Résumé

La stratégie la plus utilisée aujourd'hui pour la conception de nouveaux matériaux luminescents est basée sur des méthodes d'essais-erreurs. Cependant, ces méthodes peuvent souvent entraîner une consommation d'argent et de temps. En ce sens, un modèle théorique agissant comme un outil prédictif peut servir comme une stratégie alternative. De tels modèles sont également étudiés et utilisés par des scientifiques du monde entier, mais ils sont pour la plupart difficiles à utiliser. Dans ce travail, deux modèles semi-empiriques conviviaux et faciles à utiliser ont été proposés pour la conception de luminophores intégrés dans le développement de technologies importantes, en particulier dans les éclairages à base de LED et les cellules solaires. Ces modèles sont: 1) le modèle de facteur environnemental (EF) basé sur la théorie diélectrique de la liaison chimique proposée par Philips et 2) le modèle de transfert de charge métal-métal (MMCT) utilisées pour trouver des relations entre les propriétés structurelles d'un matériau et sa luminescence. Le modèle EF a été appliqué à la famille des composés A I B II PO 4 (A I = cation monovalent, B II = cation divalent) dopés à Eu 2+ . Il était capable d'estimer l'énergie du bord d'excitation et d'identifier les sites de dopage à ± 1000 cm -1

. Il peut donc être utilisé pour la conception de nouveaux luminophores appartenant à cette famille. Dans la deuxième partie, les deux modèles ont été utilisés pour identifier la nature de la luminescence dans les oxydes dopés à Bi 3+ . A cet effet, une méthode combinant les modèles motionnés avec les valeurs de Stokes shift a été trouvé fiable. En outre, la a été jugée utile pour expliquer la modification des propriétés luminescentes de YVO 4 :Bi 3+ sous haute pression. Dans la troisième partie, les deux modèles ont été utilisés pour explorer la luminescence des oxydes dopés à Sb 3+ par analogie avec Bi 3+ . Cependant, les résultats n'étaient pas assez bons pour identifier la nature de la luminescence dans ces matériaux. La raison pourrait être liée à la position décentrée du dopant (Sb 3+ ).

Mots Clés : modèles semi-empiriques, propriétés optiques, propriétés structurelles, Eu 2+ , Bi 3+ , Sb 3+ , LEDs.

Abstract

The most used strategy for the design of new luminescent materials today is based on trial-error methods. However, these methods may often result in consummation of money and time. In this sense, a theoretical model acting as a predictive tool can serve as an alternative strategy. Such models are also studied and used by scientists around the world but they are mostly difficult to use. In this work, two friendly and easy to use semi-empirical theoretical models were proposed as a criterion for designing phosphors integrated in the development of important technologies especially in LED-based lightings and solar cells. These models are: 1) the environmental factor model (EF) based on the dielectric theory of chemical bonding proposed by Philips and 2) the metal to metal charge transfer (MMCT) model by Boutinaud that were both used for the purpose of finding relationships between the structural properties of a material and its luminescence. The EF model was applied on the family of A I B II PO 4 (A I = monovalent cation, B II = divalent cation) compounds doped with Eu 2+ . It was able to estimate the excitation edge energy and to identify the doping sites within uncertainty of ± 1000 cm -1

. It can therefore, be used for the design of new phosphors belonging to this family. In a second part, both models were used to identify the nature of luminescence in Bi 3+ -doped oxides. For this purpose, a method combining the mentioned models along with the values of the Stokes shift was found reliable. In addition, the method was found useful to explain the change in luminescent properties of YVO 4 :Bi 3+ under high pressure. In the third part, the two models were used to explore the luminescence of Sb 3+ -doped oxides by analogy with Bi 3+ . However, the results were not good enough to identify the nature of luminescence in these materials. The reason could be related to the off-centered position of the dopant (Sb 3+ ).

Key words: Semi-empirical models, optical properties, structural properties, Eu 2+ , Bi 3+ , Sb 3+ , LEDs.

I would like to thank all people who contributed to this work.

First, I would like to thank my supervisor, the director of this thesis, Philippe BOUTINAUD for all the support he gave me during the last three years and especially at the end when I was writing this manuscript and was under pressure to finish before the deadline. Thank you for believing in my work in the first place and giving me the opportunity to be a member of the ML group at the ICCF Clermont-Ferrand. I extremely appreciate the efforts you did to make this work successful. I cannot forget all the hours you spent with me explaining, correcting or solving problems. Thanks for your care to always make me comfortable without putting any pressures on me. It was really an honor to work with you, I learned a lot from you.

I want to thank the head of the ML group, Genevieve CHADEYRON for her support, her smile and sympathy. Such a lovely person with all the positivity.

I would also like to thank all the members of the ML group and the ICCF especially the PhD students Sihem SEBAI, Mourad Seraiche, Paul Legentil, and the post-doctorants Rachod BOONSNIN, Mohamad KHAYWAH, Yasser Ahmad and Jeff NYALOSASO with whom I shared good moments. It was nice to meet you all.

Special thanks to all the persons who helped me with the experimental part: Rachid MAHIOU, Daniel ZAMBON, Anthony BARROS, Jeremey PEYROUX, Nathalie CAPERAA and Rodolphe THIROUARD.

I would like to thank all my Lebanese friends in Clermont.

I thank my parents and my family for their support and especially my father who was the first to encourage me towards a PhD degree and who supported me financially and emotionally. Thank you for being such a great father! Finally, maybe it was not meant to be without you. Your presence in my life made the last three years much easier. Thanks for all your support that I will never ever forget. In the hard times you were there to give me power and confidence. Thank you for every moment we shared together my partner, my friend and my guardian: Hussein…

General Introduction

Nowadays, luminescent materials, or phosphors, are integrated in a various range of important technologies starting from television screens, computers, mobile phones, lasers, red indicators to X-ray photography, cathode ray tubes and solid state lightings...Despite the huge production of phosphors, a lot of them still present considerable drawbacks especially under the effect of temperature. It is straightforward to improve the quality of these materials by creating new compositions of luminescent materials. This becomes inevitable basically for LED based lighting uses to help obtaining the warm white light. For this purpose broad band emitting phosphors in the visible region and excited in the UV region are of particular interest. However, the spectral range of emission and excitation of such materials is generally strongly dependent on the nature of the dopant and on that of the host lattice. For this reason, they are usually difficult to design. Most of the time, this difficulty is overcomed by preparing as much new materials as possible and study their optical properties. This approach, typically based on combinational chemistry, is time and sometimes matter consuming, with uncertain results.

The second approach is to develop methods that allow predicting the properties and then save money and time. A number of theoretical methods and semi-empirical models have been established in order to explain the properties of luminescent materials and especially to relate these properties to the crystal structure. However, these approaches were either too complicated or lack the ability to be applied in a wide range of compounds.

The purpose of this work is to introduce friendly and easy to use methods helping for the design of band-like emitting compounds in which the dopant is either Eu 2+ or the mercury like ions Bi 3+ and Sb 3+ .

The choice of these dopants is justified as follows: (1) Eu 2+ has a strategic interest for LED-based lighting in many different host lattices, including silicates and phosphates. Its choice, in this context, is obvious.

(2) Bi 3+ is not a rare earth, which is, in the context of LED-based lighting, of economical interest. The luminescence of this ion could be very bright in complex transition metal oxides like for instance YVO 4 .

Here, the broad yellow emission that is obtained, which has a charge transfer character, could eventually replace the Ce 3+ emission in YAG, thus eliminating the need of Ce 3+ . Bi 3+ is also a well-known sensitizer of trivalent lanthanide luminescence, subject that there is a maximum of overlap between the emission of Bi 3+ and the absorption of the trivalent lanthanide. In this connection, the prediction of the spectral distribution of the Bi 3+ features is of great importance. (3) Sb 3+ is a mercury-like cation with s 2 electron configuration, like Bi 3+ , but its luminescence behavior in transition metal oxides is almost unknown. There is an opportunity here to start re-investigating the optical properties of this forgotten activator at the light of our newly developed models.

This manuscript is divided into four chapters:

The first chapter is an introduction that explains luminescence of different phosphors and introduces two semi-empirical models, the environmental factor model (EF) and the metal to metal charge transfer (MMCT) model, to be investigated for being a good compromise between efficiency and ease of use.

The second chapter presents the application of the environmental factor model on the case of Eu 2+ ions incorporated in monophosphate lattices of the type ABPO 4 (A = Li + , Na + , K + ; B =Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ).

Our motivation here is to evaluate the relevancy of the model, try to identify the nature of doping sites in the materials and propose a criterion for the design of ABPO 4 :Eu 2+ phosphors. In the context of the work, the phosphate NaZnPO 4 :Eu 2+ is prepared and its photoluminescence is firstly described.

