the creation of nitrogen or zirconium vacancies -empty octahedral sites in the fcc network -leading to symmetry-broken

Conclusion

By using the variable-composition ab initio evolutionary algorithm USPEX, we explored stable and metastable compounds in the Zr-σ system at ambient pressure. τur calculations revealed that Zrσ, Zr2σ, Zr4σ3, Zr6σ5, Zrκσι, Zr15σ16, Zrισκ and Zr4σ5 are thermodynamically stable compounds at low temperatures, while Zr3σ2 is marginally metastable and could be stable at high temperatures. Particularly, we found the two controversial compounds discovered by Juza et al. could be Zr15σ16 and Zr4σ5. The newly discovered compounds have defective rocksalt structures with ordered nitrogen or zirconium vacancies. Besides, we found hardness of bulk Zr3σ4 is below 15 GPa.

Pressure-induced transitions in GaPO4

The discovery of the pressure-induced amorphization of α-quartz at pressures above 15

GPa 233 generated a great deal of interest in the origin of this phenomenon, which has been the subject of numerous experimental and theoretical studies. Amorphous material has also been reported to be present in addition to high-pressure crystalline phases, 234 which highlights the competition between transitions to crystalline and amorphous forms 235 and the different transition pathways available at high pressure, many involving metastable states. α-quartz homeotypes (Geτ2, BeF2, Pτσ, ABτ4: A = B, Al, Ga, Fe; B = P, As) provide models for the high-pressure behavior of silica and are also potential new materials with improved properties (e.g., piezoelectrics). Starting from the α-quartzor α-berlinite-type structure (space group P3121, Z = 3) for Aτ2 or ABτ4 compounds, respectively, transitions to amorphous and/or crystalline phases have been reported for various α-quartz homeotypes. 236,23ι In the case of FePτ4, 23κ for example, simultaneous transitions to a CrVτ4 structure and an amorphous form were observed at 2.5 GPa. This CrVτ4 structure with mixed 6:4 cation coordination is also the stable highpressure form for AlPτ4 and GaPτ4; 23κ-240 however, thermal activation is often necessary to obtain this form. The CrVτ4-type form is also obtained at room temperature from α-cristobalitetype GaPτ4; 241 however, the behavior at ambient temperature starting with the α-berlinite-type form is less clear with the materials obtained at high pressure not always being well crystallized. [242][243][244][245] Several solid solutions exist between α-quartz homeotypes. In the case of the AlPτ4-GaPτ4, an x-ray diffraction study indicated that there is complete static Al/Ga disorder, which may open the way to design materials with tunable piezoelectric properties. 246-24κ Up to the present, it is not known what influence preexistent static disorder has on the competition between transitions to crystalline or amorphous high-pressure phases.

Therefore, in this work, we cooperated with the experimental group and indicated, in addition to the presence of a CrVτ4-type phase, the formation of a new, unidentified, poorly crystallized phase in the Ga-rich region of the AlPτ4-GaPτ4 solid solution. 249 The combined use of x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) as a function of pressure and temperature along with Raman scattering and first-principles calculations in the present study has τur theoretical study indicates that the CrVτ4-type form of GaPτ4 (space group Cmcm) has the lowest enthalpy (G = H at T = 0 K) from roughly 5 to at least 20 GPa. At 5 GPa, we note a tiny enthalpy difference between orthorhombic Cmcm and α-berlinite-type P3121 phases. τur calculated phase transition pressure from P3121 to Cmcm is 4.ι GPa at T = 0 K, but one has to be reminded that the possibility of a large kinetic barrier for the α-berlinite-type P3121 phase to

Cmcm phase transformation has to be considered. Even if the Cmcm phase is calculated to be ground state at 5 GPa, higher pressure and/or thermal activation may be needed to reach it. As shown in Figure 5.β, the α-berlinite-type phase P3121 becomes thermodynamically unstable relative to all proposed low-enthalpy structures when external pressure is higher than roughly 12

GPa. Recall that the P3121 phase contains 4-fold coordinated Ga atoms (Gaτ4 units) while the other predicted structures possess octahedral Gaτ6 units. The Pτ4 tetrahedra remain unaltered from 4 to 20 GPa, in agreement with the well-known rigidity of these Pτ4 units. As we are looking for metastable phases at high pressure, we focus our attention on the low-enthalpy structures located on the potential energy surface of GaPτ4 at 20 GPa. At 20 GPa, the second lowest structure is P-1, located at +9 meV/atom (+20 meV/atom at 10 GPa) from Cmcm. This phase becomes lower in energy than Cmcm above 30 GPa. This P-1 structure consists of stacked infinite chains of edge-sharing distorted Gaτ6 octahedra and the linkage between Gaτ6-based chains is made by Pτ4 tetrahedra (See Figure 5.3). structure is built up of layers of corner-sharing Gaτ6 octahedra in the yz plane linked by Pτ4 tetrahedra with one shared edge between tetrahedra and octahedra in the a direction. This is very different from the Cmcm structure, which is built up of chains of edge-sharing Gaτ6 octahedra in the c direction linked by corner-sharing by Pτ4 tetrahedra in the other directions. It can be noted that in the low-pressure α-berlinite-type structure all tetrahedra share corners. The stable highpressure phase with the CrVτ4 structure is obtained on heating at high pressure from berlinitetype GaPτ4. In contrast, α-cristobalite GaPτ4, which is based on a cubic close-packed oxygen sublattice, can transform to the CrVτ4 phase at ambient temperature via a displacive mechanism.

The present results show that starting from the berlinite-type phase, a new orthorhombic Pmn21

structure is obtained at high pressure and ambient temperature or with moderate heating. This transition can be expected to be less reconstructive than the berlinite-CrVτ4 transition. This is similar to what occurs in α-quartz, where at ambient temperature transitions occur to metastable monoclinic P21/c and P2/c phases via diffusion less pathways. As the phase is metastable, its presence in a given experiment will depend on time (transition kinetics), temperature (in the absence of enough thermal activation to obtain the stable high-pressure form), and nonhydrostatic stress, which could favor certain energetic pathways.

Conclusion

A combination of experiment and first-principles calculations was used to investigate the high-pressure phase transitions in GaPτ4 and the AlPτ4-GaPτ4 solid solution. X-ray absorption and Raman spectroscopy are consistent with an increase in the coordination number of Ga to 6

with the phosphorus remaining in 4-fold coordination. Raman spectroscopy and x-ray diffraction provide evidence for a new metastable high-pressure phase in addition to the stable CrVτ4-type form. In the case of the solid solution, the sample consisted essentially of the new metastable phase, indicating that cation disorder or the smaller cation size of aluminum are favorable factors for its metastability. First-principles calculations and the evolutionary algorithm USPEX were used to predict possible high-pressure structures in this system. The experimental data were consistent with one predicted metastable orthorhombic structure, space group Pmn21. This Cτ2 is a molecular (0 D) linear structure with 2-coordinated C atoms, while Siτ2 is a solidstate compound with an extended three-dimensional Si-τ net containing four-coordinated silicon centers. Besides these two compounds, others structural properties have been experimentally observed and predicted in 16-ve AB2. In 2000, Frapper and Saillard proposed a polymeric form of Cτ2, isostructural to 1D CS2 as well as other molecular and extended polymorphs of carbon dioxide and carbon disulfide. 64 Following this work, and others, we studied silicon disulfide SiS2 under high pressure. We expect that such a compound may have new and original structures under pressure, with high Si and S coordination numbers, due to their atomic size (Cσ > 4).

In autumn 2015, we started to develop the CSP of SiS2 under pressure using evolutionary To summarize the experimental SiS2 phases, the claimed transition phase sequence is:

Ibam σP → P21/c-I HP1 → P21/c-II HP2 → P-3m1 HP4 2.κ GPa 3.5 GPa 4 GPa In Autumn 2015, we hypothesized that a 6-fold coordinated phase, and even an κ-fold coordinated phase, may dominate the higher-pressure range. Since, this "6-fold" hypothesis has been double-checked by both numerical simulations and experimental data, 259,260 let's see if our κ-fold hypothesis is valid by scrutinizing the 40-500 GPa pressure range. In this study, we used ab initio electronic structure calculations, combined with an evolutionary search for crystal structure prediction, to explore the high-pressure phases of SiS2 up to 500 GPa. In addition to the presence of the known low-pressure phases, we predicted several structures with 6-fold coordination and κ-fold coordination at higher pressures. In the following, we will detail our results obtained in the 100-500 GPa pressure range, as previous theoretical work has been published recently which looksat the 0-100 GPa region.

Computational details

The search for ground stable SiS2 structures was performed using the evolutionary algorithm, as implemented in the USPEX code. κ, 13,30 The lowest-energy structures were determined at 0, 20, 50, 100, 150, 250, 300, 400, and 500 GPa with systems containing up to 12 formula units (f.u.), thus up to 36 atoms per primitive unit cell. The first generation of κ0 structures was produced randomly; all subsequent generations contained 60 structures and were produced using variation operators such as heredity (50%), softmutation (15%), lattice mutation (15%), 20% of each new generation was produced randomly.

Each structure was fully relaxed to an energy minimum within the framework of DFT embedded in the VASP package. 4κ We employed the all-electron PAW method 49 and the PBE version of the GGA functional. 41,42 A plane-wave basis set, with a kinetic energy cutoff of 600 eV, was employed. We used uniform Γ-centered k-points meshes with a reciprocal space resolution of 2π × 0.04 Å -1 for Brillouin zone sampling. These settings enable the great convergence of total energies, forces, and stress tensors.

The dynamical stability of the statically relaxed structures was established by the absence of imaginary phonon frequencies, which were computed using the finite-displacement approach, as implemented in the PHτστPY code 9ι .

Results and discussions

First of all, the experimentally known σP-SiS2 Ibam phase was successfully found as a ground-state at 1 atm in our CSP searches, which validated the reliability of our methodology.

Above 0 GPa, the P21/c-I HP1 phase was the lowest phase thermodymically, and contains both edge-and corner-sharing SiS4 tetrahedra. At 4 GPa, 4-fold P21/c-II HP2 transforms to a 2dimensional layered P-3m1 phase (See Figure 5.9). However, the previous found HP3 phase was never a ground stable phase in our calculation. The P-3m1 exhibits an excellent pressure stability up to 1ι1 GPa. The dynamic stability of this structure at 20 and 100 GPa has been verified by phonon calculations, which showed no imaginary frequency in the entire Brillouin zone (see in Appendix). P-3m1 SiS2 is a layered structure, formed by separate sheets of edge-sharing SiS6 octahedra (Si-S bond lengths of 2.24 and 2.02 Å, at 20 GPa and 1κ0 GPa, respectively). σote that this CdIβ-type structure has already been reported in isovalent SnS2 and SnTe2 systems. This layered character is of potential interest given the possible facile frictional sliding under high-pressure shear. The stacking pattern within each SiS2 slab is /AbC/ (A, C: sulfur atoms; b, hexagonally packed silicon atoms), and is illustrated in From 0 to 500 GPa, the following transition phase sequence is computed for silicon disulfide (transition pressure in GPa):

4-fold to κ-fold Si coordinated structures are observed. σote that three high-density phases compete in energy between 200 and 400 GPa within an energy window of only ~20 meV/atom (See Figure 5.11b), namely Cm, P21/m and Pmmm.

P-3m1 SiS2 transforms to P-1 phase at 1ι1 GPa (Z=κ). Figure 5.11 shows that this P-1 phase stabilized as ground state in a concise pressure range (1ι1-199 GPa). P-1 structure is based on separate sheets of edge-sharing SiS6 octahedra (See Figure 5.1γa). Meanwhile, due to the compression at high pressure, the inter-layer distance is reduced to 1.30 Å, and silicon-sulfur single bonds are present (2.22 Å at 200 GPa). In P-1, ι-fold silicon atoms are observed, leading to a 3-dimensional network. Therefore, we propose that this P-1 SiS2 phase is the transition phase that connects the "6-fold" and "8-fold" phases. Further works are needed to support our A distorted cubic unit, where silicon or sulfur atoms are coordinated with κ sulfur atoms, is pressure favorable stacking than octahedrons. This indicates that the higher densities achieved in Cm arise from the higher coordination and denser polyhedral packings. Volume compression curves (Figure 5.1β) show that the Cm unit cell volume is smaller than the P-1 phase volume across all pressures studied. However, there are so far, no experimental and theoretical reports available. Between 263 and 3κ4 GPa, a ground stable phase with space group P21/m is the lowest structure (See Figure 5.15). The P21/m phase has similar sublattice to that found in Cm phase, thus κ-fold Si atoms are observed. Interestingly, the eight coordinated P21/m phase will transform into a layered P-3m1 like phase when pressure is released.

Conclusion

In summary, a high-pressure P-3m1 phase has been predicted as the stable phase thermodynamically above 4 GPa and up to 1ι1 GPa. As was experimentally characterized before the publication of this USPEX CSP result. Its structure presents hypercoordinated silicon atoms (6-fold) and a layered character of CdIβ-type. Between 1ι1 and 199 GPa, a P-1 phase is the stable form and has ι-coordinated silicon atoms. P-1 topology links the 6-fold and κ-fold coordinated

INCAR for projected DOS

The calculation can be divided into two steps. First, perform a static, selfconsistent calculation. σote that the ICHARG option should be true. In the second step, a non-selfconsistent calculation can be performed by using the precalculated CHGCAR file from the selfconsistent..

INCAR file for Lobster

In this case, the calculation is similar, with a single point calculation. The difference is that the number of bands has to be set up (σBAσDS). It's meaningless if the number of bands is too low. And the symmetry has to be turned off completely, i.e., ISYM=-1. -.

-.

-.

-.

Li O F -F -P a P a -.

-.

-.

-.

Li CO C / P / P / P / -.

-.

-.

-.

CO

Pa-Pa-I-d I-d

-.

-.

-.

-.

Ta le A .

Cal ulated e thalpies of the p edi ted Ba N o pou ds at diffe e t p essu e. Chapte . Ab initio (DFT) molecular dynamics simulations were performed with VASP for the case of a 3×2×4 (4κ Mg and 192 σ atoms; 240 atoms) Cmmm Mgσ4-based periodic supercell. Starting from the Cmmm Mgσ4 structure at ambient pressure, AIMD calculations were performed at 300 K, 600 K and 900 K. The timestep was 1 fs, and the total simulation time was as long as 10 ps.

Stru ture

Brillouin zone integration is restricted to the Γ point of the supercell. A canonical σVT (σ: constant number of particles, V: constant volume, and T: constant temperature) ensemble was adopted for the AIMD calculations using the algorithm of σose, as implemented in the VASP code.