The third chapter is an investigation of the nature of the luminescence of Bi 3+ in transition metallic oxides containing metal cations that have closed-shell d 0 electron configuration (i.e. titanates, vanadates, niobates, molybdates, etc…). This is achieved on the basis of EF and MMCT models. In the specific case of YVO 4 :Bi 3+ , which has a strategic importance, it appeared that energy levels of different origin are degenerated, which makes their identification impossible by the mean of standard photoluminescence spectroscopy. In this case, high hydrostatic pressure was applied in the range 0-20 GPa in order to check whether this degeneracy could be lifted. In this purpose, our models are refined to take into account the action of external pressure.

The fourth and last chapter presents the preparation and spectral measurements of new Sb 3+ -doped closed-shell transition metal oxides. Again, the EF and MMCT models are used to get better insight in the complex luminescence of these compounds.

Chapter 1: State of the art 1.1. Introduction to luminescence and its applications

Luminescence is the emission of light (electromagnetic radiation) as a result of electronic transitions in a material. It is also known as "cold body radiation" and is in contrast with the phenomenon of black or hot body radiation caused by heating the material at high temperatures for a certain time until it glows (incandescence) (1), (2) . Luminescent materials, also called phosphors, are widely present in our daily lives almost everywhere we go. They can be used in television screens, computers, mobile phones, organic LEDs, inorganic LEDs, lasers, X-ray photography, red indicators, etc….

Luminescence takes place, in principle, when the material absorbs external energy then converts it into photons (1) . According to the source of energy, luminescence can be classified into several types such as cathodoluminescence, electroluminescence, fluorescence, photoluminescence… Luminescent materials vary in the color of radiation they emit and in its intensity. It can be any color of the visible spectrum and even an infrared or ultraviolet radiation.

Luminescence was first found in nature and was thought of as a kind of magical phenomenon. An example is the famous Barite BaSO 4 stone of Bologna discovered in the 1600 which glows under exposure to sunlight and emits light for hours in the darkness. It was considered mysterious for 400 years.

Throughout the years, scientific explanations for this phenomenon had been made especially after the great evolution of material science. People now not only can understand how luminescence works, they can also create nicely color emitting phosphors.

Basic principles of luminescence

A luminescent material is usually created by doping a micron-sized or nano-sized grain of solid, called the host lattice, within some amount of cations (usually a few percent or less), called activators or center ions. The host can be an insulator, a conductor, a glass or a semi-conductor depending on the target of application. The activators used are usually rare earth ions in their divalent or trivalent oxidation states (Eu 2+ ,Eu 3+ ,Yb 2+ ,Dy 3+ ,Ce 3+ …), transition metal cations (Mn 2+ ,Mn 4+ ,Cr 3+ …) and mercury like cations/s 2 ions (Bi 3+ ,Sb 3+ ,Sn 2+ ,Pb 2+ ..). When a luminescent ion is exposed to a source of energy, it will be excited (Figure 1). In most of the cases, the activator absorbs the energy and one of its electrons moves from the ground state to a higher excited level of energy (absorption of energy). Because this excitation level is unstable, the electron returns back to the ground state by losing its energy in the form of photons (emission) (1), (3) . The energy levels of the ion can be derived from Hund's rule and are usually indicated by the general spectroscopic term 2S+1 L J , where S is the total spin, L the total orbital angular momentum and J the total angular momentum of the orbital electrons (4), (5) . L is written as S, P, D, F,G, H, I, for L=0, 1, 2, 3, 4, 5, 6 respectively. The configurational coordinate diagram is a good way to illustrate schematically the details of emission and absorption mechanisms of a luminescent center (Figure 2). It can be defined as a plot of the interatomic distance (R), which is the distance between the center and the neighboring ligand (surrounding anions), versus potential energy (E). It is generally assumed that the ligands are always vibrating back and forth towards the central ion in a symmetrical stretching mode of motion. This vibration, being harmonic, is treated as a quantum harmonic oscillator and thus, the relation between the potential energy E and R (the distance of the ligand from the ion) has a parabolic shape (1), (5) :

𝐸 = 1 2 𝑘𝑅 2 , k is the oscillator constant. ( 1
)
The solution of the Schrödinger Equation in that case that yields the sub levels of energy is:

𝐸 𝜈 = 𝜈 + 1 2 ( 2
)
Where ν is the frequency of the oscillator and h is Plank's constant. (1) Figure 2.Example of a one-coordinate configurational diagram of a luminescent center. The lower (g) and upper (e) parabolas represent the ground and excited states respectively. R 0 and R 0 ' are the equilibrium distances between the center and the ligands in the ground and excited states respectively (4) .

The lower parabola (g) in Figure 2 represents the ground state and the upper one (e) is the first (or lowest-lying) excited state. The horizontal lines in each parabola are the vibrational sub levels. R 0 is the equilibrium position of the distance between the ion and the ligands in the ground state. In the excited state, the luminescent center usually undergoes a change in the chemical bonding with the neighboring anions which results in a new equilibrium interatomic distance R 0 ' (1) .

A transition between the vibrational sub levels of the ground state or those of the excited state does not lead to the emission of light. It is called relaxation, where energy is dissipated as heat to the surroundings (1) . In most cases, excitation is followed by relaxation to lower vibrational levels of the excited state. Emission occurs only after the return of the electron to the ground state. Hence, the excitation and emission energies are not equal as shown in the figure. The difference between the two energies is known as the Stokes shift.

Vertical transitions are electronic or vibronic in nature and are responsible for the intensity of the signal. Horizontal movements, however, are nuclear because they are related to the interatomic distance. Both types of movements are combined to define the type of signal that will be displayed after excitation or emission. The probability of having an optical transition is proportional to:

𝑒 𝑟 𝑔 𝜒 𝑛 𝜒 (3)
Where r is the electronic dipole operator and 𝜒 𝑛 and 𝜒 are the vibrational wave functions of the excited state and the ground state, respectively. The first term of Equation ( 3) represents the electronic element and the second one is called the vibrational overlap.

In the first vibrational level, the wavefunction is maximum at R 0 , whereas in higher vibrational levels the wavefunction is maximum at the edges of the parabola (Figure 3). This means that the transition starting from R 0 has the maximum intensity of the signal. When the two parabolas lie exactly above each other, that is when R = R 0 , the energy of this transition dominates the spectrum and appears as a sharp line. It is called the zero-phonon transition. In the other case where R is different from R 0 , other energies could be possible and the signal appears as a broad band (Figure 4). The vibrational overlap is then responsible for the shape of the band. A broad band is always accompanied by a large Stokes shift. That can be confirmed by this Equation:

𝛥𝐸 𝑆ℏ𝜔 (4)
Where ΔE is the Stokes shift, S is the Huang-Rhys parameter that measures the dopant-lattice (electron-phonon) interactions and ℏω is the energy difference between the vibrational levels. S is proportional to:

𝑘 𝑅 ′ -𝑅 ℏ𝜔 (5)
The difference between R 0 and R 0 ' (ΔR) is a very important parameter that reflects the interactions and couplings of the ion with its ligands and informs us about the type of signal expected to be seen.

The above information can give three types of couplings:

 Weak coupling (ΔR=0) → small Stokes shift and very narrow band (sharp line).

 Intermediate coupling (ΔR>0) → large Stokes shift and broad band.

 Strong coupling (ΔR>>0) → very large Stokes shift and very broad band.

Materials that show sharp lines will be out of the context of this work because they present atomic-like luminescence that does not change much according to the crystal structure. Instead, the interest will be focused on broad band absorbing and emitting materials whose spectral features are more difficult to predict.

Non-radiative transitions from the excited to the ground state are also possible. In that case, the absorbed energy is partially or completely transformed into heat through the crystal. This is quenching (6) . It usually happens when the material is exposed to high temperatures. It may also occur after energy transfer to a luminescent quencher (for example a defect) in case of high concentration of the activator.

From the configurational diagram viewpoint, non-radiative return to the ground state takes place when the electron reaches the intersection of the two parabolas. In that case, the electron moves to a vibrational level of the ground state that is nearly resonant with that occupied by the electron in the excited state (intersystem crossing) (Figure 5).

Figure 5.Configuration coordinate diagram showing the difference between radiative and non-radiative returns to the ground state. The radiative transition is represented by the red arrow (Em). The non-radiative process is described by the black arrows (intersystem crossing).

This process is almost impossible to occur when the the excited state (e) and the ground state (g) are parallel (ΔR=0) (Figure 6). However, another form of non-radiative process exists in that case, in particular when the excitation ends at a higher excited state (e') which is not parallel to the ground state (7) . The system then relaxes to the first excited state, from where the emission takes place. This is called internal relaxation. The ratio of non-radiative to radiative transitions determines the efficiency of a luminescent material. Therefore, it consitutes one of the most important challenges for the design of phophors. In optical transitions, there are some restrictions that define allowed and forbidden transitions. They are called selection rules that limit the transitions to levels having the same spin and different parity (Laporte rule). For example, d-d and f-f transitions are parity forbidden (8) . However, sometimes these rules can be broken with the help of physical interactions like spin-orbit coupling and electron-vibration coupling. Here it is said that the selection rules are "relaxed". This affects strongly the intensity of the signal.

The activator is in most cases responsible for absorption and emission. In the configurational coordinate diagram, the activator and its neighboring sites are viewed as an isolated molecule, while in reality they are not deprived from the whole crystal. In other words, transitions to and from the energy levels of the host are possible and can have important role in the luminescence process. In fact, there are hosts that do not even need an activator to generate luminescence. Those are called intrinsic or selfactivated materials like for instance CaWO 4 that was used in X-rays for years and is considered one of the most efficient phosphors (1), (9) . Here, the WO 4 2-group acts as the activator. This is in general the case of all materials with closed shell transition metal complexes such as the titanates (TiO 4 4-, TiO 6 8-, TiO and tantalates (TaO 6 7-, TaO 8 11-) (10) . These materials are characterized by broad band emissions usually in the visible region that are, most of the time, intense only at low temperature. Emission resulting from such a mechanism is called charge transfer CT luminescence. In this case, an electron from the oxygen atoms is transferred to the metal (i. e. ligand to metal charge transfer) in the complex unit to form a self trapped exciton which is responsible for luminescence upon electron-hole recombination. In doped lattices, a transition is called metal to metal charge transfer (MMCT) when the electron is transferred from the activator to the metal (or cation) in the host lattice. This was reported in a number of transition metal oxides doped with Bi 3+ , which will be an important subject of this thesis. The occurrence of MMCT depends on the tendency of the dopant to be oxidized and the lattice metal to be reduced.

The states of energy of a host are defined as bands. Since the host (a solid) is made up of a number of atoms, the energy levels of atoms can be grouped into two bands: the valence band (VB) where the majority of electrons are located and the conduction band (CB) where most of the holes are found (Figure 7). The difference between the two bands is called the band gap or the forbidden zone where no charge carriers should be found. This is true if we consider the crystal to be perfect, but in reality defects are always found in the structures of crystals. When the host is excited, an electron from the valence band moves to the conduction band leaving a hole in the valence band. Emission occurs as a result of electron-hole recombination. This happens when an electron from the CB drops to combine with the hole of the VB. However, this is not always the case because the recombination may place take inside the band gap at energy levels induced by defects. Energy states related to activators, defects and impurities can exist inside the band gap and sometimes higher in the conduction band. They are known as traps because they can trap (capture) electrons or holes and cause quenching of the host's luminescence (11) . Levels located close to the conduction band or the valence band are known as shallow traps. Deep traps also exist and are more likely to cause quenching because they need higher energies to move an electron or a hole from the trap to the conduction or valence band. Knowing the location of the states of the activator or impurity with respect to the band gap energy is essential for the analysis of optical spectra.

Following this scheme, an electron may be excited from the ground state of an activator into the conduction band where it is moved to the host lattice during relaxation. This is known as photoionization. In that case, the electron may either delocalize in the conduction band or remain in Coulomb interaction with the center to form a trapped exciton, i.e. an electron-hole combination which can emit when the electron recombines with the hole. This changes the charge distribution on the center and affects the chemical bonding so the excited charge transfer state has a different equilibrium position and hence, broad bands are to be expected. In case of delocalization, the electron moves in the conduction band and be trapped at some distance by a defect. In that case, there is no emission but an afterglow may be observed (a delayed radiative recombination of the electron and hole), also called long lasting luminescence. Here, the recombination takes places once the electron is de-trapped by the effect of thermal or photonic stimulation.

In some hosts, the activator cannot be easily excited. An additional ion (called sensitizer) is then introduced into the host to assist to the excitation of the activator. The sensitizer S absorbs the energy then transfers it to the activator A following the scheme: S*+ A S + A*. The star indicates the excited ion.

Two conditions are necessary for this energy transfer to be possible:

 The ground states and excited states of S and A should be equal. An overlap between the emission band of S and the absorption band of A (Figure 8).

 The interactions between the centers depend on the distance between the two ions.

This can be seen in the Equation proposed by Dexter of the transfer rate P SA :

𝑃 𝑆𝐴 𝜋 ℏ 𝑆 𝐴 * 𝐻 𝑆𝐴 𝑆 * 𝐴 𝑔 𝑆 𝐸 𝑔 𝐴 𝐸 𝑑𝐸 (6)
The integral represents the spectral overlap between the two centers and the Hamiltonian H SA is the interaction. For instance, in the halophosphate Ca 10 (PO 4 ) 6 (F,Cl):Sb 3+ ,Mn 2+ that has been used in lamps for decades (12) , Mn 2+ acts as a yellow activator and Sb 3+ acts both as a blue activator and as a sensitizer for the Mn 2+ luminescence. Energy transfer is also possible when S=A (energy transfer between identical centers). In this case, energy can migrate within the host lattice depending on the concentration of the dopant. This could lead to concentration quenching if the excitation energy is transferred to a sink. Energy migration is also observed in the previously mentioned transition metal oxides when doped with rare earth or s 2 ions, where the host acts as a sensitizer. As an example, in YVO 4 :Eu 3+ , electrons of the tetrahedral (VO 4 ) 3-units are first excited, migration occurs at some distance and then energy is transferred to the Eu 3+ ion which is responsible for the emission.

Effect of crystal structure on luminescence

As stated already, many cations of the periodic table can be used for the creation of luminescent materials. A number of famous series of phosphors including rare earth ions, transition metal and mercury-like cations have been used through the years. Emission energies ranging between infra red to ultra violet can be seen under excitation of luminescent materials. The cause of this diversity from one material to another is the choice of the central ion on one hand, and the crystal structure of the host lattice on another hand. It is quite obvious that doping different ions in the same host will result in different optical characteristics because they have different energy level configurations. Moreover, combining the same ion in different hosts will not necessarily give similar optical results. When incorporated in the host, the center undergoes some perturbations and splitting of its energy levels.

Some of them are caused by the crystal field which is defined as the electric field on a cation site created due to the presence of neighboring anions, so it strongly depends on the oxidation state of the cation and the type of coordination (number and nature of ligands).

The crystal field mainly acts on the external orbitals. The d orbital is, for instance, five-fold degenerated. In the free ion, the five sub levels have the same energy. When subjected to the crystal field, these levels are perturbed and take different values of energy. The magnitude of the crystal field is different from one crystal to another depending on the type and symmetry of the ligands. Figure 9 shows how the d orbitals split under the effect of crystal field (Δ) in different symmetries. In octahedral or octahedrally-related point symmetry, orbitals of the d levels that point toward the ligands are twofold (e g ) while those that project between the ligands are three fold (t 2g ). In octahedral coordination, the crystal field is considered very strong. The shift of the e g orbitals is 60% upward of the magnitude of the crystal field and 40% downward for t 2g . The crystal field strength is also known as 10 Dq, where D and q are parameters depending on the charge of the ligands and the interatomic distance. The strength of the crystal field increases according to the following sequence:

Tetrahedral (4 fold) < dodecahedral (12 fold) < cubic (8 fold) < octahedral (6 fold). (

) 13
The magnitude of the crystal field is affected by several factors. The interatomic distance is very important, a small decrease in the interatomic distance leads to a large increase in the crystal field. The valence of the cation also plays a role, for example, trivalent cations encounter a stronger crystal field than divalent. Other factors like temperature and pressure can also affect this magnitude.

The splitting of d levels caused by the crystal field always takes place without changing the position of their barycenter of energy (Ec). However, another phenomenon intervenes in the perturbation of the d levels altering the position of the Ec to lower energies caused by covalency. This is known as the nephelauxetic effect. The interactions between electrons are reduced when the covalency is strong because they are spread out over wider orbitals and this causes the shift of the levels to lower energies (1) . As shown by Jorgensen, the nephelauxetic parameter β depends mainly on the electronegativity of the anion:

𝛽 = 𝐵(𝑐𝑜𝑚𝑝𝑙𝑒𝑥)/𝐵 𝑙𝑖𝑔𝑎𝑛𝑑 ( 7
)
Where B is the Racah parameter. This means that the most electronegative ion has the lowest nephelauxetic effect. This effect increases in the following order:

F -< O 2-< Cl -< Br -< I -< S 2-< Se 2-< Te 2-
Another important aspect is the Jahn-Teller effect. It is the case especially where the radius of the dopant is very small compared to that of the cation it is replacing. This will lead to a distortion of the site and to removing the degeneracy of energy levels. It is said that the ion goes "off center". This is familiar in s 2 ions as will be seen later.

Spectroscopy of important luminescent activators

4f elements

Rare earth ions are widely used in the production of luminescent materials especially lanthanides.

These elements have an incompletely filled 4f shell (4f elements). The intra-4f n transitions are forbidden by the selection rules but can be relaxed by several couplings like quadrupole interactions, electric dipole interactions and magnetic dipole interaction. Trivalent lanthanides Ln 3+ (Ln = Eu, Dy, Yb, Sm….) usually show sharp lines, but also intense, in their spectral behavior attributed to f-f transitions (14) . The reason is that their 4f shell is shielded from the environment by the filled 5s 2 and 5p 6 levels, and therefore, the effect of the host lattice is very weak on these ions and their spectra are similar to those of the free ion (15) . This can be seen in the Dieke diagram that shows the energy levels of trivalent lanthanides in LaF 3 . The horizontal lines represent the magnitude of the crystal field splitting (16) .