A stra t

The dis o e of o el stalli e o pou ds u e i al si ulatio is a ajo halle ge i Mate ials " ie e. Th ee fa ilies ha e ee studied: o pou ds o sisti g of lithiu a d a o dio ide; the M N it ide phases ith M = Mg, Ba, Mo a d ) ; the GaPO a d "i" s ste s. The stallog aphi st u tu es a e dete i ed i sili o usi g the e olutio a algo ith U"PEX, oupled ith DFT al ulatio s VA"P . The stud of pol o phis as a fu tio of p essu e has ee a ied out, he eas the a al sis of st u tu al a d ele t o i p ope ties o stitutes the hea t of this thesis. Ou o k lea l p ese ts the effe t of p essu e o the e e ge e of u e pe ted stoi hio et ies, su h as Li CO , MgN , a d BaN . "o e of these h potheti al ate ials e ai sta le at at osphe i p essu e. It is sho that the additio of the s-lo k ele e t allo s the "pol e izatio " of the u satu ated ole ules CO a d N to e a ied out at lo e p essu es. Thus, o alate C O -pol e izes i a i fi ite pol -dio a e hai i LiCO at GPa; the e Li CO o positio p ese ts the ethe e like -O C=C O-otif; N , N a d N fi ite hai s, N - pe tazolate a io s, a d N i gs a e ide tified i the Ae N phases, as ell as i fi ite o ale t D hai s, sta ilized the alkali e ea th atio s Ae ; the Ba N o pou d is a o du ti e ele t ide at a ie t p essu e a d a i sulato a o e GPa; the g ou d sta le st u tu e of MoN has e apsulated N u its, a d is ot the Mo" t pe a a ge e t p oposed e pe i e talists; ou p edi tio s oupled ith the XRD data allo the elu idatio of the GaPO st u tu e at GPa. Ke ords : alkali e ea th, e olutio a algo ith , o ital a al sis, theo eti al al ulatio , CO , DFT, a o dio ide, GaPO , high p essu e, i sili o, he i al o d, MoN , it ides, pe tazolate, stalli e phase p edi tio , ele t o i st u tu e, "i" , U"PEX, ) N

Résu é

La d ou e te de ou eau o pos s istalli s pa si ulatio u i ue est u d fi ajeu e " ie e des Mat iau . Aussi, t ois fa illes so t à l' tude : les o pos s o stitu s de lithiu et de dio de de a o e ; les phases it u es M N a e M = Mg, Ba, Mo et ) ; les s st es GaPO et "i" . Les st u tu es istallog aphi ues so t d te i es i sili o à l'aide de l'algo ith e olutio ai e U"PEX oupl à des al uls DFT VA"P . L' tude du pol o phis e e fo tio de la p essio est o duite ta dis ue l'a al se des p op i t s st u tu ales et le t o i ues o stitue le oeu de ette th se. Nos t a au ette t lai e e t e ide e l'effet de la p essio da s l' e ge e de stoi hio t ies i ha ituelles telles ue Li CO , MgN , et BaN . Ce tai s de es at iau h poth ti ues este t sta les à p essio at osph i ue. Il est o t ue l'ajout d'u l e t du lo s auto ise la « pol isatio » des ol ules i satu es CO et N à des p essio s plus asses. Ai si, l'o alate C O -pol ise e hai e i fi ie pol -dio a e à GPa da s LiCO ; la ou elle o positio Li CO p se te des otifs de t pe th e -O C=C O -; des haî o s N , N et N , des a io s pe tazolates N -et des les N so t ide tifi s da s les phases Ae N , ai si ue des hai es o ale tes i fi ies D sta ilis s pa les atio s al ali o-te eu Ae ; le o pos Ba N est u le t ide o du teu à p essio a ia te et u isola t au-dessus de GPa ; la st u tu e sta le de MoN a des e tit s N e apsul es, et o l'a a ge e t e feuillet de t pe Mo" p opos pa des e p i e tateu s ; os p di tio s oupl es au do es DRX pe ette t l' lu idatio de la st u tu e de GaPO à GPa.

Mots

Frapper

  12 th USPEX workshop in Poitiers "Pressure-induced polymerization of Cτ2 in lithium-carbon dioxide phases" B. Huang and G. Frapper (τral presentation & Poster) 10. 2016 The κ th multiscale materials modeling international conference in Dijon "Exploring the Real Ground-State Structures of Molybdenum Dinitride" S. Yu, B. Huang, X. Jia, Q. Zeng, A. R. τganov, and G. Frapper (Poster) 03. 2016 Day of the center-west of France chemistry society in Poitiers "Exploring the real ground-state structures of molybdenum dinitride" B. Huang and G. Frapper (τral presentation) 11. 2015 Scientific day of IC2MP, University of Poitiers 0κ. 2015 10 th USPEX workshop in Beijing, China 0κ. 2015 9 th USPEX workshop in Poitiers "Emergence of novel nitrogen covalent networks in magnesium-nitrogen under high pressure" S. Yu, B. Huang , Q. Zeng, A. R. Oganov, G. Frapper (Poster) Présentation en français du manuscrit Traduit par Dr Gilles Frapper, Directeur de thèse MC HdR Chimie théorique 31° section CσU Groupe Chimie Théorique, équipe E4, IC2MP UMR ι2κ5 Université de Poitiers-CσRS gilles.frapper@univ-poitiers.fr Prédiction in silico de phases cristallines Lix(CO2)y ou nitrures par algorithme évolutionnaire et calculs en chimie quantique La découverte de nouveaux composés cristallins par simulation numérique est un défi majeur en Science des Matériaux. Ce travail de thèse s'inscrit dans ce champ de recherche et aborde trois familles de composés : (1) les composés constitués de lithium et de dioxyde de carbone, Lix(Cτ2)y ; (2) les phases de composés binaires M-azote avec M = Mg, Ba, Mo et Zr ;

  x

5 68

 5 Composition chimique vs pression dans les diagrammes de phase Mg-σ et Ba-σ. Des chaînons σ2, σ3 et σ4, des anions pentazolates σ5 -et des cycles type benzène σ6 sont rencontrés (voir ci-après 7) ; leurs structures électroniques et géométriques ont été étudiées et rationnalisées. De même, des systèmes polymériques d'azote ont été observés, chaines covalentes infinies (1D) et feuillets (2D) stabilisées par les cations alcalino-terreux. Certains de ces matériaux ont des propriétés de conduction métallique sous pression extrême. 0D 1D 2D 7, motifs σy rencontrés dans les structures Aexσy (Ae=Mg, Ba) sous pression xi Tandis que Mg3σ2 est un composé ionique isolant de type Zintl (3Mg 2+ , 2σ 3-) à pression ambiante, Ba3σ2 présente une surprenante structure C2/m en feuillet dans sa phase la plus stable thermodynamiquement. Dans C2/m, des entités σ2 4-et des ions σ 3-sont encapsulées dans le sousréseau barium. A pression ambiente, C2/m Ba3σ2 8 est un conducteur électrique, un électride bidimensionnel, une propriété observée dans les composés riches en barium Ba3σ et Ba2σ. De plus, sous pression, deux transformations de phase sont prédites pour Ba3σ2 : C2/m (5 GPa)P-1 (β5 GPa)C2/c, avec la séquence électride conducteur (C2/m) isolant (P-1) métal (C2/c). La localisation d'électrons dans l'espace inter-feuillets explique les propriétés de conduction métallique de C2/m Ba3σ2, voir l'illustration en 8, tandis que la phase P-1 stable comprend des nitrures σ 3-encapsulés dans un réseau cationique, caractéristique structurale d'un composé Zintl qui lui confère un caractère d'isolant électrique. Ba3σ2 constitue une exception structurale et électronique parmi ses composés isoélectroniques Ae3σ2 à pression ambiante (Ae=Be, Mg, Ca). Structure cristalline Ba3σ2 C2/m et densité de charge (à droite) : une localisation d'électrons est observée dans l'espace inter-feuillet Finalement, et pour la première fois à notre connaissance, des composés de l'état solide de type Ae(σ5)2 sont proposés dans la série des alcalino-terreux Ae = Be, Mg, Ca, Sr et Ba. Chaque structure cristalline comprend deux cycles σ5 -pentazolates, soit 10 atomes d'azote par métal. Pour le beryllium dipentazolate Beσ10 9, une structure à charpente ouverte de type Metal Inorganic Framework MIF autorise le stockage de 1,5 molécules de diazote par formule unitaire ; sur la base d'une simulation dynamique (AIMD), il est montré que la charpente pentazolate-Be 2+ xii du composé hypothétique Be(σ5)2(σ2)1,5 est stable au moins jusqu'à 600 K. Ces sels de pentazolates peuvent libérer une quantité d'énergie et de moles gazeuses lors de la réaction de décomposition hypothétique Ae(σ5)2 = Ae3σ2 + 8 σ2, et à ce titre, s'invitent dans la famille des matériaux à haute densité d'énergie et des explosifs.

9 ,

 9 phase Be(σ5)2(σ2)1,5 10, phase Moσ2 : P63/mmc est la plus stable La dernière classe de composés nitrurés abordée dans cette deuxième partie concerne les matériaux Moxσy et Zrxσy. Au quatrième chapitre est traité un sujet en réponse à des travaux expérimentaux qui conduisaient en 2015 à la proposition d'une structure en feuillet de type MoS2 pour le composé dinitrure de molybdène Moσ2 10. σotre recherche de la structure cristalline de Moσ2 par approche évolutionnaire couplée à des calculs DFT bat en brèche les conclusions des expérimentateurs : la phase stable thermodynamiquement de Moσ2 présente une structure cristalline où des entités σ2 sont encapsulées dans des cages prismatiques d'atomes de Molybdène. La phase « expérimentale » de structure-type MoS2 est instable dynamiquement du fait de la présence de branches de phonons imaginaires dans son spectre de phonons. De plus, elle est instable thermodynamiquement. Il est montré que l'optimisation géométrique de Moσ2 type-MoS2 conduit au couplage des atomes d'azote donc à l'existence d'unités σ2 encapsulées dans le réseau métallique, du fait du caractère radicalaire de l'azote dans la phase de type MoS2. Ces résultats invitent à une nouvelle démarche expérimentale pour la détermination de la composition chimique et la structure cristallographique du matériau catalytique Moσ2. xiii Puis une étude théorique des liaisons chimiques et des arrangements structuraux présents au sein de nitrures de zirconium Zrxσy à pression ambiante est menée en collaboration avec une équipe de l'université σPU de Xi'an. Un modèle théorique basée sur une analyse orbitalaire est proposé pour l'analyse de la perturbation électronique liée à la création de défaut en azote ou en zirconium au sein de phases Zrxσy de type σaCl. La troisième partie de ce manuscrit traite deux thèmes relatifs à la prédiction de la structure cristalline de composé de composition fixé, l'un GaPτ4 11 et l'autre SiS2 12, tous deux isoélectroniques à la silice Siτ2. Le premier est le fruit d'une collaboration avec une équipe d'expérimentateurs en Sciences des matériaux sous haute pression. Les travaux expérimentaux et théoriques portent sur l'élucidation de la structure cristallographique d'une phase métastable de GaPτ4 obtenue à haute pression. σotre étude évolutionnaire/DFT conduit à la proposition d'une transformation de phase de la forme -quartz vers une forme métastable orthorhombique de groupe d'espace Pmn21, en très bon accord avec les données expérimentales (DRX & IR). Le deuxième thème envisage les différentes transformations de phase du disulfure de silicium SiS2 induites par la pression (0 à 500 GPa).

11 ,

 11 GaPτ4 à 20 GPa 12, SiS2 à 500 GPa ; Si et S sont en coordinence κ Finalement, nous présentons une conclusion générale.Ces travaux de thèse ont fait l'objet de 4 articles publiés dans des revues internationales à comité de lecture et de plusieurs communications orales(3) et par voie d'affiche(4). Deux autres publications sont soumises pour publication et deux sont en cours de rédaction.xiv Mots-clés : alcalino-terreux, algorithme évolutionnaire, analyse orbitalaire, calculs théoriques, Cτ2, DFT, dioxyde de carbone, GaPτ4, haute pression, in silico, liaison chimique, matériaux nitrurés, Moσ2, pentazolate, prédiction de phases cristallines, structure électronique,Listof pu li atio s ........................................................................................................................................ iii List of s ols .................................................................................................................................................Prése tatio e fra çais du a us rit ................................................................................................... ii Ge eral i trodu tio .................................................................................................................................... . Methods ...................................................................................................................................................... . C stal st u tu e p edi tio ethods .................................................................................................. . . Glo al opti izatio ................................................................................................................. . . U"PEX algo ith ...................................................................................................................... . Fi st p i iples al ulatio s ................................................................................................................ . . . De sit fu tio al theo ....................................................................................................... . . The e ha ge-o elatio fu tio al ...................................................................................... . . Pseudopote tials ..................................................................................................................... . . Pe iodi it a d B illoui zo e sa pli g .................................................................................. . Che i al o di g a al sis i stals ................................................................................................ . . Ba d st u tu e a d de sit of states ...................................................................................... . . C stal o ital Ha ilto populatio ........................................................................................ . . Bade ha ge a al sis .............................................................................................................. . . Ele t o lo alizatio fu tio .................................................................................................. . . E te ded Hü kel theo .......................................................................................................... . Pressure-i du ed pol erizatio of CO i lithiu -ar o dio ide phases ........................... . I t odu tio ....................................................................................................................................... . Co putatio al details ........................................................................................................................ . Results a d dis ussio ........................................................................................................................ . . The od a i sta ilit ......................................................................................................... . . "t u tu e a d o di g i Li CO phases .............................................................................. xvi . . Que ha le high-p essu e Li CO phases? .......................................................................... . . A etasta le CO -i h Li CO phase o tai i g t o-di e sio al CO ets ......................... . Co lusio a d pe spe ti es .............................................................................................................. . Alkali e earth etal itrides ............................................................................................................... . I t odu tio ....................................................................................................................................... . Co putatio al details ........................................................................................................................ . Mag esiu it ides ........................................................................................................................... . . Phase sta ilit of the Mg-N s ste u de p essu e ............................................................... . . Co pou ds o tai i g D pol it oge a hai hai s a d o ugated D la e s ............. . . Co pou ds o tai i g ole ule-like o ale t a io i u its ................................................. . . Co pou ds o tai i g o l isolated it oge ato s .......................................................... . . Que ha le high-p essu e MgN ? A oute to a ds HEDM .................................................. . . Co lusio ............................................................................................................................. . Ba iu it ides ................................................................................................................................. . . Phase sta ilit of Ba-N s ste u de p essu e ..................................................................... . . Ba-i h ele t ides: Ba N a d Ba N ......................................................................................... . . A p essu e-i du ed o du ti g ele t ide-i sulato t a sitio i Ba N ............................... . . Mole ula -like u its a d e te ded ets i it oge -i h Ba N o pou ds ........................ . . Co lusio ............................................................................................................................. . Pe tazolate alkali e-ea th etal salts ............................................................................................. . . Moti atio s ........................................................................................................................... . . Methods ................................................................................................................................ . . Results a d dis ussio ........................................................................................................... . . Co lusio ............................................................................................................................. . Tra sitio etal itrides ..................................................................................................................... . I t odu tio ..................................................................................................................................... . Co putatio al details ...................................................................................................................... . Mol de u it ides ....................................................................................................................... xvii . . E pe i e tal la e ed R MoN st u tu e .......................................................................... . . NiAs-t pe MoN pe it ide a d othe etasta le st u tu es at a ie t p essu e ............. . . P essu e effe t o MoN ....................................................................................................... . . Me ha i al p ope ties .......................................................................................................... . . Co lusio ............................................................................................................................. . ) -N o pou ds ............................................................................................................................... . . Phase sta ilit of the ) -N s ste at fi ite te pe atu es ..................................................... . . Ele t o i p ope ties a d he i al o di g ......................................................................... . . Co lusio ............................................................................................................................. . Sili a-like o pou ds u der pressure: GaPO a d SiS ............................................................. . P essu e-i du ed t a sitio s i GaPO ............................................................................................ . . Co putatio al details ........................................................................................................... . . Results a d dis ussio s ......................................................................................................... . . Co lusio .............................................................................................................................. F o -to -oo di ated sili o ato s i solid-state "i" u de p essu e ..................................... . . Co putatio al details ........................................................................................................... . . Results a d dis ussio s ......................................................................................................... . . Co lusio ............................................................................................................................. Ge eral o lusio .................................................................................................................................... Refere es ................................................................................................................................................... Appe di es ................................................................................................................................................. The discovery of new crystalline compounds by numerical simulation is a significant challenge in materials science. My Ph.D. work has contributed to this research and focused on three families of compounds: (i) compounds consisting of lithium and carbon dioxide; (ii) the Mnitrogen binary compounds with M = Mg, Ba, Mo, and Zr; and (iii) GaPτ4 and SiS2 systems.

Figure 1 . 1 ,

 11 while the black point represents the equilibrium state.

Figure .

 . Figure . Lo al pote tial e e g su fa e of H O ole ule F o Wikipedia .

Figure

  Figure . a T o-di e sio al, o e-di e sio al pote tial e e g su fa e.

Few

  minimal inputs are required to initiate a USPEX calculation: a. The number of atoms of each element b. Pressure-temperature conditions c. The size of the population in one generation d. Constraints (Inter-atomic distance, cell angle) e. The percentage of structures are reproduced by variation operators Initialization of the first generation, a set of fair points in the search space that satisfy the constraints. Then determine the quality for each member of the population using the fitness function (Interface with DFT or MD codes). The program selects the fine structures from the current generation as parents, from which the algorithm creates new offspring in the search space, by applying specially designed variation operators to them. After that, an evaluation of the quality for each new member of the population is undertaken. It selects the excellent offspring to build the new generation of the population. Finally, repeat the last three steps until the halting criteria is achieved.

Figure .

 . Figure . Blo k s he e of U"PEX.

Figure 1 . 4 .

 14 Figure 1.4. Thus, the fitness in variable composition searches can be defined as the minimum vertical distance from the convex hull.

Figure .

 . Figure . Bi a o e hull.

Figure .

 . Figure . Co pa iso of a a efu tio i the Coulo pote tial of the u leus lue , to the o e i the pseudopote tial ed . The eal, a d the pseudo a efu tio a d pote tials at h a o e a e tai utoff adius. F o Wikipedia

Figure .

 . Figure . " he ati ep ese tatio of a o eptual o di g a al sis.

Figure 1 .

 1 8 represents the ELF of Baσ2 at 0 GPa.The ELF indicates that the electrons are localized near the atom forming, ironic system. The strong covalent bonding between nitrogen atoms and the lone pair electrons is revealed clearly by the ELF value of 0.κ.

Figure .

 . Figure . ELF of BaN at GPa.

  appeared from the condensed-state community. Let's describe these molecular and extended Cτ2 phases (See Figure β.1).

Figure .

 . Figure .P edi ted CO p essu e-i du ed phase t a sitio s at K.

  )y phases based on Cτ2 building blocks. Application of an evolutionary algorithm (EA) structure search method designed to locate the global energy minimum structures may lead to chemical decomposition, i.e., cleavage of carbon-oxygen bonds and formation of more stable Li-τ and Li-C based structures. Therefore, only structures with Cτ2 units were kept during the EA search, while the others were rejected (no n C neighbors for a n-coordinated C atom with n = 3, 4 and no peroxide τ-τ bonds). Therefore, constraining the EA search prevented decomposition to avoid formation of graphene-like, diamond-like and Li-τ networks. Under these conditions, a structural search approach based on the variable-composition evolutionary algorithm (VC-EA) implemented in USPEX was employed to scan the configurational, structural and compositional spaces of Lix(Cτ2)y (where x and y are positive integers) and seek the local minima on the enthalpy landscape. Thus, the VC-EA enabled identification of specific compositions in the pseudo-binary Li-Cτ2 system at a given pressure, in this case, atmospheric pressure and 10, 50 and 100 GPa (see Figure β.γ). Finally, fixed-composition EA structural searches were undertaken for given (x, y) compositions and pressures to check the lowest energetic phase on the potential energy surface (PES) had been found.

  structure, as indicated by an absence of imaginary frequencies in all phonon dispersion curves. The corresponding stationary point on the PES is a local enthalpy minimum. Secondly, a thermodynamic criterion was employed. Strictly speaking, all Lix(Cτ2)y structures are metastable in the ternary Li-C-τ phase diagram with respect to the global minimum lithium carbonate Li2Cτ3, but may appear experimentally for kinetic reasons, such as experimental squarate Li2C4τ4 99 and oxalate Li2C2τ4 (see the computed ternary phase diagram in Figure β.β).

Figure . a

 . Figure . Co puted te a o e hulls of the Li-C-O s ste at a ie t p essu e.

Fm

  -3m), and I-43d and Cmca of Li 104,105 at their respective stability ranges were used. The structure and phase transition findings were in good agreement with the experimental and theoretical results. Compounds located on the convex hull are thermodynamically stable against decomposition into any other Lix(Cτ2)y or Li and Cτ2, while compounds above the convex hull are unstable. The convex hull of all known ΔHf values at a given pressure connects the enthalpically stable phases. in Figure β.β we show the relative ground-state enthalpies of formation (ΔHf) of the selected phases at ambient and high pressures. The solid lines in Figure β.γ indicate that in addition to the well-characterized 1:1 composition found in experimentally synthesized lithium oxalate Li2C2τ4 κ3 , two stable compositions, 2:1 and 3:2, emerge at atmospheric pressure. Interestingly, composition diversity does not increase with pressure up to 100 GPa; only the 2:1, 1:1 and 3:2 compositions are located on the convex hull at 10 and 50 GPa, while the 3:2 phase lies slightly above the convex hull at 100 GPa. Although other compositions are not on the convex hull but are dynamically stable and have negative enthalpies of formation, see the Cτ2-rich phases with 3:4 and 1:2 stoichiometry in Figure β.γ.

Figure .

 . Figure . Co e hulls fo the Li-CO s ste at sele ted p essu es. E pt s ua es ep ese t etasta le st u tu es, solid s ua es de ote sta le st u tu es.

Figure .

 . Figure . P essu e-o positio phase diag a fo the Li-CO s ste f o to GPa. The sta le phases a e i di ated old li es a d etasta le phases a e depi ted thi dashed li es. The itali u e s ep ese t the the od a i sta ilit a ges, hile la k u e s de ote the phase t a sitio p essu es.

2 . 3 . 2

 232 depicted in Figure β.4. Seven different Lix(Cτ2)y phases are proposed, and their geometrical and electronic structures are subsequently discussed. Structure and bonding in Lix(CO2)y phases σext, we examined the electronic and geometrical structures of the proposed Lix(Cτ2)y phases to better understand the bonding within these structures. The covalent carbon-oxygen nets encountered within the predicted phases are summarized in Table β.1. Carbon-carbon bonds are roughly 1.54 Å long for the single bond and 1.34 Å for the double bond at ambient conditions (while the C-τ bond is 1.40 Å and C=τ bond is 1.1κ Å; see Figure β.5). For calibration, the C-C single bond distance in diamond C (Fd-3m) is 1.46 Å at 100 GPa (C-τ = 1.34 Å at 100 GPa in high-pressure quartz-like solid Cτ2). These crude structural parameters were used to guide our assignment of C-C and C-τ bond order to rationalize the local structural environments using VSEPR theory and the Zintl-Klemm concept.

Figure .

 . Figure . I te ato i dista e as a fu tio of p essu e.

Figure .

 . Figure . C stal st u tu es of Li C O a P / -I at GPa a d P / -II at GPa. Le is st u tu es of the pla a C O -a io a d V"EPR a otatio .

  Ta le . C stal st u tu es of LiCO P / phases at GPa a -I is the most stable phase calculated for LiCτ2 in the 0-10 GPa pressure range (Figure β.4). As shown in Figure β.6, this structure is a molecular solid formed by isolated C2τ4 2-oxalate anions embedded in a sublattice of Li + cations. Each oxygen atom of the oxalate is coordinated to two Li atoms and each lithium atom is coordinated via ionic interactions to four oxygens at 1.95-2.00 Å, forming a tetrahedral environment. Therefore, the structure of lithium oxalate consists of planes of corner-shared Liτ4 tetrahedra connected by planes of oxalate anions. Within the carboxylate Cτ2 -group, a unique C-τ distance of 1.2ι Å was computed, comparable to the experimental C-τ separation of ~1.2ι Å observed in oxalate salts. 105 Four π electrons are delocalized over each -C(=τ)(τ -) group, leading to a formal Cτ bond order of 1.5, as illustrated by the resonant structures presented in Figure β.6b. The C-C bond length (1.5ι Å at ambient pressure) is approximately 0.03 Å too long for two 3-coordinate C atoms (~1.54 Å). Slightly elongated C-C bonds are also encountered in experimental oxalates structures, 105 indicating an associated single bond order, or lower. Therefore, the extent of π conjugation between two Cτ2

  Figure β.6b, the coordination number of the lithium cation increases from 4 in the low-pressure P21/c-I phase (Liτ4 tetrahedra) to 6 in the high-pressure oxalate phase (Liτ6 octahedra), in which fused Liτ6 octahedra form packed 2D lithium-oxygen slabs. σote that the oxalate motif in P21/c-II remains intact up to higher pressures, and the Zintl P21/c-II phase is thermodynamically stable up to 33 GPa (see Figure β.4). P21/c remains an insulator at 33 GPa, with a calculated Eg of 3.0 eV. Above 33 GPa, the P2/c phase becomes the most stable structure. A drastic phase transformation occurs, as the P2/c Cτ2-based covalent net topology is quite different to that of P21/c oxalate. σevertheless, breaking and creation of bonds during the P21/c → P2/c phase transformation may be prevented due to the high kinetic barriers between these structures. σext, we discuss the P2/c phase. b. High pressure P2/c LiCO2 with infinite 1,4-dioxane chains The monoclinic P2/c structure (Z = 4, Figure β.7) was calculated to be the most stable LiCτ2 phase above 33 GPa. This Zintl phase is particularly interesting; 6-membered rings can be identified, consisting of four C and two τ atoms, as illustrated in Figure β.8. Each 1,4-C4τ2 ring adopts a chair conformation, typical of related structures in 1,4-dioxane and cyclohexane. These 1,4-C4τ2 rings form chain-like arrangements along an axis, where each repeating unit (ring) is fused to its neighbor via a common C-C edge. Each ring junction contains two stereocenters (Figure β.8a), with terminal τ atoms in a cis arrangement, whereas the terminal τ pairs along the chain are in trans with one 'up' and the next 'down' (Figure β.8b). Thus, this ionic LiCτ2 crystal contains chiral Cτ2-based anionic chains without chiral centers. This unusual stereo configuration minimizes repulsive interactions between the charged terminal τ atoms and maximizes the ionic forces between the chains and the Li + sublattice. Together, these effects lead to the overall packed-shape of the poly-Cττ2/2 -chain shown in Figure β.7 and Figure β.8. All of the bond angles within the infinite Cτ2-based chain are close that of a tetrahedral arrangement (106.2-113.1° at 50 GPa), as expected for bent oxygen and tetrahedral carbon atoms. The calculated C-τ and C-C bond lengths within the rings at 50 GPa are 1.42 Å and 1.51 Å, respectively, indicating classical single bonds at this pressure. A shorter terminal C-τ separation of 1.30 Å is observed, as expected for C(sp 3 )-τ(sp) -. Bird's-eye views of the crystal structure along the b and ac directions are presented in Figure β.7a and b, respectively. The ionic bond length for Li-τ is approximately 1.κ4-1.9ι Å, and connects the separated chains into layers along the crystal direction [101]. In P2/c LiCτ2, each Li is coordinated to six τ atoms from four neighboring chains, and fused Liτ6 octahedra form packed 2D lithium-oxygen slabs. The calculated DτS indicates P2/c is an insulator at 50 GPa (Eg ~6.5 eV). The P2/c LiCτ2 structure has potential as a high-pressure polymorph and should be obtained at pressures above 33 GPa. A chemical mechanism can be proposed to connect the molecular oxalates and infinite poly-dioxane chain found in LiCτ2, as displayed in Scheme β.4. In this model, pressure favorizes the formation of new C-C and C-τ single bonds, which thermodynamically stabilize the LiCτ2 P2/c phase at high pressure.

Figure .

 . Figure . High-p essu e P / LiCO phase at GPa. a Vie pe pe di ula to the o e-di e sio al CO -ased hai s, ie alo g the [ ] di e tio a d o ugated fused o tahed a LiO la e s sta ki g alo g the a is a o ato s ha e ee e o ed fo la it .

Figure .

 . Figure . Rep ese tatio s of the C-O o ale t et i the high-p essu e P / LiCO phase at GPa. I a a d , the e te ded fused -e e ed i g hai is highlighted i ed. The , -dio a e C O i gs fo a oat o fo atio ith all te i al O ato s i a is a a ge e t. I , the o pa t o fo atio al a a ge e t is sho .

Figure

  Figure . a C stal st u tu e of the C / phase of Li C O . Le is st u tu e of the pla a C O -a io ethe elike a d V"EPR a otatio .

(

  see Figure β.4). As shown in Figure β.10, all C atoms are tetracoordinated (sp γ ), each to two terminal τ atoms and two C atoms. The calculated C-C and C-τ bond lengths at 50 GPa are 1.54 Å and 1.36/1.3κ Å, respectively, comparable to the interactomic separations (single bonds) in cubic diamond carbon (C-C = 1.50 Å) and the silica-like Cτ2 phase (C-τ = 1.36 Å). The covalent carbon-oxygen net forms unidimensional chains, whereas the infinite carbon chain has an alltrans skeleton running along the b axis. The closest computed Li--τ separations ranged from 1.ι5 to 2.10 Å, characteristic of ionic bonding (computed Li-τ = 1.ι5~2.0ι Å in Li2τ at 50 GPa). In the condensed state, poly-(C(τ -)2)n-forms a 21 helix stabilized by ionic interactions and chain packing. Based on these structural parameters, we confidently conclude that poly-(C(τ -)2)n-is an isoelectronic analogue of poly(tetrafluoroethylene) -(CF2)n-. The Pnma Li2Cτ2 phase is an insulator at both 15 and 50 GPa (computed Eg of 3.5 eV and 3.6 eV, respectively).

Figure .

 . Figure . P a Li CO phase at GPa. a P oje tio alo g the di e tio . E te ded i fi ite CO -ased hai s u i g i the Li ha els. Rep ese tatio of a f ag e t of the pol -C O --hai . Dista es a e i di ated i a gst o s a d the V"EPR AX E a otatio is gi e . Le is eso a t st u tu e fo a epeat u it of the pol -C O --hai .

  describe a new composition, Li3(Cτ2)2 with n = 2, which exhibits two stable phases at the pressure range studied: one phase contains finite C4τκ6-motifs and the other infinite extended -(C(τ)2)n-chains. e. Li3(CO2)2 with the finite C4O8 6-motif and infinite -(C(O)2)n-chains A 3:2 composition emerged on the convex hull at 1 atm and is thermodynamically stable up to 65 GPa (Figure β.γ and Figure β.4). Two phases were identified, with a P-1 to C2/m transition phase pressure of 56 GPa. At low pressure, Li3(Cτ2)2 crystallizes in a triclinic structure, with the space group P-1 and Z = 2 (Figure β.11). In this structure, carbon and oxygen atoms form molecular-like C4τκ units which occupy cavities in the channels running along the c axis. In C4τκ, two C atoms are 3-coordinated and the other two are 4-coordinated, while all oxygen atoms are terminal. This C4-anion possesses an inversion center and two planar terminal carboxylate groups. The calculated C-C and C-τ separations around the tetrahedral carbon atoms are 1.4ι/1.5ι Å and 1.33/1.3κ Å at 50 GPa, respectively. The anion possesses a peculiar structural characteristic, namely the slightly elongated central C-C bonds along the C4-unit due to steric repulsion between oxygen lone pairs, a phenomenon also observed in oxalate anions. The C-C bond strength should be weaker than that of other C4 alkanes. Shorter C-τ bond lengths of ~1.26 Å were predicted for the terminal Cτ2 groups, a signature of the delocalized double bond character in carboxylate units. Thus, the encapsulated C4τκ 6-anion may be formulated as -τ2C-C(τ -)2-C(τ -)2-Cτ2 -. All atoms fulfill the octet rule, as depicted in the Lewis structure (Figure β.11c), which explains the insulator property of the P-1 Li3(Cτ2)2 phase at 50 GPa (see the DτS in Figure β.11).Above 56 GPa, the C2/m space group phase has the lowest enthalpy. This novel Li3(Cτ2)2 phase is thermodynamically stable in the narrow pressure range of 56-65 GPa. Interestingly, the carbon-oxygen covalent net of this phase forms unidimensional chains with a chemical topology analogous to Pnma Li2Cτ2. The infinite carbon chains contain an all-trans skeleton running in a lithium channel(Figure β.11d, e) and can be viewed as the result of polymerization of the C4τκ 6- species found in the low-pressure P-1 Li3(Cτ2)2 phase. At 60 GPa, the C-C bond length is 1.61 Å, longer than in Pnma Li2Cτ2 (1.4κ Å), while the C-τ distance was computed at 1.2ι-1.29 Å, smaller than in Pnma Li2Cτ2(1.33-1.36 Å). These structural differences are understandable if one considers that each poly-Cτ2 chain in C2/m Li3(Cτ2)2 contains one less electron per (Cτ2)2 unit than Pnma Li4(Cτ2)2, which results in weaker C-C bonds and stronger C-τ bonds. Due to this non electron-precise and delocalized picture, one expects C2/m Li3(Cτ2)2 to be metallic; this prediction was confirmed by the DτS.