The property of sharp lines of 4f n in rare earth ions is very important for obtaining bright red, blue or green colors. This advantage has played a big role for the design of luminescent materials in different fields of application such as the red emitting YVO 4 :Eu 3+ (17) or Y 2 O 2 S:Eu 3+ in televisions and lamps, in addition to the blue colored Sr 3 (PO 4 ) 5 Cl:Eu 2+ and green colored GdMgB 5 O 10 :Ce 3+ ,Tb 3+ also used in lamps (1) . However, as stated before, our interest is in ions that show broad band spectra. This is the case of the lanthanides that show 4f n to 4f n-1 5d 1 transitions. These are mostly divalent Ln 2+ (Ln = Eu, Yb, Sm….) in addition to some trivalent Ln 3+ like Ce 3+ that tend to be oxidized (18) . Ce 3+ has the simplest excitation spectrum (one electron in its outer shell). It has especially been used to understand the perturbations of the lanthanide energy levels in a crystal and to study the relation of 4f n to 4f n-1 5d transitions with the crystal structure (19) . Analogies are often made with the case of Eu 2+ (20), (21) . Figure 10 shows that the excited state (4f n-1 5d) is shifted with respect to the ground state (4f n ) which is the reason of broad band formation and presence of Stokes shift in these ions (22) . This is due to the fact that the 5d levels of a lanthanide are very sensitive to the crystal environment. Figure 11 shows the 5d energy levels of Eu 2+ /Ce 3+ in a cubic crystal (23) . In addition to the crystal field splitting, the barycenter position (Ec) of the 5d levels is lowered by the neuphlexautic effect.

The Ec energies and the 10 Dq of Eu 2+ in a series of different hosts having cubic structures are compiled in Table 1. Both parameters show significant variations from one host to another. In the series of sodium halides, the energy Ec is minimum in NaI (31950 cm -1 ) and maximum in NaF (38010 cm -1 ). Knowing that the energy of Ec in free Eu 2+ is 46340 cm -1 , the lowering in the Ec is less in NaF than NaI which obeys the nephelauxetic order. The difference in Ec and CFS values explains the diversity of colors that Eu 2+ can generate. Table 1 Values of the barycenter energy Ec of 5d levels and the crystal field splitting 10Dq of Eu 3+ in different cubic hosts.

(

) 24
Host Ec (10 3 cm -1 ) CFS 10Dq (10 The nephelauxetic effect and crystal field are known to act independently of each other (25) .

However, both phenomena combine to create the so called red shift of a lanthanide in a solid. It is indicated as D parameter in Figure 11. The red shift is defined as the difference between the f-d energy of a lanthanide of charge Q as a free ion and that of the same lanthanide in a crystal A (26) :

𝐷 𝑄, 𝐴 = 𝐸 𝑓𝑑 𝑛, 𝑄, 𝑓𝑟𝑒𝑒 -𝐸 𝑓𝑑 𝑛, 𝑄, 𝐴

According to the work of Dorenbos, the red shift is the same for all divalent lanthanides in the same crystal on one condition that they substitute for the same cation site (27) . This means that if we know the transition energy of one divalent lanthanide, it is possible to know the energies of all the others because the f-d energies of the free ions are already known. The explanation of this model can be illustrated in Figure 12 that shows a representative scheme of the locations of 5d and 4f energy levels of divalent lanthanides with respect to the conduction and valence bands of the CaF 2 host (28) . The abscissa indicates the number of electrons in the 4f shell. The energies of 5d levels are considered constant for all divalent lanthanides. The 4f energies, in contrast, show a characteristic pattern. The same pattern was proved to exist for a huge number of hosts as well as in the gaseous state (free ion). With the help of this pattern, if we know the energy E f-d (n,Q,A) in Equation ( 8) for one lanthanide in a solid we can predict the energies f-d of all the lanthanides in this solid. .

For trivalent lanthanides, the red shift model is also true (Figure 13) (29), (30) . The red horizontal line represents the 5d state of Ln 2+ . As seen in the figure, the f-d energy is usually larger in trivalent lanthanides as compared to the divalent. In fact, the wavelength of the 4f n to 4f n-1 5d transition is located far in the ultraviolet UV region in trivalent lanthanides. In divalent lanthanides, however, this transition shifts to lower energies and is located in the UV-visible region. The 4f n energies are lowest for Ln 2+ with half filled orbitals (Eu 2+ ) and completely filled 4f orbitals (Yb 2+ ), and highest for those with one electron (La 2+ ) and eight electrons (Gd 2+ ).

This work will deal precisely with Eu 2+ that shows near UV absorption and visible emission. It is the most stable among divalent lanthanides and is widely studied (31) . .

Luminescence as a result of the 4f n-1 5d to 4f n transition in lanthanides exhibits intense emission.

Examples are the blue emitting Ca 2 PO 4 Cl:Eu 2+ and the green emitting Sr(SCN) 2 :Eu 2+ . Ce 3+ also shows visible emission as the case of the famous YAG:Ce 3+ that emits in the yellow region and is used in solid state lightings (32) .

Rare earth ions were also reported to act as sentitizers to other rare earth activators or to transition metal ions as in BaMgAl 10 O 17 :Eu 2+ ,Mn 2+ . Ln 3+ show transitions with charge transfer character especially for these that tend to become divalent like Eu 3+ and Yb 3+ where an electron is transferred from the ligand to the center (LMCT) (33)-(35) . This was rarely noticed in the optical behavior of Ln 2+ . On the other hand, energy transfer between Ln 2+ identical ions is possible under the effect of concentration quenching.

Transition metal ions

Transition metals like Mn 2+ and Cr 3+ are known for their broad emissions in a number of hosts, for example, Zn 2 SiO 4 :Mn 2+ that emits a band of red to green colors and was used in lamps and cathode ray tubes. These ions have an incompletely filled d shell, having for example, d 1 ,d (36) for an octahedral or tetrahedral symmetry that show the effect of the crystal field on the perturbation of d n configurations. This results from the fact that d n levels are the outermost energy shells and are in direct contact with the ligands (surrounding anions), and this explains why transition metals doped materials usually have broad band absorption and emission spectra except Mn 4+ and Cr 3+doped materials in high crystal field, that are characterized by sharp spectral lines (37) .

Mercury like ions (s 2 )

Post-transition metal ions (Tl + , Sb 3+ ,Pb 2+ ,Bi 3+ ,Sn 2+ ) have also been used as luminescent centers for decades. The spectroscopy of these ions was investigated in the 60-70's, mostly to develop phosphors for lamps, and then almost forgotten. The last past years have evidenced a renewal of interest on s 2 mercury-like activators like Bi 3+ for their efficient luminescence action in the UV or visible region and for their aptitude to serve as sensitizers for the trivalent lanthanide emission. The so-obtained phosphors show interesting properties for a possible use in LED-based lighting devices or for integration in solar cells, for instance YVO 4 :Bi 3+ ,Eu 3+ (38) .

The s 2 configuration generates an intense parity allowed s 2 to sp absorption in the ultraviolet leading to a wide range of visible emission. Most s 2 ions are characterized by their broad band spectra and their considerably very large Stokes shifts especially in Sb 3+ -doped materials. Their optical behavior is considered to be strongly affected by the type of crystal structure (case of strong coupling with the host). However, the influence of crystal field on these ions is not widely studied as for rare earth and transition metals. Figure 14 shows the energy levels of s 2 free ions. The ground state is 1 S 0 and the excited states resulting from spin orbit coupling are the triplets 3 P 0 , 3 P 1 , 3 P 2 , and the singlet 1 P 1 . These states generate 3 possible transitions:

 1 S 0 to 3 P 1 called the A band  1 S 0 to 3 P 2 called the B band  1 S 0 to 1 P 1 called the C band

The transition to 3 P 0 is strongly forbidden. The B band is however a forbidden transition but can be relaxed by coupling with unsymmetrical lattice vibrations. It has however a very low intensity. The C band is an allowed transition and A becomes allowed by the spin orbit coupling. 3 P 1 and the underlying level 3 P 0 are usually the emitting levels with an emission ratio largely depending on temperature. They lead to a wide range of radiations from UV to visible. Table 2

presents indicative Stokes shift values of

Bi 3+ in various crystals. The variation is of one order of magnitude which is considered large. The reason, although not always clear, is (at least for a part) related to the amount of the space available for Bi 3+ in the lattice and the fact that s 2 lone pairs induce a stereochemical activity. When incorporated in a crystal, the s 2 ion is usually supposed to go off center in its ground state to obtain the preferred asymmetrical coordination. This is referred to as a pseudo Jahn-Teller effect. In the excited state, the ion relaxes to the center of the coordination polyhedron which results in a large Stokes shift. This phenomenon is observed in the spectra of most of s 2 -doped compounds except in those having six coordination structures, where the s 2 ion is too large and cannot be relaxed to a different equilibrium distance because there is no enough space. Here a narrow Stokes shift is expected, which is the case of Cs 2 NaYCl 6 :Bi 3+ . The large Stokes shift of Bi 3+ -doped compounds was also found to be related to the trap depth, an aspect that is not well understood. The 3 P excited state is subject to both Jahn-Teller and spin orbit couplings. In structures where the spin orbit coupling is strong the Jahn-Teller effect is supposed to decrease. In addition to the three bands shown in Figure 5, a D band also appears in the excitation and emission spectra of Bi 3+ -doped compounds especially in oxides with d 0 or d 10 metal cations. The energy of the D-like excitation band is usually close to that of the A band, while the emission, owing to a larger Stokes shift, is usually located at lower energy relative to the A-like emission, i.e. in the visible region of the spectrum. Different explanations on the origin of the D band have been made. It was previously ascribed to a ligand to metal charge transfer. Although recent studies suggest two other hypotheses:

 The D band is a result of the formation of Bi 3+ pairs or clusters. After excitation, an electron is transferred from one Bi 3+ to another nearby Bi 3+ within an intervalence charge transfer (IVCT) process. This creates Bi 2+ and Bi 4+ ions (39) .

 The D band is a result of metal to metal charge transfer (MMCT) from Bi 3+ to the metal M n+ in the host. The transition is expressed as Bi 3+ (6s 2 )/M n+ (d 0 ) → Bi 4+ (6s 1 )/M (n-1)+ (d 1 ) or Bi 3+ (6s 2 )/M n+ (d 10 ) → Bi 4+ (6s 1 )/M (n-1)+ (s 1 ). This is also viewed as an impurity trapped exciton (bound of electron hole pair) with the hole being localized on Bi 3+ and the electron delocalized on the nearest cationic neighbors.

The first hypothesis was confirmed in a number of hosts by the work of Srivastava, for instance (Y,Gd) 3 Ga 5 O 12 :Bi 3+ and GdAlO 3 :Bi 3+ , where the luminescence was shown to be strongly dependent on the concentration of Bi 3+ ions (39) . After Dorenbos, for an electron to move from Bi 3+ to Bi 2+ , the ground state of Bi 2+ should be located lower than the3 P 1 state of Bi 3+ as shown in Figure 15 represented by the red line. This causes the quenching of the A band (40) . This model, however, supposes that one Bi 3+ acts as an electron donor while the other Bi 3+ acts as an electron acceptor within the pair. This would lead to consider that both ions in the pair do not have the same electronegativity. The reason for that is at the moment somewhat unclear for us. .

The MMCT, being the transition of photoionization of Bi 3+ , seems to have higher energies than the transition from Bi 3+ to Bi 2+ . However, the MMCT model, proposed by Boutinaud suggested that this energy is also located lower than 3 P states in a number of oxides like in the case of YVO 4 :Bi 3+ , where the emission spectrum shows a broad visible band related to the D state. As a matter of fact, three configurations can be found for Bi 3+ states in oxides (Figure 16):  The MMCT state is located lower than the 3 P 0,1 states. The MMCT state is directly excited and is emitting. Excitation in the 3 P 0,1 states can be followed by the autoionization of Bi 3+ , depending on the coupling strength between 3 P 1,0 and the conduction band states. In the case of photoionization, the absorbed energy can be lost nonradiatively or transferred to the MMCT state, with subsequent emission from this latter state (Figure 16.a).  3 P 0,1 and MMCT energies are equal. This means that the 3 P 1,0 states are located close to the bottom of the conduction band. Here, both MMCT and 3 P 1,0 emit with different intensity ratios depending on the temperature (Figure 16.b). This was supposed to be the case of YPO 4 :Bi 3+ in ref. (41). At low temperatures, this compound emits a narrow band upon excitation at 246 nm as well as the broad band of the MMCT state extending from 280 to 450 nm. At higher temperatures the D band becomes more intense relative to the A band (Figure 17). However, in other studies, the second emission was ascribed to Bi 3+ pairs (43) . This question is therefore still open. .

 MMCT is higher than 3 P 0,1 , like in the case of La 2 Zr 2 O 7 :Bi 3+ (44) . Emission from the 3 P 1,0 states follows a direct 1 S 0 → 3 P 1 transition. Excitation of the MMCT state is still possible and can be followed either by MMCT emission or by 3 P 1,0 → 1 S 0 depending on temperature. (Figure 16.c).

Sb 3+ also shows similar properties as those of Bi 3+ . However, it is much less studied due to its relatively unusual large Stokes shift that causes stray quenching of the emission at room temperature.

The halophosphate Ca 10 (PO 4 ) 6 (F,Cl):Sb 3+ :Mn 2+ (45) that shows intense blue emission from Sb 3+ at room temperature despite a Stokes shift as large as 19000 cm -1 possesses, in this connection, an exceptional behavior that is still not understood today. In Sb 3+ -doped compounds the JT effect is more important than Bi 3+ -doped compounds because Sb 3+ has a smaller radius. For instance in YPO 4 :Sb 3+ , two emissions from the 3 P excited state are observed. This is typically due to the JT effect which induces two minima on the 3 P 1 state. This behavior is not observed in YPO 4 :Bi 3+ [START_REF] Grafmeyer | Luminescence properties of Sb 3+ in Yittrium phosphates[END_REF] .

LED-based lighting materials

Luminescent materials are integrated in solid state lighting packages especially in LEDs (light emitting diodes). Figure 18 shows the structure of a LED lamp. The LED chip is made up of two semiconductors, one of n-type (that has more electrons) and the other of p-type (that has more holes). When a voltage is applied through the p-n junction, electrons from the n-type recombine with holes from the p-type resulting in light emission. In addition to indoor and outdoor lightings, LEDs are used as indicators in electronic devices, calculators ,clocks, DVD players, traffic signs…Early LEDs were mainly red or infra-red emitting materials and were used in the 1960's as replacements for incandescent and neon lamps in electronic indicators.

In the 1970's blue LED's appeared made up of GaAs and GaN but they were not very bright until 1990's when the first InGaN was used. The evolution of LEDs continued to improve until white LED was produced.

The production of LED lightings is still widely open and there are many concerns, worldwide, for the development of these materials. These new lighting products are gradually entering our homes and offices forcing the incandescent lamps out of the lighting markets. It is expected that the number of consumers of LED lights will increase in the following years. This is due to LEDs being considered friends with the environment, having longer lifetimes up to 60,000 hours compared to incandescent lamps (1000 hours) and being able to save energy at least 75% less energy than other type of lightings. They also emit less infra red radiation so their surface is not hot. However, they can be affected by heat inside the semiconductor originating from the electric current, which can reduce the light output. Other disadvantages for the usage of LED lamps are their price that is still considerably higher than that of incandescent and halogen lamps. The brightness of these devices has been taken into consideration because they produce less lumens/W than incandescent lamps. That's why there is an urge now to develop this kind of technology. The aim is to find new compositions of luminescent materials in a faster and cheaper manner than trial-error -based methods and that can lead to more efficient, brighter and lower price phosphors especially for the improvement of white LED lightings. Depending on the purpose of application of the white light, four different strategies are available (Figure 19):

 Three RGB (red, green, blue) LED chips. The three colors emitted from the three different LEDs are mixed together to generate a combined spectrum that appears white to the eye.

 One blue chip with one yellow chip.

 One blue chip with one phosphor. The phosphor absorbs the energy emitted from a blue LED and emits a yellow light. Combined blue and yellow give white light. The color temperature can be turned from cold to warm white by adding a red component to the spectrum (i.e. a second phosphor).

 One UV chip with three phosphors. Each of the three phosphors can be excited by UV coming from the LED. One red, one green and one blue emitting phosphor should be used.

The third method is, for the moment, the most attractive one because blue based LEDs for white light have better luminous efficiency. The most used system is blue InGaN (460 nm) or GaN mixed with Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ) [START_REF] Ronda | Challenges in Application of Luminescent Materials[END_REF] . This has a high efficiency but a low color rendering index (CRI) and gives cool white light due to the lack of a red component. As a matter of fact, all phosphors used to combine with the blue LED to produce warm white light result either in low CRI or low efficacy. The color rendering index is defined as the ability of the system to produce light similar to the natural light (daylight). This cannot be easily reached with blue-based LEDs because of the high intensity of blue color. In this sense, considerations of the UV-based LEDs are now taken into account by using near UV InGaN based LEDs hoping to create warm white light with better efficiency and color rendering index [START_REF] Zhang | Tunable luminescence in Ce 3+ , Mn 2+ -codoped calcium fluorapatite through combining emissions and modulation of excitation: a novel strategy to white light emission[END_REF] . In this context, the development of white LEDs is centered on the improvement of phosphors absorbing in the near-UV region and emitting broad band visible light. It is assumed that this type of systems for white light will be dominant in the coming years.