  Figure . a P-Li CO phase at GPa ith e apsulated C O -a io s a d the asso iated Le is st u tu e. d C / Li CO phase at GPa ith e i fi ite pol -C O hai s.

Figure .

 . Figure . Pho o f e ue ies a P a Li CO , C / Li CO , P / LiCO at GPa.

Figure .

 . Figure . CO -ased ets e ou te ed i a P / LiCO -pol -, -dio a e-, P a Li CO -pol -C O -a d C / Li CO -fi ite C O --stal st u tu es at a ie t p essu e. Dista es a e i di ated i a gst o s.

Figure .

 . Figure . " apshots of the LiCO supe ells at the e d of the AIMD si ulatio s fo P / LiCO -ased supe ells at a ie t p essu e a d , , a d K.

  Figure β.γ). All of these structures have one-or two-dimensional Cτ2-based nets. Here, we focus our discussion on the structural and electronic properties of the high-pressure P-1 Li(Cτ2)2 phase, which possesses highly symmetrical and aesthetical 2D covalent Cτ2 layers. At 100 GPa, our evolutionary crystal structure search identified a low-energy stationary point on the PES of the 1:2 composition. P-1 Li(Cτ2)2 is a local minimum (Z = 4) with an enthalpy of formation of -1.ι44 eV/atom. Its structure contains a covalent Cτ2-based slab sandwiched between Li layers. The C-τ layer is built from repeated C4τκ 2-units, as illustrated in Figure β.15. Each unit possesses a non-planar seven-membered C4τ3 ring. Typical interatomic separations are observed, and a tentative Lewis structure is depicted in Figure β.15c; all local structuralenvironments can be rationalized with the help of simple VSEPR theory. As expected for an electron-precise count in the Zintl phase, P-1 is an insulator at 100 GPa. σote that the C-τ bonds which connect two chains are broken when pressure reduces to 1 atm. Hence P-1 is not a quenchable phase.

  Figure . a P-Li CO at GPa. Vie alo g the a is. Rep ese tatio of the D CO -ased et. C O - epeati g u it.

  Figure . P edi ted a,d C / Na CO , ,e P / NaCO , a d ,f P Mg CO phases at GPa.

  these predicted phases. They are shown in Figure β.16: a one-dimensional chain is encountered in C2/m σa2Cτ2, isostructural to poly-C(τ -)2 found in Pnma Li2Cτ2, while a two-dimensional layered Cτ2 net appears in P21/c σaCτ2; P21212 Mg(Cτ2)2 presenting layered Cτ2-nets, whereas each slab is made of 10-membered C6τ4 rings.

Figure .

 . Figure . " he ati e e g p ofile of ole ula N to pol it oge o pou d ea tio .

(

  Figure . N ets fou d i solid-state it oge allot opes. E pe i e tall ha a te ized phases a e i di ated EXP hile theo eti al o es o espo d to the lo est g ou d-states.

Figure

  Figure γ.γ, i.e., octaazacubane σκ derived from cubane CκHκ, dipentazole (σ5)2 analog to dicyclopentadienyl (C5H5)2, and nitrogen fullerene σ60 from C60, which has been proposed for the first time by Indian chemist Riad Mana of Livermore lab. 131

Figure .

 . Figure . H potheti al o taaza u a e, dipe tazole N a d N pol it oge ole ules.

Figure γ. 4

 4 gives an illustration of this family of compounds, focusing on the encountered σn topologies.

Figure .

 . Figure . E a ples of e pe i e tal N otifs fou d i ole ula spe ies a-e a d solid-state salts f-h . Azido RN ole ules a R=H, CF , o ga o etalli f ag e t,… , Me "i N-N=N-N "iMe , the lo gest it oge hai i Ph-N=N-N Ph -N=N-N Ph -N=N-Ph , o ga o etalli [ Cl W -N N -WCl ] + d a d a idoazido [Os p Cl N ] -e o pou ds. I lue a e ep ese ted the th ee e pe i e tall ha a te ized N io s i stalli e salts; N -azide f, pe taze iu N + g a d y lo-pe tazole a io N -h.

Figure .

 . Figure .N otifs e ou te ed i A N io i salts listed i Ta le . .

Figure . a

 a Figure . a Mo o li i lithiu azide C / -LiN at at , P / -LiN at GPa, C / -LiN at GPa, d Lithiu pe tazolate P / -LiN at GPa.

3. 3 . 1

 31 Phase stability of the Mg-N system under pressure Stable compounds, which form the thermodynamic convex hull (Figure γ.7), were identified at selected pressures and their stability fields are shown in the pressure-composition phase diagram (Figure γ.8). It is noteworthy that our evolutionary searches successfully reproduced experimentally known Mg3σ2 structures. Moreover, the searches revealed that Mg3σ2 transforms from the Ia-3 to Pbcn phase at 21 GPa, to the C2/m phase at 24 GPa and the Pnma phase at 43 GPa. The high-pressure Pnma phase has been proposed by Wei et al. based on first-principles particle swarm methodology implemented in Calypso code. ι,2ι Besides Mg3σ2, eight hitherto unknown stoichiometries emerged to be stable under pressure, including five σ-rich stoichiometries, namely, Mg5σι, Mg2σ3, Mgσ2, Mgσ3, and Mgσ4, and three Mg-rich compositions, Mg5σ3, Mg4σ3, and Mg5σ4. As shown in Figure γ.8, Mg3σ2, Mg2σ3, Mgσ2, Mgσ3, and Mgσ4 undergo a series of phase transitions in their stability domains, while other stoichiometries only possess a single phase.

Figure .

 . Figure . Co e hulls of the Mg-N s ste at sele ted p essu es. "olid s ua es de ote sta le st u tu es, hile e pt o es ep ese t etasta le st u tu es.

Figure . 1 →

 .1 Figure . P essu e-o positio phase diag a of Mg-N s ste f o to GPa. The sta le phases a e i di ated old li es, a d etasta le phases a e depi ted thi dashed li es. The lue u e s ep ese t o pou d sta ilit a ges, hile la k itali u e s de ote phase t a sitio p essu es.

Figure

  Figure . a C stal st u tu e of C MgN . The la ge o a ge i les ep ese t Mg ato s, hile s all lue o es de ote N ato s. " he e illust ati g ele t o t a sfe f o ele t opositi e Mg ato s to the D pol N hai a d V"EPR AX E otatio A, e t al ato ; X, liga d a d , its u e ; E, lo e pai s a d its u e s; + is the ste i u e . Mole ula o ital diag a of the ope N -π s ste . The ight side sho s the s he ati a d st u tu e of the -N --hai ithout left a d ith ight a d o e lap. d Cal ulated de sit of states of C MgN atGPa. e Reso a t st u tu es.

Figure .

 . Figure . The al ulated ele t o lo alizatio fu tio ELF of the pla e of C MgN at GPa.

  Figure γ.1βa. At 100 GPa, a chair conformation is observed with a calculated dihedral angle of 150°. When pressure is released to 1 atm (the Mg-σ Madelung energy and the packing forces decrease), covalent hexazine-σ6 anion remains intact and becomes planar with six nearly identical σ-σ bonds of 1.39 Å, which are slightly shorter than a single σ-σ bond(1.45 Å) but longer than a double one(1.25 Å). Formally, magnesium atoms transfer all of their valence electrons to σ6 rings, leading to σ6 4-unit. This ten π-electron 6-membered ring follows the 4n + 2 Hückel rule (aromatic), and it is isoelectronic to the well-known inorganic cyclic S3σ3 -and P64-species. A molecular orbital diagram of the benzene-like σ6 4-anion is displayed in Figure γ.11. Among the six π levels of the planar σ6 ring, the three bonding π molecular orbitals (Mτ) and the two degenerated antibonding π * ones are fully occupied, leading to only one π-bond delocalized over the six nitrogen atoms. This electronic situation explains well the structural feature encountered in σ6 4-, e.g., σ-σ bonds slightly stronger than usual single σ-σ bond. An energy gap separates the degenerate occupied π4 * and π5 * Mτ (top of the valence band) and the empty π6 * Mτ (bottom of the conduction band); Mgσ3 is a semiconductor (see Figure γ.1βc).

Figure .

 . Figure . At ight, ole ula o ital le els of pla a D h N -i g. Ea h le el leads to lo alized π a d la k o es, o upied le els; e pt o es, u o upied le els . At left, e pt s le el of Mg + ato s ill lead to a e pt s a d. A a d gap sepa ates the ale e a d the o du tio s a ds.

  Figure .a C stal st u tu e of P-I MgN .Le is st u tu e of li N -a io . Cal ulated de sit of states fo P-I MgN at GPa.

  Figure . a C stal st u tu e of P-II MgN . Le is st u tu e of the N -ased u it N -ith V"EPR AX E otatio A, e t al ato ; X, liga d a d , its u e ; E, lo e pai s a d its u e s; + is the ste i u e . Cal ulated de sit of states at GPa.

Figure .

 . Figure .Cal ulated de sit of states DO" of P-MgN at GPa at H"E h id fu tio al le el.

Figure γ. 8 ,

 8 Mgσ2 is stable between 1ι and 94 GPa, and has two phases, with the transition Cmcm to P63/mcm pressure of 43 GPa. The Cmcm structure consists of σ2 dimers sitting in the channels of a 3D magnesium network(Figure γ.15a). At 40 GPa, the σ-σ distance is 1.25 Å, indicating σ-σ double bond character. According to the ZKC, Mgσ2 can be described as Mg 2+ σ2 2-. Thus, diazenide σ2 2-has 12 valence electrons and its electronic ground-state configuration is 1 g 2 2 u *2 1πu 4 2 g 2 1πg *2 . The σ2 2-dumbbell possesses a bond order of 2 and the σ2 2-unit is a solid-state equivalent of molecular triplet τ2. As shown in the calculated DτS (Figure γ.15d), 1πg * degeneracy is broken by the static electric field created by the surrounding Mg 2+ ions. Therefore, a small gap is created between these two antibonding HτMτ-LUMτ 1πg * levels. Cmcm Mgσ2 is a very narrow-gap semiconductor with a band gap of 0.25 eV at 40 GPa.

  Figure . a, C stal st u tu es of C a d P / MgN . Reso a t Le is st u tu e of the pla a N N -u it ith V"EPR otatio . d, e Cal ulated de sit of states fo the C phase at GPa a d P / phase at GPa.

  Figure . a-C stal st u tu es of C / a d P Mg N , a d C Mg N . d Le is st u tu es of the e t e N -a d e N -a io s. Delo alized π ele t o s o pai s a e sho i ed. e, f Cal ulated de sit of states fo P Mg N at GPa a d C Mg N at GPa.

steric number 2 ,

 2 giving linear molecule geometry, while the 1ι-20 ve AB2 systems have steric number 3 or 4, which will give bent molecule geometry with one or two lone pairs on the central atom. The calculated electronic DτS also corroborates our findings (Figure γ.16e): the Fermi level crosses a region mainly based on nitrogen π antibonding states, i.e., the πx * of molecule-like σ3 4-. Half of the πx * band is filled. Thus Mg2σ3 is metallic.

Figure .

 . Figure . "i plified o elatio diag a of the ai o itals of the li ea a d e t N -ole ula s ste .

σ3 5 -

 5 conforms to the VSEPR prediction: AX2E2 deriving from a tetrahedron(Figure γ.16d).Molecular orbital analysis of linear and bent triatomic σ3 species revealed the antibonding πx * level (HτMτ) is occupied by two electrons in σ3 5-; thus, the associated band would be filled (see the Walsh diagram in Figure γ.17).

  A gap separates the valence and conduction bands (Figure γ.18), in perfect agreement with ZKC.

Figure .

 . Figure . C stal st u tu es a d ele t o i de sit of states of a C a-Mg N at GPa, P / -Mg N at GPa a d I-d-Mg N at GPa.

Figure .

 . Figure . Radial dist i utio fu tio s RDF fo the N-N sepa atio s o se ed du i g AIMD si ulatio s fo a C MgN -ased supe ell at a ie t p essu e a d te pe atu es of , , a d K. The la k a o i di ates the a e age N-N dista e i pol -N -.

  around each cation center. As shown in Figure γ.β1, these Baxσy compounds undergo a series of phase transitions in their stability domains.

Figure .

 . Figure . Co e hulls of the Ba-N s ste at sele ted p essu es. "olid s ua es de ote sta le st u tu es, hile e pt o es ep ese t etasta le st u tu es. The spa e g oup is i di ated fo ea h o el sta le st u tu e.

Figure .

 . Figure . P essu e-o positio phase diag a of Ba-N s ste f o to GPa. The sta le phases a e i di ated old li es, a d etasta le phases a e depi ted thi , ed, dashed li es. The lue u e s ep ese t the the od a i sta ilit a ges, hile la k, itali u e s de ote phase t a sitio p essu es.

  Figure γ.β1) up to 100 GPa. τne of their common features is the presence of isolated nitrogen nuclei trapped inside barium sublattice cavities, as depicted in Figure γ.ββ. Ba3σ and Ba2σ have VEC values of 11 and 9, respectively, higher than that of κ (octet rule). Therefore, extra electron e -is expected over the whole structure.

Figure

  Figure γ.ββ. P63/mcm structure presents a hexagonal close packing of [Ba6/2σ] chains. Within

4. 3 Å

 3 Figure . C stal st u tu e of a Ba N P / a d Ba N Rat GPa.

  Figure γ.β5a, Ba2σ is a metal. The broad density of states peak around EF is mainly composed of a non-atom centered orbital located at X (voids) with small contributions from Ba and σ, as shown in the total and projected DτS displayed in Figure γ.β5a.

  a

Figure .

 . Figure . C stal st u tu es of Ba N, asso iated ELF o tou s Isosu fa e alue = . . a Rphase at GPa; Pphase at GPa; I / phase at GPa.

  164

aFigure .

 . Figure . Total a d p oje ted DO" of Ba N phases. a Rphase at GPa; Pphase at GPa; I / phase at GPa.

Figure .

 . Figure . The al ulated e thalp diffe e es of a ious Ba N st u tu es elati e to the C / st u tu e as a fu tio of p essu e.

Figure .

 . Figure . Cal ulated st u tu es of Ba N . a C / Ba N at at looki g at Ba N sla ased o t o he ago al Ba o ola e s ith e apsulated N u its a d N ato s i o tahed al sites.

Figure .

 . Figure . " he ati ole ula o ital diag a of N -. The solid a d dashed li es i di ate the p i a a d se o da pa e tage of the o itals, espe ti el s-pz i i g . O l the ale e o itals a e sho .