Semi-empirical models

Importance of semi-empirical models

In reference to the studied behaviors of the different luminescent materials, not a lot of anticipation can be done when creating new phosphors. Only after synthesis and spectroscopy could we know the optical properties and the importance of the new produced compounds. An approach to favor the development of luminescent materials is to test as much as possible of new compositions. Looking through the periodic table, a plenty of compounds can be created. In this sense, some people use trialerror methods like for instance, combinatorial screening chemistry that allows the synthesis of thousands of compounds using a single process. However, another approach for the mission is to find and develop methods that could serve as predictive tools for the production of phosphors. In this way we can save a lot of time and work…

Overview of some theoretical models

A number of theoretical models were investigated to understand the structure-luminescence relationships. Some scientists used density functional theory (DFT) ab initio calculations and others used semi-empirical models ranging from very easy approaches to very complicated ones. For instance, only one structural parameter such as the radius (or size) of the ion or the distance between ion and ligands, may be used to establish relations between the optical properties and the structure of the material. On the other hand, models involving a series of structural parameters are also found in the literature like Judd-Ofelt theory [START_REF] Judd | Optical Absorption Intensities of Rare-Earth Ions[END_REF], [START_REF] Ofelt | Intensities of Crystal Spectra of Rare-Earth Ions[END_REF] or Dorenbos polarizable ligand model [START_REF] Dorenbos | Relating the energy of the [Xe]5d 1 configuration of Ce 3+ in inorganic compounds with anion polarizability and cation electronegativity[END_REF] . Although none of these models can be considered perfectly applicable for the design of phosphors, some of them have shown interesting aspects that may form a good base for better explanations of luminescence-structure complexes.

Dorenbos found a correlation between the energy shift of 5d configuration of Ce 3+ with a parameter called the spectroscopic polarizability related to the polarization of the ligands in oxides, and a parameter called the average cation electronegativity in Fluorides. Judd and Ofelt succeeded in finding a relation between structural parameters and spectral intensities especially of rare earth ions in solids.

The environmental factor (EF) model and the MMCT model, that we wish to introduce and test in the present manuscript, are two recent semi-empirical models that we found friendly and easy using.

The Environmental factor (EF) model

The environmental factor model, developed by Shi and Zhang (24) , is based on the dielectric theory of chemical bonding proposed by Pauling and developed by Philips, Van Vechten, and Levine ( 53)- [START_REF] Levine | d-Electron Effects on Bond Susceptibilities and Ionicities[END_REF] . This theory has been successfully applied to a wide variety of physical problems before being extended to phosphors as a possible designing tool. The concept of the EF model is to decompose the crystal lattice into binary units, each representing one type of cation-anion bond, then treat each of them as one separate crystal. This enables us to know the physical properties of every single bond in the whole crystal, and thus, study the environmental factors of each cation site available for the dopant to occupy in the crystal.

Going through the calculations, let us take as an example the monophosphate of formula ABPO 4 , where A us a monovalent cation and B a divalent cation, to show how binary crystals can be obtained. = 𝑜. In the case of ABPO 4 , for instance, we would have a=b=p=1 and o=4. In these expressions, m i (A), m i (B), m i (P) and m i (O) are the multiplicity of the different Wyckoff sites (i) occupied by atoms A, B, P and O, respectively and Z is the number of formula units per unit cell. The compound will then be expressed as a sum of oxo-units:

Such compounds will be investigated in

A(i) ) ( ) ( ) ( i A i A i O i N N a  O(i) ) ( ) ( ) ( i O i O i A i N N o  , B(i) ) ( ) ( ) ( i B i B i O i N N b  O(i) ) ( ) ( ) ( i O i O i B i N N o  and P(i) ) ( ) ( ) ( i P i P i O i N N p  O(i) ) ( ) ( ) ( i O i O i P i N N o  , where N A(i)-O(i) , N B(i)-O(i)

and N P(i)-O(i) indicate the number of oxygen atoms O(i) surrounding A(i), B(i) and P(i), respectively, while N O(i)-A(i) , N O(i)-B(i) and N O(i)-P(i) indicate the number of cations A(i), B(i) and P(i)

surrounding oxygen O(i), respectively. N A(i) , N B(i) and N O(i) are the coordination numbers of the considered atoms in the lattice. This information requires perfect knowledge of the crystal structure of the compound. In practice, they are extracted from ICSD databases and treated using VESTA software [START_REF] Momma | VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis[END_REF] .

For ease of use, matrices containing all the necessary information are settled. As an illustration, we show in Table 3 the matrix of an ABPO 4 lattice in which one crystallographic site is present for A, B and P (i.e. i=1) and two sites are present for the oxygen atoms (i.e. i=2):

Table 3 Example of coordination matrix of an ABPO 4 lattice with one cation site A(1), B(1), P(1) and two oxygen sites O(1), O(2).

A(1)

B( 1) P( 1) O( 1) O( 2)

O(1) N A(1)-O(1) N B(1)-O(1) N P(1)-O(1) O(2) N A(1)-O(2) N B(1)-O(2) N P(1)-O(2) A(1) N O(1)-A(1) N O(2)-A(1) B(1) N O(1)-B(1) N O(2)-B(1) P(1) N O(1)-P(1) N O(2)-P(1) N Total N A(1)-O(1) + N A(1)-O(2) N B(1)-O(1) + N B(1)-O(2) N P(1)-O(1) + N P(1)-O(2) N O(1)-A(1) +N O(1)-B(1) + N O(1)-P(1) N O(2)-A(1) +N O(2)-B(1) + N O(2)-P(1)
The matrices of all the investigated lattices in this work are given in Appendix B. Taking the example of KSrPO 4 , that contains atoms K(1) in 4c Wyckoff site, Sr(1) in 4c Wyckoff site, O(1) in 4c Wyckoff site, O(2) atom in 8d Wyckoff site and O(3) in 4c Wyckoff site, and knowing that Z=4, we write KSrPO 4 as

K(1) 1 Sr(1) 1 P(1) 1 O(1) 1 O(2) 2 O(3) 1 .
The corresponding matrix provided by the crystal structure of this compound is given in Table 4. This matrix gives information on the coordination numbers of each atom.

Reading the matrix gives the following:

N K(1) = 10, N Sr(1) = 9, N P(1) = 4, N O(1) = 4, N O(2) = 5, N O(3) = 4, N K(1)-O(1) = 3, N K(1)-O(2) = 6, N K(1)-O(3) = 1, N Sr(1)-O(1) = 2, N Sr(1)-O(2) = 4,N Sr(1)-O(3) = 3, N P(1)-O(1) = 1, N P(1)-O(2) = 2,N P(1)-O(3) = 1, N O(1)- K(1) = 1, N O(2)-K(1) = 2, N O(3)-K(1) = 1, N O(1)-Sr(1) = 2, N O(2)-Sr(1) = 2, N O(3)-Sr(1) = 2, N O(1)-P(1) = 1, N O(2)-P(1) = 1 and N O(3)-P(1) = 1.
This allows the decomposition of K(1)Sr( 1 The environmental factors he and Fc are then calculated for each cation site in these binary units. For a given binary crystal A(i) α O(i) β , the two factors he (A(i)) and Fc(A(i)) corresponding to the A(i) site are calculated as:

𝑒 𝐴 𝑖 = 𝑓𝑐 (𝐴 𝑖 -𝑂 𝑖 ) 𝑁 𝐴 (𝑖) 1 𝛼 (𝐴 𝑖 -𝑂 𝑖 ) 𝑄 𝑂(𝑖) 2 (9)
and 𝐹𝑐(𝐴 𝑖 ) =

In these expressions, fc (A-O) and fi (A-O) represent respectively the fractional covalency and ionicity of

each A(i)-O(i) bond in the unit, α (A(i)-O(i)) is the volume polarization of each A(i)-O(i) bond, d (A(i)-O(i)) is the A(i)-O(i) distance in the unit and Q O(i)
is the effective charge carried but the oxygen atoms in site (i) of the binary unit. For a given A(i

) α O(i) β unit, Q O(i) is calculated as 𝑄 𝑂(𝑖) = 𝛼 𝛽
𝑄 𝐴(𝑖) , where Q A(i) is the effective charge of cation A(i), taken as its bond valence sum. The bond valence sum is known from the bond valence model which is a development of Pauling's rules that estimates the oxidation state of the central atom only from bond lengths determined by X-ray or neutron diffraction [START_REF] Momma | VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis[END_REF] .The bond valence sum V of a central atom can be determined from:

𝑉 = exp ( 𝑙 𝑖 -𝑙 0 𝑏 ) 𝑛 𝑖=1 ( 11
)
Where b is a constant equal to 0.37Å, l i is the bond length between the central atom and another neighboring atom and l 0 is a bond valence parameter of the central atom. In this work, the bond valence sum was determined directly from VESTA by inserting the bond valence parameter taken from the work of O'KEEFFE [START_REF] Brese | Bond valence parameters for solids[END_REF] .