Figure . a

 . Figure .a Total a d p oje ted DO", a d -COHP of C / Ba N at GPa.

Figure .

 . Figure . Total a d p oje ted DO" of C / Ba N at GPa

.

  a The pa tial ha ge de sit aps fo the pla e of C / Ba N al ulated usi g the e e g a ges of -. eV E -. eV a d -. eV E . eV. The Fe i le el is lo ated at E = . eV.

Above 5 . 5 aFigure .

 55. Figure . Cal ulated st u tu es of Ba N . a P-Ba N at GPa, C / Ba N at GPa.

  a

Figure .

 . Figure . BaN st u tu es a d asso iated total a d p oje ted DO". a P / at GPa P / at GPa C / at GPa.

Figure .

 . Figure . BaN phases: a,d C / at GPa a d DO", ,e C / at GPa a d DO", ,f P / at GPa a d DO".

  Figure . C Ba N phase at GPa. La ge sphe es a e a iu ato s, a d s all sphe es a e it oge ato s. G ee a d lue sphe es lie i the a pla es, hile N u its i o a e alo g di e tio . G ee o ta ts a e d a as a e e-guide.

  Figure .a C stal st u tu e of P / BaN , a d its DO".

  Figure . C / BaN st u tu e at GPa. a ie of the i fi ite it oge hai i Ba ha els, sele ted st u tu al pa a ete s a d Le is st u tu e of pol -N -hai dista es i Å, a gles i °; V"EPR AX E otatio .

  a

  Figure .a I a BaN st u tu e. Ba-Ba g ee li es a e d a as a e e-guide.Le is st u tu e of N - pe tazolate. P oje ted DO".

Figure γ. 40 ,

 40 one linear σ3

Figure .

 . Figure .Ba N st u tu es ith highlighted isolated N a d N u its. Ba-Ba g ee li es a e d a as a e e-guide.

Figure . Figure .

 .. Figure .Ba N phase at at . Ni e N -liga ds a e oo di ated to a Ba + atio .

  Ta le . P edi ted spa e g oup of solid-state MN o pou ds at a d GPa. a. the od a i al etasta le; . a se t of data; . the od a i al sta le To compare the relative enthalpy stability of Mσ10 polymorphs, Figure γ.44 represents the enthalpy difference of each phase, relative to I-42d (II) phase at 0 GPa, and Ibam phase at 50

  Figure . C stal st u tu e of Be N : a I-d I at GPa a d Fdd at GPa.

Figure γ. 46

 46 Figure γ.46 displays the P43212 and I-42d (II) Mgσ10 structures at ambient pressure and 50 GPa, respectively. In P43212, Mg forms an octahedron with 6 σ5 rings, and Mg-σ distance varies from 2.215 Å to 2.2ι0 Å, typical ionic bonds. Interestingly, the sublattice of Mg has the same I41/amd space group found in Beσ10 I-42d (I). Above ι GPa, I-42d (II) phase becomes the lowest phase. The I-42d (II) contains κ-fold coordinated Mg 2+ cation with eight different σ5 rings at the corners. The Mg-σ distance varies from 1.969 to 2.230 A.

Figure .

 . Figure . C stal st u tu e. a. MgN P at GPa; . MgN I-d II at GPa.

  a

  Figure .a Pa d C / BaN at GPa.

Figure

  Figure γ.50).

Figure .

 . Figure . " apshot of AIMD al ulatio s at GPa a d K, the a o e figu es ep ese t the stati st u tu es efo e the AIMD al ulatio sta ts; the figu es elo a e aptu ed at ps, a d all of the st u tu es a hie ed the e uili iu states.

Figure .

 . Figure . Be N N . phase.

Figure 4 . 3 . 1

 431 Figure . a E pe i e tal lai ed Mo" -t pe R MoN a d fo al ele t o att i utio o t i oo di ated X ato s i MoX X = ", N; Mo + , d . Full opti ized R stal st u tu e at the PBE dW-D le el. The asso iated pho o dispe sio u es of the full ela ed R st u tu e ith/ ithout a de Waals o e tio .

Figure .

 . Figure . P edi ted sta le a d etasta le MoN st u tu es a d thei o espo di g e e gies.

Figure

  Figure . a C stal st u tu es of the g ou d-state P / MoN . The o espo di g a d st u tu e, DO", a d -COHP plots o puted at GPa. The ight pa t sho s a s he ati ole ula o ital e e g le el diag a of N -u it.

Figure .

 . Figure . " he ati ole ula o ital diag a of N -. The solid a d dashed li es i di ate the p i a a d se o da pa e tage of the o itals, espe ti el s-pz i i g . O l the ale e o itals a e sho . N -a io has ale e ele t o s a d the o d o de is o e.

Figure .

 . Figure . C stal field splitti g of the d le els of the Mo ato s d o figu atio i a t igo al p is ati MoN e i o e t of the P / MoN . Ea h Mo ato is oo di ated to si N ato s, fo i g a t igo al p is ati MoN e i o e t.

4. 5 )

 5 . Because pernitride P63/mmc Moσ2 has a filled dz 2 band and unfilled dxy/dx 2 -y 2 bands, Moσ2 should be a semiconductor, consistent with an energy gap found at the Fermi level (DτS, indirect gap, Figure 4.γ). Moreover, an analysis of the chemical bonding between Mo and σ in the crystal Hamilton population (CτHP) plots displayed in Figure 4.γ shows that all bonding levels have been filled with electrons, and all Mo-σ antibonding states show up in the unoccupied crystal orbitals, well above the Fermi level. With respect to Mo-σ bonding and the octet rule for σ2 4-, P63/mmc pernitride structure is ideal.

Figure 4 . 6 .

 46 Formation enthalpies per atom of the Moxσy phass.

Figure .

 . Figure . Co e hull diag a s of the Mo-N s ste at a d GPa o side i g ze o-poi t e e g o e tio solid s ua es, sta le; ope s ua es, etasta le .

Figure .

 . Figure . C stal st u tu es of the Mo N . a P -Mo N, I -Mo N , P / -MoN a d d R -Mo N . The la ge pu ple a d s all lue sphe es ep ese t Mo a d N ato s, espe ti el .

Figure . a

 a Figure . a The al ulated e thalp diffe e es of a ious MoN st u tu es elati e to the lai ed R st u tu e as a fu tio of p essu e togethe ith de o positio i to the i tu e of Mo+N a d MoN+ / N . E e g diffe e e of Pa d P / st u tu es as a fu tio of p essu e.

  2Zr4σ5 (s) = κZrσ (s) + σ2 (g) will become negative above 300 K, which means Zr4σ5 should decompose into Zrσ and σ2 gas at roughly room temperature if associated kinetic barrier allows this process, perfectly consistent with the results ofJuza et al. 206 Crystal structures of the representative zirconium-rich Zr6σ5 and nitrogen-rich Zr4σ5 are schematically shown in Figure 4.1β. σo imaginary phonon frequencies are found, indicating their dynamical stability. The computed lattice parameters for Zrσ and Zr3σ4 are in good agreement with those obtained from other theoretical and experimental investigations, which confirms the accuracy of our calculations. From the structural point of view, Zrσ has the ideal cubic rocksalt structure, while Zrn+1σn (n = 2, 3, 5, ι) and Zrmσm+1 (m = 4, ι, 15) are versions of the rocksalt structure with ordered nitrogen or zirconium vacancies (Zr2σ has rutile-type structure).

  metal carbides Mn+1Cn (M = Hf and n = 2, 5; 229 M = Zr and n = 1, 2, 3, 6; 229 M = Ti and n = 1, 2, 5) 230 and nitrides Mn+1σn (M = Ti and n = 1, 2, 3, 5). 231 For σ-rich phases, one eighth of the metal atoms are replaced with vacancies in Zrισκ, while one fifth in Zr4σ5.

Figure .

 . Figure . C stal st u tu es of the ep ese tati e a ) N a d ) N o pou ds.

Figure 4 .

 4 1γ) of zirconium nitrides at ambient conditions in order to study their chemical bonding. Let us start first with Zr3σ4 and then discuss the effect of zirconium or nitrogen vacancies on the electronic properties of rocksalt based-structures. In DτS of o-Zr3σ4 displayed in Figure4.1γa, three main regions may be described with three kinds of molecular orbital overlaps: nonbonding, bonding and antibonding.231 Firstly, a rather sharp peak at roughly -13 eV appears and it is mainly of σ(2s)

  Zrn+1σn and Zrmσm+1 structures. Similarly to o-Zr3σ4, DτS of Zrxσy phases can be decomposed into three well-separated energy regions as shown in Figure 4.1γ, but here no gap separates the valence and conducting bands: (1) a deep lowest valence band, sσ; (2) hybridized Zr(4d)/σ(2p) band, dMpσ; (3) a partially filled higher-energy Zr(4d) band, dM. The sσ band is dominated by the 2s orbitals of the nitrogen atoms and is nonbonding. The next group of valence bands, dMpσ, results from strong hybridization of the 4d states of zirconium atoms with 2p states of nitrogen atoms. Also, one may see that for Zrxσy the bottom of the dM band, dominated by 4d orbitals of zirconium atoms, responsible for metallicity. When nitrogen vacancies are created in substoichiometric Zrn+1σn (n = 1, 2, 3, 5 and ι), notice that obviously the formal oxidation state of Zr decreases as the number of nitrogen vacancies increases, going from Zr 3+ d 1 in Zrσ to Zr 1.5+ d 2.5 in Zr2σ. Therefore, one may expect the occupation of the Zr 4d levels in substoichiometric Zrn+1σn compounds. This is what happens: Zr-Zr bonding and nonbonding (slightly antibonding) Zr-σ levels appear just below the Fermi level, mainly metal 4d in character (See Figure 4.1γd). In Zrn+1σn, Zr atoms are no longer all in the Mσ6 octahedral environment; some of them are in Mσ5 square pyramidal configurations.Therefore, one may expect the stabilization of antibonding Zr-σ levels when going from formally octahedral Zrσ6 to square pyramidal Zrσ5 environment due to the lack of a Zr(4d)-σ(2p) antibonding component. The occupation of these Zr-σ nonbonding levels may explain the mechanical properties of these substoichiometric Zrn+1σn compounds.

Figure .

 . Figure . The al ele t o i de sit of states of a o-) N , ) N a d ) N. d C stal o ital Ha ilto populatio -COHP u es of ) N.

Figure 4 .

 4 Figure 4.1γc displays the total and projected DτS of Zr2σ, but also the projected d states of a hypothetical Zrσ structure within the Zr2σ structure (all σ vacancies are filled in the so-called perfect structure). τne can see that nitrogen vacancies give rise to additional states just below the Fermi level compared to its corresponding perfect structure, which originates from the Zr-Zr bonds passing through a nitrogen vacancy site. Such "vacancy states" usually lead to a drastic increase in the density of states at the Fermi level (0.0ι6 in Zrσ; 0.090 in Zr6σ5; 0.112 in Zr2σ, states per eV per electron). The increasing density of the d state at the Fermi level can be interpreted as an increase in the Zr(4d )-Zr(4d ) bonding or metallic bonds between the zirconium atoms.

Figure .

 . Figure . "tati g ou d-state e thalpies pe ato of a ious GaPO st u tu es elati e to the C VO -t pe C phase as a fu tio of p essu e. "t u tu es o tai -oo di ated dashed li e o -oo di ated Ga ato s solid li es .

Figure . Figure .

 .. Figure . P-GaPOstal st u tu e a d GaO -ased hai s i Pphase at GPa.

5. 2

 2 From 4-to 8-coordinated silicon atoms in solid-state SiS2 under pressure At ambiant conditions, the 16-valence electron triatomic species have different ground-state covalent structures. Examples are given in Scheme 5.1. S he e . ale e ele t o t iato i AB s ste s. CN, oo di atio u e ; D, ole ula ; D pol e i ; a d D e te ded s ste s.

  262 a few months later (August 2005). Let's say right now that our CSP searches did not confirm their hypothesis; no FeS2 pyrite-like phase has been located on the SiS2 PES as a global minimum. Above ~45 GPa, the phase diagram of SiS2 has not been explored experimentally.

Figure .

 . Figure . Pap ite Fe" phase. Coo di atio pol hed a of " ato s a ou d Fe ato s a e sho as o tahed al, hile " du ells a e i ello .

Figure .

 . Figure . C stalli e st u tu es of the e pe i e tal "i" phases. a I a NP , P / -I HP , P / -II HP . d I-d HP .

Figure 5 Figure .

 5. Figure . C stal st u tu e of P-"i" . a sta ked "i" sla s; he ago all pa ked sili o la e lue sti ks: "i-"i sepa atio of . Å at GPa , a d CdI -t pe st u tu e. Coo di atio pol hed a of " ato s a ou d "i ato s a e sho as o tahed al lue .

.

Figure .

 . Figure . E thalp -p essu e elatio s fo solid "i" : a -GPa a ge; e thalpies a e sho pe ato a d elati e to the -fold oo di ated Pphase; -GPa a ge ; e thalpies a e sho pe ato a d elati e to the -fold oo di ated Pphase; fo us of the -GPa a ge; e thalpies a e sho pe ato a d elati e to the -fold oo di ated P phase.

Figure .

 . Figure . Volu e e olutio upo p essu e fo p edi ted solid-sate "i" phases f o to GPa: The olu e diffe e e ΔV of C , P / a d P is slightl lo e tha ou a u a ( ΔV < 0.02 Å 3 ). Volu e is gi e i Å a d p essu e i GPa.

  as the exploration of the transition phase pathways using Variable-Cell σudged-Elastic-Band (VCσEB) method, implemented in USPEX.263

Figure .

 . Figure . C stal st u tu e of "i" Pst u tu e at GPa. a et ithout i te la e "i-" o d "i-"< . Å ;et ith the "i-" i te la e o ds "i-" < . Å .

  a

Figure .

 . Figure . C stal st u tu e of C "i" phase at GPa. a all&sti k ie , oo di atio pol hed a of " ato s a ou d "i ato s a e sho as disto ted " u es lue , a d the ed li e i di ates the "" es.

  a

Figure .

 . Figure . C stal st u tu es of "i" : a P / at GPa a d P at GPa Yello a d g a ep ese t " ato s a d lue ep ese ts "i ato s.

Figure .

 . Figure . De sities of states DO" fo the P-, C P / a d P phase at GPa, GPa, a d GPa, espe ti el .

1. 2

 2 INCAR and POSCAR for projected DOS of empty sphere (Chapter 3.2)To plot the projected DτS in a vacuum sphese, the IσCAR and PτSCAR are provided below.The projected vacuum DτS is in the last column of DτSCAR. You may use split_dos.py from http://theory.cm.utexas.edu/vtsttools/ to get the split DτS.

  CutoffNeigh o Fi de utoff, data # Ite ate o e all i put pa ti les: a _list = list fo i de i a ge u _pa ti les : # Ite ate o e the eigh o s of the u e t pa ti le: fo eigh i fi de .fi d i de : a li e = '% d % d %. f %s\ '% i de , eigh[ ], eigh[ ], eigh[ ] a _list.appe d a li e # Read the PO"CAR to Get the u e of ea h ele e t #This step is o l fo Na-C-O s ste pos a = ope 'CONTCAR', ' ' pos_ o t = pos a . eadli es li e_ u = pos_ o ti t ele .split [ ] ele _ = i t ele .split [ ] dista e = float ele .split [ ] # Fo ato _id i , if the e is o o d ith ato _id i , o ti ue if ato _id != ele _ : o ti ue # I do 't a e the pu e A o pu e BC. elif A == -a d B == -a d C == -: <= B a d ele _ > A a d ele _ <= B a d ele _ > A a d dista e < dis_BB: o d_BB = o d_BB + output. ite ele # u e of C--O o d elif ele _ <= B a d ele _ > A a d ele _ > B a d dista e < dis_BC: o d_BC = o d_BC + output. ite ele elif ele _ > B a d ele _ <= B a d ele _ > A a d dista e < dis_BC: ite io + TF if o d_BB == a d o d_BC == : C ite io = TF = C ite io + TF TF = TF + TF + TF + TF if o d_BB != o o d_CC != o o d_AX != o o d_BC != : output. ite 'Ato ID:|% d| O-O:|% d| C-C:|% d| C-O:|% d| Li-X|% d| \ '% ato _id, o d_CC, o d_BB, o d_BC,

Figure A .