For instance in the K(1) 3/10 O(1) 1/4 unit, we have Q K(1) = 1.368 and Q O(1) = 1.64. The quantity

d 𝐴 𝑖 -𝑂 𝑖 -2.48
(in eV) represents the homopolar part of the energy gap E h(A(i)-O(i)) separating the bonding and anti-bonding states in the A(i)-O(i) bond. This energy comes from the interaction of dipole moments and other multiple moments and is the origin of bond covalency [START_REF] Levine | d-Electron Effects on Bond Susceptibilities and Ionicities[END_REF] . Values of fc (A(i)-O(i)) and fi (A(i)-O(i)) are calculated as:

) /( 2)) ( ) ( ( 2)) ( ) ( ( 2)

) ( ) ( ( ) ( ) ( i O i A i O i A h i O i A h i O i A C E E fc       (12)
and ) /( 2)) ( ) ( ( 2)) ( ) ( ( 2)

) ( ) ( ( ) ( ) ( i O i A i O i A h i O i A i O i A C E C fi       (13)
where

C (A(i)-O(i))
is the heteropolar parts of the energy gap characterizing the A(i)-O(i) bond. The volume polarization α (A(i)-O(i)) of each A(i)-O(i) bond is obtained using:

𝛼 𝐴 𝑖 -𝑂 𝑖 = 𝑣 𝐴 𝑖 -𝑂 𝑖 3 4𝜋 𝜺 𝑨 𝒊 -𝑶 𝒊 -𝟏 𝜺 𝑨 𝒊 -𝑶 𝒊 +𝟐 (14)
where

v (A(i)-O(i)) is the volume (in cm 3 ) of the A(i)-O(i) bond and ε (A(i)-O(i)) is the dielectric constant of this bond.
The heteropolar parts of the A(i)-O(i)bond energy gap is calculated as:

𝐶 𝐴(𝑖)-𝑂(𝑖) = 14.4𝑏 ′ 𝐴 (𝑖)-𝑂 (𝑖) 𝑟 𝐴 (𝑖)-𝑂 (𝑖) exp -𝑘 𝑠 𝐴 (𝑖)-𝑂 (𝑖) 𝑟 𝐴(𝑖)-𝑂(𝑖) 𝑍 𝐴(𝑖) * - 𝑛 𝑚 𝑍 𝑂(𝑖) * 𝑖𝑓 𝑛 ≥ 𝑚 𝐶 𝐴(𝑖)-𝑂(𝑖) = 14.4𝑏 ′ 𝐴(𝑖)-𝑂(𝑖) 𝑟 𝐴(𝑖)-𝑂(𝑖) exp -𝑘 𝑠 𝐴 (𝑖)-𝑂 (𝑖) 𝑟 𝐴(𝑖)-𝑂(𝑖) 𝑚 𝑛 𝑍 𝐴(𝑖) * -𝑍 𝑂(𝑖) * 𝑖𝑓 𝑛 ≤ 𝑚 With r (A(i)-O(i)) = d (A(i)-O(i)) /2 and b' (A(i)-O(i)) = 0.089 < N (A(i)-O(i)) > 2 . The quantity <N (A(i)-O(i) )> is the average coordination number of the A(i)-O(i) bond.
It is defined as: The term exp -𝑘 𝑠 𝐴 (𝑖)-𝑂 (𝑖) 𝑟 𝐴(𝑖)-𝑂(𝑖) represents the Thomas-Fermi screening factor in which :

𝑘 𝑠 (𝐴 (𝑖)-𝑂 (𝑖)) = (4𝑘 𝐹 (𝐴 (𝑖)-𝑂 (𝑖)) 𝜋𝑎 𝑏𝑜 𝑟 ) 2 (17)
𝑘 𝐹 (𝐴 (𝑖)-𝑂 (𝑖)) = (3𝜋 2 𝑁 𝐴(𝑖)-𝑂(𝑖) * ) 1 3 (18)
In Equation ( 17), a bohr is the Bohr's radius. The quantity 𝑁 𝐴(𝑖)-𝑂(𝑖) * in Equation ( 18) is the number of effective valence electrons per cubic centimeter in the A(i)-O(i) bond, calculated as:

𝑁 𝐴(𝑖)-𝑂(𝑖) * = ( 𝑍 𝐴 (𝑖) * 𝑁 𝐴 (𝑖) + 𝑍 𝑂 (𝑖) * 𝑁 𝑂 (𝑖) ) 𝑣 (𝐴(𝑖)-𝑂(𝑖)) (19)
where Z* A(i) and Z* O(i) are the number of effective valence electrons on the A(i) and O(i) atoms of the bond and v (A(i)-O(i)) is the volume of the bond. Z* A(i) and Z* O(i) are calculated as: Z*=Z.q, where Z is the number of valence electrons of the atom and q is the effective charge of the electron in the considered bond which can be obtained for each atom as Q/(formal valence state). For instance in K(1) 3/10 O(1) 1/4 unit, we would have q K =1.368/1= 1.368, q O =1.64/2=0.82, Z* K =Z K .q K =1.368 and Z* O =Z O .q O =0.82x6= 4.92.

The bond volume v A(i)-O(i) is obtained from:

𝑣 𝐴(𝑖)-𝑂(𝑖) = 𝑑 (𝐴(𝑖)-𝑂(𝑖)) 3 𝑑 (𝐴(𝑖)-𝑂(𝑖)) 3 𝑖 𝑁 𝐴 𝑖 -𝑂 𝑖 𝑏 ( 20
)
where V ZN N i O i A b i O i A / )) ( ) ( ( )) ( ) ( (   
is the number of chemical bonds of type A(i)-O(i) per cubic centimeter. Z is the number of formula per unit cell and V is the volume of the unit cell. Coming back to KSrPO 4 , we have Z=4 and V=394 (Å -3 ), then for K(1) 3/10 O(1) 1/4 , we have N b (K1-O1) = 0.03 (Å -3 ). The dielectric constant ε (A(i)-O(i)) of the A(i)-O(i) bond required in Equation ( 14) is obtained from:

𝜀 (𝐴(𝑖)-𝑂(𝑖)) = 1 + 4𝜋𝜒 𝐴 𝑖 -𝑂 𝑖 (21)
χ (A(i)-O(i))
is the susceptibility of the A(i)-O(i) bond, defined as:

𝜒 (𝐴(𝑖)-𝑂(𝑖)) = (ℏ𝛺 𝐴(𝑖)-𝑂(𝑖) ) 2 4𝜋𝐸 𝑔 (𝐴 (𝑖)-𝑂 (𝑖)) 2 (22)
The quantity

Ω (A(i)-O(i))
is the plasma frequency of the A(i)-O(i) bond, calculated as :

ℏ𝛺 𝐴(𝑖)-𝑂(𝑖) = 37.16 𝑁 (𝐴(𝑖)-𝑂(𝑖)) * (23)
The term E g(A(i)-O(i)) represents the average value of the energy gap, calculated as

𝐸 𝑔 (𝐴 (𝑖)-𝑂 (𝑖)) 2 = 𝐸 (𝐴(𝑖)-𝑂(𝑖)) 2 + 𝐶 (𝐴(𝑖)-𝑂(𝑖)) 2 (24)
In this work, we have designed an excel program that helps to calculate the environmental factors in an easy and automatic way. After filling the data in the coordination matrix with the multiplicity of each atom and Z in KSrPO 4 (Tables 5 and6), the formula allowing the decomposition into binary crystals is automatically obtained (Table 6). Table 7 8).

Table 5 Input data of the coordination of the different atoms with the corresponding multiplicity in KSrPO 4 .

Table 6 Formula of KSrPO 4 obtained automatically by the excel program.

Z Formula 4 K 1 Sr 1 O1 1 O2 2 O3 1 P 1 Based on the investigation of a series of cubic host lattices including fluorides, bromides, chlorides, iodides, oxides, sulfides and selenides consisiting of one or more cation sites with different coordination symmetries, Zhang has established a correlation between he and the centroid energy Ec of the 4f n-1 5d 1 electron configuration of ions like Ce 3+ and Eu 2+ (24) (Figure 20).