 A Figure A . DO" a d a d st u tu e fo Ba N ele t ide phases al ulated lo al fu tio ethod CRY"TAL Chapte .

Figure A .

 A Figure A . De sit of states DO" fo MN phases Chapte .

Figure A .

 A Figure A . Pho o dispe sio u es of the "i" o pou ds at sele ted p essu es. No i agi a pho o odes a e fou d i the hole B illoui zo e, hi h o fi s thei d a i al sta ilit . Chapte .

Figure A .

 A Figure A . Radial dist i utio fu tio s RDF fo the a C-C, C-O, a d Li-O sepa atio s o se ed du i g AIMD si ulatio s fo * * -ased supe ell at a ie t p essu e a d T = K Li CO P a phase , K, K, K a d K.

Figure A .

 A Figure A . Radial dist i utio fu tio s RDF fo the a C-C, C-O, a d Li-O sepa atio s o se ed du i g AIMD si ulatio s fo * * -ased supe ell at a ie t p essu e a d T = K Li CO Pphase , K, K, K a d K.

Figure A .

 A Figure A . Radial dist i utio fu tio s RDF fo the a C-C, C-O, a d Li-O sepa atio s o se ed du i g AIMD si ulatio s fo * * -ased supe ell at a ie t p essu e a d T = K LiCO P / phase , K, K, K a d K.

  -lés : al ali o-te eu , algo ith e olutio ai e, a al se o italai e, al uls th o i ues, CO , DFT, dio de de a o e, GaPO , haute p essio , i sili o, liaiso hi i ue, MoN , it u es, pe tazolate, p di tio de phases istalli es, st u tu e le t o i ue, "i" , U"PEX, ) N Adresse du La oratoire : IC MP -UMR U i e sit de Poitie s -"FA Bâti e ts B , ue Mi hel B u et -T"A , Poitie s ede

  Part of the Ba-σ and SiS2 topics have been partially developed by two co-supervised students from σorthwestern Polytechnic University, during their international research exchange program in 2016, Mr. Le Wang and Mr. Shipeng Gong, respectively (6 months in Dr. Gilles Frapper's Lab).

	obtained at high pressure. τur theoretical study leads to a phase transformation of the quartz form
	to a metastable orthorhombic form with Pmn21 space group, in agreement with the experimental
	data (XRD & IR). The second theme considers the different phase transformations of silicon
	disulfide induced by pressure from 0 to 500 GPa. Above 200 GPa, κ-coordinated Si phases are
	proposed as stable structures.

  Therefore, different types of crystal structure prediction methods, with inherently different ways of approaching this difficult task, are proposed.κ-15 Most methods designed for crystal structure prediction, such as simulated annealing, metadynamics, minima hopping, and genetic algorithms, have focused on overcoming the free energy barriers. Usually, in these methods, one starts within a suitable region of configuration space, then explores its neighborhood by moving across a barrier into a nearby local minimum, hoping to discover a new good minimum. Below is an overview of several successful CSP algorithms.

  .5ι to 1.40 Å) and elongation of the C-τ bonds (1.2ι to 1.3ι Å) at 0 GPa. During the phase transition from experimental LiCτ2 to predicted Li2Cτ2, formal chemical reduction of the C2τ4 motifs occurs without CC/Cτ bond breaking (Scheme β.5).

4-

units, two electrons are added to a CC bonding (C-τ antibonding) π molecular orbital, which explains the contraction of the CC bond (

1

S he e . " he ati edo e ha is fo Li C O he e = , .

  bonds are observed along the zig-zag carbon net (~1.ι Å), which weaken structural stability. The Pnma structure possesses an enthalpy of decomposition to solid lithium and carbon dioxide of 0.6ι1 eV/atom, slightly higher that that of oxalate (0.654 eV/atom). However, the enthalpy of 0.053 eV/atom is greater than that of the lowest metastable Li2Cτ2 phase at 1 atm,

	the C2/m space group that contains ethene-like C2τ4 units (see Figure β.9). If the barrier to
	isomerization of C2/m Li2Cτ2 (or decomposition to elements or other Lix(Cτ2)y compounds) can
	protect Pnma, then perhaps the barrier could be detected at low temperatures. τur AIMD
	calculations show that the polymeric units are broken above 600 K, but their carbon net is
	maintained at 300 K (See Appendix).

6-

observed in C2/m Li3(Cτ2)2 illustrated in Figure β.1γb and Figure β.1γc. At ambient pressure, the carbon backbones of these phases remain intact, though elongated CC Finally, we finish by discussing the quenchability of the finite C4τκ chains in Li3(Cτ2)2. At ambient pressure, the enthalpy of formation is exothermic (-1.596 eV/atom). The elongated central CC bond of 1.6ι Å observed at 0 K and 1 atm weakens the molecular C4τκ 6-anion. σevertheless, no carbon-carbon bond cleavage or other decomposition was observed during a AIMD simulation at 600 K and 1 atm. The C4τκ motif within P-1 Li3(Cτ2)2 is maintained. At 900 K, structural instability is observed as C-C bonds are broken. To conclude, in addition to the experimentally proven oxalate 1:1 phase, we propose three novel phases in the pseudo-binary Li-Cτ2 phase diagram are viable solid-state structures at ambient pressure based on our numerical CSP searches, namely C2/m P-1 Li2(Cτ2)2, P-1 Li3(Cτ2)2 and P2/c poly-1,4-dioxane LiCτ2. These structure predictions should generate interest in the synthesis of these crystals.

3.1 Introduction σitrogen

  forms about ικ% of the Earth's atmosphere, making it the most abundant, uncombined element accessible to man, and a potential natural resource in chemistry. The characterization of nitrogen is credited to Scottish physicist Daniel Rutherford in 1ιι2, 111 but at the same time, it has also been studied by several scientists such as Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley.112 Its name was given by the French scientist Jean-Antoine-Claude Chaptal, comte de Chanteloup, in 1ι90 (in French, nitrogène, from Greek roots νἰ ρον, nitre ; εννᾶν, to form). σevertheless, Antoine Lavoisier gave this inert gas another name, azote -mephitic air-, from the Greek word άζω ός (azotikos), "no life".

At standard conditions, two nitrogen atoms bind to form a dinitrogen molecule, a tasteless, colorless and odorless diatomic gas. At 1 atm, molecular nitrogen liquefies at ιι K

(-195.κ°C)

, and freezes at 63 K (-210 °C) into the beta hexagonal close-packed crystal allotropic form.

  Table γ.1 is an up-to-date list of Axσy, where A is an s block

element, alkaline or alkaline earth metal. Figure γ.4 summarizes the topology of σn units observed in experimentally characterized and theoretically predicted Axσy phases:

  3-in Be3σ2 and Mg3σ2). Eckerlin et al. 154 report a polymorph Be3σ2 phase transition around 1400 °C. Chang et al. investigated the structural stability and mechanical properties of Be3σ2 crystallizing in α and phases.

  Table γ.1. Lite atu e epo t o A N i a phases A, s-lo k ele e ts; alkali e a d alkali e ea th etals . These st u tu es a e lassified i to fou fa ilies elati g to the di e sio alit of the it oge su latti es:e te ded D it oge la e s, i fi ite D it oge hai s, e apsulated ole ule-like D a io i u its, a d isolated u lei N -io s. E pe i e tall ha a te ized st u tu es a e e pli itl i di ated EXP ; if ot, this is a st i tl h potheti al phase.

	Cs2N (C2/m) Peng 145 Phases CsN (C2/m) Peng 145 CsN (P-1) Peng 145 Phases Li13N (Immm) Shen 151	>18 Isolated nitrides N 3-7/1 Molecular anions P (GPa) VEC a /ACN b Nitrogen framework, dumbbell N2 4-pernitride electrical property 7.5-44 6/1 dumbbell N2 2-diazenide >44 open-chain N4 4-tetrazadiene, VEC a /AC Nitrogen framework, P (GPa) N b electrical property 6/1 semiconductor 43-76 18/0 metallic
	Li3N2 (P4/mbm) Shen 151 CsN2 (C2/m) Peng 145 Li13N (C2/m) Shen 151	30-39 4-40 >76	6.5/1 5.5/1 18/0	dumbbell N2 3-, metallic dumbbell N2 -metallic
	Li3N2 (C2/c) Shen 151 CsN3 (I4/mcm) Peng 145 Li5N (P6/mmm) Shen 151	39-89 <6 >80	6.5/1 5.33/1.33 10/0	dumbbell N2 3-, metallic linear N3 -azide, insulator metallic
	Phases Li2N2 (Pmmm) Shen 151 , P (GPa) Zhang 174 CsN3 (C2/m) Peng 145 Li3N (Pm-3m) Shen 151 CsN3 (P21/m) Peng 145 Li3N (P6/mmm) Shen 151 Li2N2 (Immm) Shen 151 CsN3 (C2/m) Peng 145 Li3N (P63/mmc) Shen 151 LiN3 (C2/m) Prasad 149 190-500 Li2N2 (Pnma) Shen 151 CsN3 (P21) Wang 173 Li3N (Fm-3m) Shen 151 Li2N2 (Cmcm) Shen 151 Li2N2 (P63/mmc) Shen 151 CsN5 (Cmc21) EXP Peng,Steele 140,141 Na3N (Pm-3m) Vajenine 178 Na3N (P6/mmm) Vajenine 178 1-2.3 0-8.2 6-13 0-0.2 16-26 0.2-VEC a /ACN b Nitrogen framework, electrical property 6/1 5.33/1.33 linear N3 -azide 8/0 insulator dumbbell N2 2-, metallic 5.33/1.33 linear N3 -azide 8/0 insulator 8.2-8.9 6/1 dumbbell N2 2-, metallic >81 5.33/2 benzene-like N6 2-, metallic 0.2-8/0 insulator Two-dimensional (2D) network 5.33/2.6+2 8.9-66.4 6/1 dumbbell N2 2-, metallic 13-51 5.33/1.33 linear N3 -azide 0.2-8/0 insulator layers of fused puckered 10-embered decagon rings + 1D zigzag chains, metallic >66.4 6/1 dumbbell N2 2-, metallic 0-56 5.5/1 dumbbell N2 2-, metallic >14 cyclopentadiene-like N5 -0-1 8/0 insulator 5.2/2 pentazolate, metallic 8/0 insulator One-dimensional (1D) network LiN2 (P-1) Shen 151 >56 5.5/2 LiN3 (P-62m) Shen 151 0-0.9 5.33/1 dumbbells N2 -, metastable CaN (C2/m) Zhu 164 0-14 7/1 dumbbell N2, metallic Na3N (P63cm) Vajenine 178 2.3-25 8/0 insulator polyacetylene-like armchair chains poly-N4 2-, semiconductor LiN3 (P-1) Prasad 149 metastable 5.33/2.33 LiN3 (C2/m) Shen 151 0-36 linear N3 -azide, CaN (Cmc21) Zhu 164 14-40 7/1 dumbbell N2, metallic Na3N (P63cm) Vajenine 178 >25 8/0 insulator 5.33/1.33 metastable below 49 GPa CaN (C2/m) Zhu 164 40-76 7/1 dumbbell N2, metallic Na3N (Fm-3m) Steele 152 >10 8/0 insulator linked chair six-membered rings poly-N6 2-, metallic LiN3 (P21) Wang 169 >375 5.33/2.67 LiN3 (P6/m) Shen 151 , Wang 169 >49 36-375 5.33/2 CaN (Pbam) Zhu 164 76-100 7/1 dumbbell N2, metallic K3N (P63/mmc) Fischer 179 metastable 8/0 insulator benzene-like N6 2-, metallic Ca2N3 (P-62m) Zhu 164 18-44 6.33/ dumbbell N2, metallic Cs3N (Cmcm) Peng 145 18-64 8/0 insulator puckered fused 5-membered rings poly-KN3 (I4/mcm) Zhang 171 0-22 5.33/2 linear molecular N3 -anions CaN2 (Pnma) Zhu 164 0-3 6/1 dumbbell N2 2-Be3N2 (Ia-3) Xia 175 0-118 8/0 insulator N6 2-, insulator NaN2 (Cmmm) Steele 152 50-100 polyacetylene-like zigzag chains poly-N2 -, KN3 (C2/m) Zhang 171 22-40 5.33/2.5+2 linear molecular N3 -anions CaN2 (I4/mmm) Zhu 164 3-18 6/1 dumbbell N2 2-Be3N2 (P-3m1) Xia 175 118-150 8/0 insulator 5.5/2 metallic NaN3 (C2/m) Zhang 170 152-200 5.33/2.5+2 LiN5 (P21/c) Shen 151 15-65 5-membered ring N5 -CaN2 (P21/c) Zhu 164 18-92 6/1.5 N4 4-, metallic Be3N2 (P63/mmc) Xia 175 metastable 8/0 insulator 5.2/2 pentazolate, insulator CaN2 (Pbam) Zhu 164 92-100 6/1.5 N4 4-, metallic Be3N2 (Pn-3m) Xia 175 metastable 8/0 insulator puckered fused six-membered rings poly-N4 -and zigzag chains poly-N2 -, metallic KN3 (P6/mmm) Zhang 171 40-100 5.33/2.67 LiN5 (C2/c) Shen 151 >65 5-membered ring N5 -CaN3 (Pmma) Zhu 164 8-36 5.67/1 dumbbell N2, metallic Be3N2 (R3m) Xia 175 metastable 8/0 insulator 5.2/2 pentazolate, insulator CaN4 (P4/mbm) Zhu 164 5-19 5.5/1 dumbbell N2, metallic Ca2N (R-3m) Zhu 164 0-10 9/0 insulator Fused chair-like six-membered rings poly-N6 2-, metallic NaN (Pmmm) Zhang 175 0 6/1 dumbells N2 2-, metallic CaN3 (C2/c) Zhu 164 36-100 5.67/2 N6 4-, metallic Ca2N (Fm-3m) Zhu 164 10-84 9/0 metallic
	KN3 (C2/m) Zhang 171 NaN (Cmcm) Steele 152 >298.6 CaN5 (Pm) Zhu 164 Ca2N (I4/mmm) Zhu 164 NaN (Cmmm) Steele 152 CaN6 (Fddd) Zhu 164 EXP Ca3N2 (Ia-3) Römer 180	10-30 33-100 84-100 5.33/2.5+2 28-100 ambient 0-0.5	puckered fused six-membered rings poly-6/1 dumbells N2 2-, metallic 5.4/2 N5 2-, metallic 9/0 metallic N4 -and zigzag chains poly-N2 -, metallic 5.33/1 dumbells N2 2-, metallic 5.33/1.33 linear N3 -azide 8/0 insulator
	KN3 (C2/m) Zhang 171 NaN2 (Cmmm) Steele 152 >274 SrN6 (Fddd) Zhu 177 Ca3N2 (R-3c) Römer 180 RbN3 (P-1) Wang 172 30-50 Na2N5 (Pbam) Steele 152 BaN6 (Fddd) Zhu 177 Ca3N2 (Pbcn) Römer 180 Ca3N2 (C2/m) Römer 180 RbN3 (C2/m) Wang 172 >200 NaN3 (C2/m) Steele 152 Ca3N2 (P-3m1) Römer 180 NaN3 (R-3m) Zhang 170 Ca3N2 (C2/m) Römer 180 NaN3 (I4/mcm) Zhang 170 CsN3 (P-1) Wang 173 51-200 NaN3 (P6/m) Zhang 170 a I A N , VEC is the ale e ele t o o e t atio ; VEC= + / ith : u e of ele t o s gi e Fused chair-like six-membered rings poly-30-100 5.5/1 dumbells N2 2-, metallic ambient 5.33/1.33 linear N3 -azide, insulator metastable 8/0 insulator 5.33/2.67 N6 2-, insulator polyacetylene-like trans-cis chains poly-30-100 Five-membered ring N5 2-, ambient 5.33/1.33 linear N3 -azide, insulator 0.5-10 8/0 insulator 5.4/1 pentazolate, metallic 10-27 8/0 insulator 5.33/2 N12 4-, metallic 5.33/2.67 Fused chair-like six-membered rings poly-<0.85 5.33/1.33 linear N3 -azide, insulator 27-38 8/0 insulator 0.85-6.5 5.33/1.33 linear N3 -azide, insulator 38-90 8/0 insulator 6.5-58 5.33/1.33 linear N3 -azide, insulator N6 2-, metallic 5.33/2 58-152 5.33/2 benzene-like N6 2-, metallic polyacetylene-like trans-cis chains poly-N12 4-, metallic CsN2 (C2/c) Peng 145 >40 5.5/2 CaN4 (P41212) Zhu 164 19-67 5.5/2 KN3 (I4/mcm) Babu 176 0-20 5.33/1.33 linear N3 -azide insulator armchair chains poly-N4 2-, metallic NaN5 (Cm) Steele 152 20-100 pentazolate, insulator 5.2/2 five-membered ring N5 -infinite spiral chains poly-N8 4-, insulator NaN5 (P2/c) Steele 152 16.5 pentazolate, insulator 5.2/2 five-membered ring N5 -u e of A ato s; : u e of it oge ato s.	A; :
	KN3 (C2/m) Zhang 171	15.7-20	5.33/1.33	linear N3 -azide, insulator
	KN3 (Cm21) Zhang 171	20-41	5.33/1.33	linear N3 -, azide; metastable
	KN3 (P6/mmm) Zhang 171	>41	5.33/1.33	benzene-like N6 2-, metallic
	RbN3 (I4/mcm) Wang 172	0-30	5.33/1.33	linear N3 -azide, insulator
	RbN3 (P6/mmm) Wang 172	50-200	5.33/2	benzene-like N6 2-, metallic

Ta le .

  Above 5ι GPa, a tetragonal phase becomes the ground-state structure (space group I4/mmm, MoSi2-type), in which the coordination number of nitrogen increases from six to eight. Ba2σ structure presents a bcc-substructure character as displayed in Figure γ.β4c. Alternatively, this phase may be described as AB-staked Ba2σ layers, made of fused distorted Baκ/4 cubes, centered by σ atoms. In a distorted σBaκ cube, Ba-Ba is 2.ι2 Å, and 3.00 Å and Ba-σ are 2.52 Å at 100 GPa. The interspacing slab is computed at 2.94 Å. As displayed in Figure γ.β4c, no localized electrons are seated in the void layer. Moreover, a s → d transition has been identified in I4/mmm Ba2σ by a DτS analysis, a well-characterized feature in solid alkaline and earth alkaline metals under high pressure. A metallic bond character is present in high-pressure I4/mmm Ba2σ.

). P-3m1 Ba2σ maintains the 2D electride property; see its ELF plot in Figure γ.β4b.

Table γ .

 γ 6.

  The calculated density of states shows that all predicted stable and metastable compounds are insulators with a wide band gap at ambient pressure and 50 GPa. The band gap energies are listed in Table γ.7. Hence, they would be optically transparent, if observed. For example, the calculated band structure of the P-1 phase of Csσ5 confirms an optically transparent insulator in experiments.153,145

	Ta le . Ba d gap eV ithi PBE-GGA le el
	Ba d Gap eV	BeN	MgN	CaN	" N	BaN
	GPa	.	.	.	.	.
	GPa	.	.	.	.	.
	Noti e: GGA-PBE u de esti ate the a d gap.

Table 4

 4 Cal ulated st u tu al pa a ete s fo MoN phase at sele ted P essu e Ǻ, GPa

	.1. The calculated

Ta le .

  3-). The energetic preference of the pernitride over the dinitride could be related to (i) covalent σ-σ bonds being more energetic than Mo-Mo and Mo-σ ones and (ii) the high Mo oxidation state (VI) in the latter, as proposed for other transition metal Mσ2 phases (Feσ2 216 , Reσ2 21ι

  ). Mo (VI) can only be found in solid-state extended structures under extremely oxidizing conditions such as in Moτ3 and polymolybdates. 21κ In addition, the three van der Waals-type structures, Cm, P63/mmc, and R3m(10-12), are by far the less stable ones in our predicted Moσ2 structures (ΔH > 0.6 eV/atom). This result confirms that R3m Moσ2 is definitively not a viable structure. Recently,Wu et al. 219 proposed a nonmagnetic 2D T-type Moσ2 structure which contains octahedral Mo atoms. τur evolutionary searches locate the corresponding 3D T-type phase (space group C2/m) at 0.ι01 eV/atom higher in energy than the ground-state one. Moreover, its phonon dispersion curves present strong imaginary frequencies; thus, this octahedral Mo-containing layered structure is dynamically unstable.

  technics and DFT calculations. A σorthwestern Polytechnical University (σPU) exchange Finally, in September 2016, Evers, Schnöckel, Medvedev et al submitted an article in Inorganic Chemistry entitled "More Than 50 Years After Its Discovery in Siτβ, τctahedral Coordination Has Also Been Established in SiSβ at High Pressure". 260 Their high-pressure phase is one that we predicted a few weeks before (space group P-3m1, Z=1, CdI2 type), among others 259 . Then, at 2ι.5 GPa, a CdIβ-type structure characterized by X-ray powder diffraction as well as by Raman spectroscopy. This structure features octahedral SiS6 coordination. σote that these authors claimed that the "σP-SiSβ phase remains stable up to approximately 7 GPa, at which pressure, the appearance of the new diffraction peaks indicates the onset of the structural phase transition". Moreover, in their conclusion, they wrote: "a SiSβ polymorph with FeSβ structure (Cσ 6 + β) could probably be synthesized in a diamond anvil at less rigorous experimental conditions than for Siτβ". The authors proposed this phase by making an analogy with poststishovite Siτ2 phase proposed by τganov et al. 261 in February 2005, and characterized above

26κ GPa and 1κ00 K by

Kuwayama et al.

  Li (0.756, 0.514, 0.061), (0.743, 0.488, 0.569), (0.244, 0.507, 0.875), (0.254, 0.494, 0.383); C (0.953, 0.200, 0.297), (0.546, 0.805, 0.647), (0.955, 0.203, 0.804), (0.542, 0.795, 0.154), (0.040, 0.787, 0.630), (0.460, 0.217, 0.270), (0.036, 0.777, 0.320), (0.461, 0.223, 0.957); O Li ( 0.420, 0.244, 0.364), (0.921, 0.234, 0.380), (0.415, 0.750, 0.355), (0.416, 0.995, 0.591), (0.420, 0.494, 0.588), (0.919, 0.497, 0.590); C (0.753, 0.986, 0.129), (0.085, 0.003, 0.104), (0.086, 0.502, 0.113), (0.753, 0.488, 0.104), (0.248, 0.248, 0.851), (0.583, 0.233, 0.852), (0.668, 0.744, 0.851), (0.160, 0.755, 0.839); O (0.185, 0.069, 0.328), (0.172, 0.333, 0.100), (0.665, 0.419, 0.334), (0.665, 0.157, 0.105), (0.658, 0.662, 0.092), (0.178, 0.828, 0.082), (0.174, 0.576, 0.335), (0.675, 0.913, 0.351), (0.150, 0.824, 0.609), (0.656, 0.906, 0.863), Ta le A . Cal ulated e thalpies of lithiu , a o , o ge , a o dio ide, lithiu o ide Li O, lithiu a o ate Li CO phases at diffe e t p essu e. Chapte Ta le A . G a i et i e e g de sities fo it oge -ased ate ials Chapte .

	Li(CO2)2 4 50 Co pou ds P1 Li	a=4.410, b=7.965, c=5.121 α= . , β= . , γ= . E thalpies eV/ato GPa GPa (0.010, 0.342, 0.146), (0.490, 0.662, 0.568), GPa GPa (0.011, 0.341, 0.650), (0.485, 0.657, 0.069), (0.979, 0.639, 0.776), (0.520, 0.366, 0.341), I -BCC F -FCC I-d C a -. -. . .
			C O		P / -. C / O	(0.978, 0.620, 0.292), (0.520, 0.379, 0.849), (0.675, 0.117, 0.379), (0.825, 0.889, 0.687), Fd-Fd--. -. (0.167, 0.117, 0.890), (0.331, 0.882, 0.196), (0.335, 0.890, 0.686), (0.165, 0.115, 0.379), (0.824, 0.878, 0.198), (0.674, 0.119, 0.889) C / O8 Fd--. C / O8 C / O8
	Li3(CO2)2 2 100	C2/m	a=8.756, b=2.328, c=7.591 α= . , β=143.3, γ= .	Li (0.144, 0.000, 0.028), (0.000, 0.500, 0.500); C (0.179, 0.500, 0.388); O (0.055, 0.500, 0.781), (0.731, 0.000, 0.270)
	Li3(CO2)4 2 100	P1	a=4.352, b=7.850, c=4.688 α=89.5, β=90.4, γ=89.9
		Stoi hio etr MgN	Spa e group P /	(0.674, 0.163, 0.628), (0.149, 0.081, 0.851), (0.175, 0.587, 0.855), (0.671, 0.409, 0.859), E erg de sit E thalp eV/ato eV/f.u kJ/g (0.164, 0.325, 0.622), (0.665, 0.672, 0.622) Li (0.498, 0.505, 0.248), (0.995, 0.495, 0.751); -. . .
	Li(CO2)2 4 100 MgN MgN MgN MgN	P-1	a=4.314, b=7.542, c=4.582 P--. -. α= . , β= . , γ= . C / C -. P-I -.	C (0.177, 0.210, 0.575), (0.822, 0.791, 0.929), . . (0.321, 0.783, 0.845), (0.679, 0.218, 0.486); O (0.206, 0.322, 0.295), (0.205, 0.322, 0.795), . . (0.278, 0.616, 0.972), (0.720, 0.385, 0.529), . . (0.900, 0.113, 0.601), (0.099, 0.888, 0.906), (0.406, 0.113, 0.093), (0.595, 0.887, 0.400) . .
	Li2CO2 LiCO2	4 4	0 Quench MgN MgN 0 Quench Mg N Mg N Nit oge	Pnma P2/c	C a=5.896, b=2.813, c=8.833 -. α= . , β= . , γ= . P / -. P -. a=7.298, b=5.559, c=4.257 α= . , β= . , γ= . C / -. Pa--.	Li (0.892, 0.250, 0.901), (0.794, 0.250, 0.205); . . C (0.950, 0.250, 0.540); O (0.510, 0.250, . . 0.805), (0.789, 0.750, 0.023) Li (0.000, 0.102, 0.750), (0.500, 0.205, 0.250); . . C (0.143, 0.623, 0.923); O (0.233, 0.409, . . 0.204), (0.775, 0.172, 0.866) --
			Mg N		Ia-	-.	-	-

Acknowledgments

Calculations were performed on the THτR supercomputer at the Mesocentre SPIσ (Poitiers University) and at HPC GEσCI (TGCC Curie and CIσES; grant 20160κι539 ; 2 millions cpu). The work was supported by Région Poitou-Charentes (PhD fellowship 2014-201ι), CσRS and Poitiers University, GDRI RFCCT CσRS (DσM-evol program), and Hubert Curien Partnerships PHC XU GUAσGQI 2015 (σo. 34455PE) program of the French Ministry of Foreign Affairs. τur Institute, « Institut de Chimie des Matériaux et Milieux de Poitiers » UMR ι2κ5, and our research team E4 "Catalyse et Milieux σon Conventionnels" are also acknowledged for their financial support (operating & HPC equipment, CPER 201ι).

Methods

Goals

As a crucial thermodynamic parameter, pressure has emerged as a powerful tool to investigate the physical and chemical behaviors of materials, and this has lead to new and unexpected products.

In this study, we investigate the possible formation and stability of polymeric nitrogen compounds in the binary Mg-σ system, and the Ba-σ system, at high pressure. Structure searches for stable Mg/Ba-σ compounds were performed using an unbiased evolutionary structure prediction algorithm in conjunction with density-functional calculations. After which, electronic properties and chemical bonding of the identified compounds were systematically investigated, with the aim of exploring the properties and potential applications of Mg/Ba-σ compounds as high energy-density materials.

Barium nitrides

In this study, we have investigated the possible formation and stability of molecular and polymeric nitrogen motifs in the Ba-σ binary system upon pressure. In comparison with Mg 2+ , the increase of the Ba 2+ size should allow more nitrogen atoms around the cation. We have focused on the pressure range from 0 up to 100 GPa, as these pressures can nowadays be easily reached in diamond anvil cells. Structure searches for (meta)stable Ba-σ compounds were performed using an unbiased structure prediction method, based on the evolutionary algorithm, in conjunction with first principles density-functional approaches, to evaluate total energies and forces. We predicted, upon compression, the emergence of several (meta)stable nitrogen networks in different compositions, such as 3:1, 2:1, 1:1, 3:2. τur calculation indicated that nitrogen atoms would form, besides isolated nitride, σ2 dumbbells, linear σ3 azides, σ4 zigzag units, pentazolate σ5 rings, 5-membered rings with terminal nitrogen σ6 motifs, benzene-like σ6 rings, infinite nitrogen-based chains, and nitrogen layers at high pressure. We rationalized the nitrogen shapes through chemical bonding analysis, and thus explained the electronic properties of the identified compounds. Finally, we explored the properties and potential applications of Ba-σ compounds as high energy-density materials.

Phase stability of Ba-N system under pressure

All proposed solid-state Baxσy compounds are stationary points on the potential energy surface, located on the Ba-σ convex hull or above. We verified that each phase meets two criteria of stability: (1). Dynamical criterion: a structure is dynamically stable, as indicated by the absence of imaginary frequencies, in all of the phonon dispersion curves. The corresponding stationary point on the PES is a local enthalpy minimum. (2). Thermodynamic criterion: a Baxσy compound is thermodynamically stable when its free energy (at T = 0 K, enthalpy) of formation, from the component elements and any other Baxσy According to the structural arrangement and dimensionality of the covalent nitrogen network, the great variety of crystal structures are classified into four different categories (see Tables γ.γ and γ.4): structures with (1) isolated nitrogen atoms, as found in Ba3σ, Ba2σ, and Ba3σ2; (2) encapsulated molecule-like covalent anionic units, such as σ2 dumbbells, linear triatomic σ3 units, bent σ4 finite chain, five-membered rings σ5 and σ5-σ, six-membered rings σ6 as found in Ba3σ2, Baσ, Baσ2, Ba2σ11, Ba5σι, Baσ5, or Baσ6 or Baσ10; (3) 1D infinite nitrogen chains (Baσ3, Baσ4 and Baσ5); and finally (4) 2D covalent nitrogen layers alternating with barium sheets (Baσ3). This structural classification can be applied to other alkaline and alkaline earth nitrides Axσy compounds, such as σ2 dumbbells, linear σ3 units, five-membered σ5 rings, cyclic σ6 rings, and 1D linear σx. Atmospheric R-3m Ba2σ has an anti-CdCl2-type structure. The hexagonal layered unit is composed of Ba6/3 edge-sharing octahedra centered by a nitrogen atom, with a Ba-σ distance of 2.ι9 Å typical of ionic bonding. In this phase nitrogen exists as σ 3-: the closest σ-σ separation is 4.06 Å, much longer than a 1.45 Å σ-σ bond length, as in H2σ-σH2 hydrazine. 1κ9 Thus, each Ba2σ unit can be regarded as a positively charged ionic slab, [(Ba 2+ )2(σ 3-)] + . These cationic

Zr-N compounds

According to the phase diagram provided by Gribaudo et al. 210 , Zrσ and Zr3σ4 can be stable at ambient conditions. In 2003, c-Zr3σ4 with a Th3P4 structure was synthesized by Zerr et al. 201 using diamond-anvil cell experiments at 16 GPa and 2500 K. This compound was expected to exhibit a very high Vickers hardness around 30 GPa, similar to that of -Si3σ4. However, Kroll showed that hardness is just slightly harder than 14 GPa. in the Zr-σ system at ambient conditions, and then their phase stability at finite temperatures are evaluated.

Phase stability of the Zr-N system at finite temperatures

Thermodynamic stability of zirconium nitrides in the temperature range of 0-2000 K was quantified by constructing the thermodynamic convex hull, which is defined as the Gibbs free energy of formation of the most stable phases at each composition:

Any phase located on the convex hull is considered to be thermodynamically stable (at T = 0 K, G = H) and at least in principle synthesizable. 210 In the case of zirconium nitrides, a series of stable compounds at various Zr:σ ratios, i.e., 2:1, 4:3, 6:5, κ:ι, 1:1, 15:16, ι:κ, 4:5 have been discovered by our evolutionary searches at 0 K, shown in C2/m) have also been found to be thermodynamically stable. For the missing composition Zr3σ2, the lowest-energy structure is Immm with the enthalpy of formation lying very close to the convex hull at only 0.005 eV per atom, i.e., Zr3σ2 is a metastable phase at 0 K. Additionally, Juza et al. in 1964 have discovered two nitrogen-rich phases with rocksalt structures. Unfortunately, detailed stoichiometries and crystallographic information were not determined. From our evolutionary searches, we found these two compounds could be Zr15σ16 (x = 1.0ι, SG: P-1) and Zr4σ5 (x = 1.25, SG: C2/m). Their structures are composed of edge-sharing Zrσ6 and □σ6 (□ means Zr vacancy) octahedra, similar to the rocksalt Zrσ structure. To the best of our knowledge, such nitrogen-rich nitrides have never been reported in other TM-σ systems.

For Zr3σ4, the most stable structure has the orthorhombic Pnma symmetry, 22κ which is energetically more favorable than the Th3P4-type structure by 0.019 eV per atom at 0 GPa and 0 K. We found it is thermodynamically metastable considering decomposition into Zrσ and σ2 at ambient conditions. First-principles calculation shows that o-Zr3σ4 will transform into c-Zr3σ4 at 2 GPa.

Silica-like compounds under pressure: GaPO 4 and

SiS 2

enabled us to better understand the competition between this new phase and other crystalline or amorphous forms. The detail of experiments can be found in the appendix.

Computational details

To identify the (meta)stable structures in the GaPτ4 system, we have employed the firstprinciples evolutionary structure prediction method USPEX. κ, 13,30 It searches for the global minimum of free energy of the crystal concerning its structural parameters. At a given pressure (P = 5 and 20 GPa), a fixed stoichiometry crystal structure search is performed using up to 4 formula units per primitive unit cell. The first generation had κ0 randomly produced candidate structures; all subsequent generations contained 60 structures. Evolutionary variation operators were applied to search for the global energy minimum; 50% of the new structures were produced by heredity, with the remaining structures were produced by atomic mutation (10%), lattice mutation (10%), permutation (10%), and random generator (20%). Density functional total energy calculations and geometry optimizations were performed using the VASP program 4κ with the Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation (GGA/PBE). 41 We used projector-augmented wave (PAW) potentials 49 for Ga (Ga_d), P, and τ with radii 2.3 a.u. for Ga ([Ar] core), 1.9 a.u. for P ([σe] core), and 1.52 a.u. for τ ([He] core). A plane-wave basis set with a kinetic energy cutoff of 520 eV was employed. We used uniform Γcentered meshes with a reciprocal space resolution of 2π × 0.06 Å -1 for Brillouin zone sampling to ensure the error bars of the total energies were less than 1 meV/atom. Structure relaxations proceeded until all forces on atoms were less than 1meV/Å and the total stress tensor had deviations from target pressure within 0.01 GPa.

Results and discussions

Compression of the α-berlinite-type phase and high-pressure phase transition

From the experiment of view, the initial compression behavior of the α-berlinite-type phase (P3121) was in good agreement with previous work. 249 Beginning at between 9 and 12 GPa, new diffraction features were observed (Figure 5.1). A new crystalline phase was obtained, which was characterized by broad diffraction lines with a highly inhomogeneous intensity distribution in the 2D diffraction images indicating poor crystallinity and strongly preferred orientation effects. This renders structural determination difficult. The most prominent diffraction lines of this phase at 21

GPa and are distinct from those expected for an orthorhombic CrVτ4-type structure. However, above 20 GPa, some very weak lines at positions where those of an orthorhombic CrVτ4-type form could be expected were observed. This phase remained at an impurity level concerning the new unidentified phase even after heating to 340 ℃ at 25 GPa. Upon pressure release, the sample retransformed to the α-berlinite-type phase. To determine the effect of temperature on the competition between these two structures GaPτ4 was studied isothermally at 320 ℃ up to 32 GPa. σew diffraction features appeared beginning near 9 GPa, which is slightly lower than in previous ambient-temperature studies. Again, the data exhibited poor crystallinity and strong preferred orientation. However, in contrast to Al0.3Ga0.ιPτ4, strong diffraction features corresponding to the orthorhombic CrVτ4-type form were observed in addition to the principal lines of the new unidentified phase observed for the solid solution. The unit cell of the CrVτ4type form at 32GPa and 320 ℃ was refined, and the following values obtained: space group Cmcm, a = 5.15ι(1ι) Å, b = ι.030( 14) Å, c = 5.6κ0( 12) Å. This is consistent with previous measurements on this phase. 240,241,250 The pressure was decreased to 15 GPa at 320 ℃ and then final decompression was performed at ambient temperature. τn pressure release, a mixture of the berlinite-type and these two high-pressure phases was retained.

phases. The increase in Ga-τ bond length, by about ι% due to the increase in coordination number of gallium from 4 to 6 is consistent with previous work on GaAsτ4 at high pressure. 23ι,244

First-principles evolutionary crystal structure prediction

We used the evolutionary algorithm USPEX to investigate candidates for high-pressure (meta)stable structures of gallium orthophosphate (GaPτ4). We performed fully unconstrained variable-cell USPEX simulations at 5 and 20 GPa, using no experimental information such as lattice parameters, space group symmetry, etc. Among the predicted structures at 5 and 20 GPa, the structures with the lowest enthalpies are used to investigate their relative stability as a function of pressure (See Figure 5.β). Lattice constants and structural parameters of the most interesting candidate structures are given in the Appendix.

Ta le . Latti e pa a ete s of C phase at GPa.

P essu e "ou e Latti e pa a ete s GPa U"PEX a = . , = . , = . α = β = γ = ° GPa E pe i e t a = . , = . , = . α = β = γ = ° σote that the calculated lattice constant of the CrVτ4-type Cmcm phase at 32 GPa (T = 0 K) is in excellent agreement with the experimental value (T = 593 K), which gives a benchmark of the typical accuracy to expect of density functional theory (DFT) simulations for this system (See Table 5.1).

structure is found to be present at ambient temperature and after heating at moderate temperatures up to 320-340 ℃ indicating the presence of a non-negligible activation energy for a reconstructive phase transition to the stable CrVτ4-type form. This can be understood by the major difference between the structures of the CrVτ4-type form based on chains of edge-sharing Gaτ6 octahedra and the orthorhombic Pmn21 structure built up of layers of corner-sharing Gaτ6 octahedra.

program student has been trained in this research field under the direction of G. Frapper and under my supervision. Mr. Shipeng Gong wrote a training research report in April 2016 on our preliminary results. Then, in σovember 2016, while we were writing our SiS2 manuscrit, Plašienka, Mortonak and Tosatti 259 published an article entitled "Creating new layered structures at high pressures: SiSβ". They presented predictive ab initio calculations combined with an evolutionary structure search (UPSEX code) of SiS2 up to 100 GPa. And, of course, all of their predicted phases are identical with ours. Thus, we decided here to focus our discussion on results of high pressure SiS2 phases from 100 to 500 GPa.

In 1935, the ambient pressure (σP) phase of silicon disulfide had been reported by Zintl and Loosen and Büssem et al. 253,254 σP-SiS2 structure presents 1-dimensional infinite chains which interact via weak van der Waals forces (orthorhombic Ibam space group, Z=4 ). 255,256 Each extended SiS2 chain consists of distorted, edge-sharing SiS4 tetrahedra that run in the same direction, as displayed in Figure 5.8. This "covalent" topology is encountered in isoelectronic BeCl2 phase shown in Scheme 5.1.

In 1965, Silverman et al. 25ι reported the first high pressure phase HP3 at 6.0-6.5 GPa and 114κ-15ι3 K (tetragonal space group I-42d , Z = 4). This experimental crystal structure is shown in Figure 5.8. It consists of distorted, corner-sharing SiS4 tetrahedra. This HP polypmorph is similar to that of silica-like 3D phases with a silicon coordination number of 4 and is quenchable at ambient conditions. Forty years later, two novel high-pressure phases have been characterized by Evers, Schnöckel and co-workers. 25κ These structures are claimed to be the "missing links between the extremes of only edge-sharing and only corner-sharing tetrahedra". A monoclinic P21/c-I phase emerges at 2.κ GPa as the ground state phase. This structure may be viewed as the interconnection of polymeric tetrahedral SiS4/2 chains through Si-S bonds. Here, a 2D layer is formed and silicon is four-coordinated. At slightly elevated pressure (3.5 GPa), a phase transition occurs, again with P21/c space group. P21/c-II has a covalent Si-S net with edge-and corner-sharing tetrahedral, as found in P21/c-I, however, with a 3D connectivity net with voids. metal (See Figure 5.10, DτS of SiS2 at 50 GPa). σote that PBE DFT levels underestimate the band gap, therefore this band gap closure may have appeared at higher pressure. S he e . Le is st u tu e of "i" / u it i solid-state HP P-"i" . The V"EPR AX E otatio is i di ated, as ell as the lo al st u tu al e i o e t of "i a d ".

Moreover, our CSP searches determined several low-energy metastable phases in 20-100

GPa pressure range, all of them present layered /AbC/ SiS2 slabs, but with different packing.

Therefore, a van de Waals correction was introduced to consider the long-term interactions between these slabs. Thus, we performed a full structural optimization by using the vdW-D2 method of Grimme. 215 Taking into account such vdW correction, the enthalpy difference between P-3m1 and low enthalpy metastable phases is reduced by roughly 50 %, but the P-3m1 phase still Si atom families in SiS2. Finally, above 199 GPa, three stable structures are based on compact SiSκ and SSκ cubic units. These phases are metallic.

With the increase of pressure, SiS2 undergoes an interesting evolution. The covalent dimensionality changes from 1D to 2D to 3D. The associated high barriers, and the substantial reorganization demanded by the transformation, suggests that the P-3m1 phase may recover back to ambient conditions as a metastable phase, in contradiction with experiments. However, all of the other predicted phases have been proved to be unstable at ambient pressure. Also, the P-3m1

phase is a semiconductor with a band gap of 0.4κ eV at 20 GPa. It metallized around 50 GPa and remains a relatively poor metal up to the 100 GPa.

Finally, according to current knowledge, high-pressure phases of SiS2 do not present a thermodynamically stable pyrite-like structure. 260 Further investigations are underway to complete the following results -including a comparison with Siτ2. τur aim is to present a definitive and clearer picture of SiS2 structural evolution upon pressure.

General conclusion

The Appendix 2. Crystal structures information , , .

Mg N C a a= . , = . = .

Mg . , .

Mg N P a= . , = . = .

Mg . , , . N .

, . , . , . , .

MgN

C

a= . , = . , = . Mg , . , . N .

, .

, .

MgN P / a= . , = . Mg . , . , N , , .

, . , .

MgN P-II a= . , = . = .

, α= β= . , γ= Mg . , .

, . ; N .

, .

MgN P-I a= . , = . = .

, α= . β= . , γ= .

Mg .

, . , . N .

, .

MgN C a= . , = . = .

Mg . , , N .

, .

, .

MgN P-a= . , = . = .

, α= . β= . , γ= . Mg , , . ; N .

, . , . . , .

, .

Metasta le o pou ds

Mg N C a= . , = . , = .

Mg , .

, . , . , . , .

, .

; N , . , . , . , .

Mg N

P-a= . , = . a= . , = . Mg . , . , . , , N .

, . , .

Mg N C a= . , = . = . ,

. ; N . , . , . . , . , .

. , .

, .

Mg N R a= . = .

Mg , , . ,

Mg N I a= . = .

, = .

Mg . , . , . . , . , . . , , .

; N . , . , . . , . , .

. , . , . . , , .

Mg N C / a= . , = . = .

, ; N . , .

, .

BaN P / a= . , = . , = . α= . , β= . ,

BaN P / a= . , = . , = . α= . , β= . , ; N . , . Mo , , ; N , , . , , , .

Ta le A . "t u tu al pa a ete s of the p edi ted ) N at GPa Chapte .

dista es i Å, a gles i °, p essu e i GPa .

Phase Spa e group Latti e para eter Positio

) N P / a= . , = .

) . , . , N . , . , .

) N I a= . , = . , = .

) . , , ; , , N .

, , .

) N C / a= . , = . = .

, β= .

) . , , . ; . , . , . N . , . , . ; , . ,

) N C / a= . , = . = .

, β= .

) . , . , . ; . , . , . N . , .

, ; . , . , . ; , . , .

) N C / a= . , = . = .

, β= .

) . , . , .

; . , , . ; . , , . N . , , . ; . , . , . ; , , . ; , . ,

) N F -a= .

) , , N . , . , .

) N P-a= . , = . , = . , α= . ,β= . , γ= . -.

-. , .

-.

-.

, .

-.

-.

Ta le A . "t u tu al pa a ete s of the p edi ted "i" at sele ted p essu es Chapte .

dista es i Å, a gles i °, p essu e i GPa . Si (0.337, -0.000, 0.500), (0.000, 0.000, 0.000), (0.000, 0.000, 0.500); S (0.326, -0.000, 0.000), (0.166, 0.500, 0.253), (0.500, 0.500, 0.242) -.

Phase

Appendix 3. Energy information

Li CO C / -.

-.

-.

Li CO P / -.

-.

-.

Li CO P / -.

-.

-.

Li CO P / -.

-.

-. In our DFT calculations on Lix(Cτ2)y phases, the PBE functional is used throughout this work. The projector-augmented wave (PAW) method was used for the core-electron representation. After preliminary convergence tests, we used a cutoff energy Ecut= 550 eV for the valence basis set. ab initio (DFT) molecular dynamics simulations were performed with VASP for the periodic supercell of three phases. Starting from each structure at ambient pressure, AIMD calculations were performed at 300 K, 600 K, 900, and 1200 K. The timestep was 1 fs, and the total simulation time was as long as 10 ps. Brillouin zone integration is restricted to the Γ point of the supercell. In our DFT calculations on Mgxσy phases, the PBE functional is used throughout this work.

The projector-augmented wave (PAW) method was used for the core-electron representation.

After preliminary convergence tests, we used a cutoff energy Ecut= 550 eV for the valence basis set.