A fitting of the data then led to the following semi-empirical Equation that allows calculating Ec (in cm -1 ) from the knowledge of he:

𝐸𝑐 = 𝐴 + 𝐵𝑒 -𝑘 𝑒 (25)
For Eu 2+ , A = 27130, B = 16450 and k = 1.817. For Ce 3+ , A = 12490, B = 37930 and k = 0.662. In the case of Eu 2+ , the accuracy of the model is estimated by us at ± 360 cm -1 . Equation ( 25) is basically valid for lattices having cubic-related symmetry. 2+ in various cubic lattices (24) In another work, an empirical relationship has also been established between Fc and the crystal field strength E 10Dq (in cm -1 ) for Eu 2+ and Ce 3+ in cubic halide lattices [START_REF] Shi | Dependence of crystal field splitting of 5d levels on hosts in the halide crystals[END_REF] (Figure 21):

𝐸 𝐷𝑞 𝐴 𝐵 • 𝐹𝑐 (26)
For Eu 2+ , A = 4990 and B = 18770. For Ce 3+ , A = 3270 and B = 14340. In the case of Eu 2+ , the accuracy of the model is estimated by us at ± 470 cm -1 . 2+ in cubic halides (59) In the case of s 2 ions like Bi 3+ and Pb 2+ , semi-empirical Equations have also been established between he and the excitation energy of the A transition band in a number of host lattices having different crystal structures [START_REF] Wang | Investigation and application of quantitative relationship between sp energy levels of Bi 3+ ion and host lattice[END_REF], [START_REF] Sun | Prediction and assignment of site occupation and energy levels for Pb 2+ ions[END_REF] (Figure 22). For Bi 3+ , two relationships have been introduced to estimate the energy of A and C transitions. For the A transition, we have [START_REF] Wang | Investigation and application of quantitative relationship between sp energy levels of Bi 3+ ion and host lattice[END_REF] :

𝐸 𝐴 𝐵𝑖 𝑒 -𝑒 (27)
and for the C transition [START_REF] Wang | Investigation and application of quantitative relationship between sp energy levels of Bi 3+ ion and host lattice[END_REF] :

𝐸 𝐶 𝐵𝑖 𝑒 -𝑒 (28)
In the case of Pb 2+ the relation was found to be linear. The energy of the A transition is calculated as [START_REF] Sun | Prediction and assignment of site occupation and energy levels for Pb 2+ ions[END_REF] :

𝐸 𝐴 𝑃𝑏 - 𝑒 (29)
All energies are given in cm -1 .

MMCT model

The metal to metal charge transfer model, proposed by Boutinaud (42), [START_REF] Boutinaud | Predicting metal-to-metal charge transfer in closed-shell transition metal[END_REF] , aims to calculate the MMCT energy of dopants in transition metal oxides from the knowledge of few structural and electronic parameters. The model was especially applied for ions with s 2 configuration like Bi 3+ and Pb 2+ in d 0 /d 10 transition metal oxides. An empirical Equation has then be introduced on the basis of the investigation of about 60 different transition metal oxides with various crystal structures:

𝑀𝑀𝐶𝑇 𝑐𝑚 -1 = 𝑘 𝐶𝑁 ′ 𝜒 𝐶𝑁 (𝐵𝑖 3+ ) -𝛼 𝐶𝑁 ′ 𝐶𝑁 𝜒 𝐶𝑁 ′ (𝑀 𝑛 + ) 𝑑 𝑐𝑜𝑟𝑟 (30)
This Equation allows the calculation of the energy of the charge transfer between Bi 3+ and the closest-lying transition metal M n+ in the host lattice (i. e. D-like transition). In Equation ( 30), CN and CN' are the coordination numbers of Bi 3+ and M n+ in their respective crystallographic sites. Parameters χ CN (Bi 3+ ) and χ CN' (M n+ ) are the electronegativities of Bi 3+ and of the transition metal ion M n+ , respectively extracted from the work of Li and Xue [START_REF] Li | Estimation of Electronegativity Values of Elements in Different Valence States[END_REF] (see Appendix A). The quantity d corr represents the shortest Bi 3+ -M n+ distance in the host lattice. This value is corrected for the doping effect (i. e. difference of ionic radius between Bi 3+ and the substituted cation in coordination number (CN) following:

𝑑 𝑐𝑜𝑟𝑟 = 𝑑 𝑜𝑠𝑡 + 1 2 𝑟(𝐵𝑖 3+ ) -𝑟(𝑀 𝑛+ (31)
where r(Bi) and r(M n+ ) are the corresponding crystal radii, as deduced from the work of R. D. Shannon [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF] and d host is the shortest distance between M n+ and the substituted cation site in the lattice. This data is obtained from ICSD data bases by using VESTA.

The value of χ CN (Bi 3+ ) can be calculated for any value of CN following the procedure given in Li and Xue [START_REF] Li | Estimation of Electronegativity Values of Elements in Different Valence States[END_REF] :

𝜒 𝐶𝑁 𝐵𝑖 3+ = 0.105𝑛 * 𝑟(𝐵𝑖 3+ ) 𝐼 𝑚 13.6 + 0.863 (32)
Where n * is the effective principle quantum number (4.36 for Bi 3+ ) and I m is the ultimate ionization energy (25.56 eV for Bi 3+ ). r(Bi 3+ ) is the coordination-dependent ionic radius of Bi 3+ .The two parameters k CN' and 𝛼 𝐶𝑁 ′ 𝐶𝑁 in Equation ( 30) are crystal-structure related quantities. Values are given in Tables 9 and10.

Table 9 Useful structure-related parameters for Equation (30) (42) .

Table 10 Useful structure-related parameters for Equation (30) (42) .

From the above Tables, it is clear that lattices in which the transition metal is tetra-coordinated (CN' = 4) will behave differently from lattices in which the transition metal is hexa or higher coordinated (CN' ≥ 6). This is clearly exemplified in Figure 23 where the experimental excitation energies of Bi 3+ in transition metal oxides are plotted against

𝜒 𝐶𝑁 ′ (𝑀 𝑛 + ) 𝑑 𝑐𝑜𝑟 .
For ease of use and fast evaluation of MMCT energies, one may consider average values of χ CN (Bi 3+ ), k CN' and 𝛼 𝐶𝑁 ′ 𝐶𝑁 as follows: χ CN (Bi 3+ ) = 1.31 (corresponding to frequently encountered 8-fold coordinated sites), k 4 = 52000 cm -1 , k CN'≥6 = 41000 cm -1 and 𝛼 𝐶𝑁 ′ 𝐶𝑁 = 1. We see from Figure 23 that the accuracy of the method is about ± 1500 cm -1 . 30) and appropriate parameters given by Tables 9 and10.

Chapter 2 The doping sites in A I B II PO 4 :Eu 2+ and their consequence on the photoluminescence excitation spectra

Introduction

Among divalent lanthanides, Eu 2+ and Yb 2+ exhibit the lowest energy values for the 4f-5d transitions (Chapter 1). Being more stable, Eu 2+ ion is widely used in designing phosphors for different applications especially for the LED lighting. To match the needs of the LED lighting market today, phosphors absorbing in the near UV or blue and emitting in the visible region are highly required. Browsing the literature, we can find that a number of Eu 2+ doped compounds have been studied for this purpose ( 1)- (6) .

The family of ABPO 4 where A I and B II are monovalent and divalent cations respectively, also serve as relevant hosts for Eu 2+ in extensive research studies owing to their attractive photoluminescent properties upon blue or near-UV excitation in addition to their excellent thermal stabilities ( 7)- (13) . Figure 1 shows the excitation spectra of a group of ABPO 4 :Eu 2+ hosts showing broad band signals extending to the UV and blue regions. .

The crystal field splitting of the 5d states of Eu 2+ in a crystal and correspondingly the red shift depend mainly on the type of coordination of the cation site in which Eu 2+ accommodates. In this connection, the A I B II PO 4 family which includes many members with many possible crystal structures and doping sites depending on the nature of A I (typically Li + , Na + , K + , Cs + , Rb + , Ag + , NH 4 + ) and B II (typically Be 2+ , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Zn 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ ), gives a variety of luminescent outputs that can be explored. This can be seen from Figure 1 where the excitation edge attributed to Eu 2+ in these compounds can vary by 100 nm. Note that the excitation edge in this chapter will be taken as the wavelength where the excitation has lost 50% of its lowest-lying maximum intensity. It marks the limit above which the excitation in the first spin and dipole allowed 4f 7 → 4f 6 5d 1 transition of Eu 2+ becomes inefficient.

However, not all of these members have been studied in the literature. The series of studied compounds are ABPO 4 with A = Li + , Na + , K + , Cs + , Rb + and B = Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ( 7)- (16) . For most of these compounds, the excitation edge can easily be picked up from the archival literature. This is what we have done in the present work. Doing so, we found important discrepancies in the edge position for a few lattices. In CsMgPO 4 :Eu 2+ , for instance, the excitation edge is reported either at 395 nm (17) or at 460 nm (7) , which makes a mismatch of 65 nm. In NaMgPO 4 :Eu 2+ , the excitation edge is found at 377 nm for a corresponding blue emission (18) or close to 500 nm for a corresponding red emission (11) , depending on the preparation method. There are no clear explanations for these behaviors. One possible explanation could be related to the fact that Eu 2+ may be inserted in either A I or B II sites, with concomitant difference in the luminescence signals. As a matter of fact, this lack of information on the nature of the doping sites for Eu 2+ prevents the prospective design of materials and restricts the preparation of phosphors to trialerror procedures that are matter and time-consuming. Therefore, a better understanding of the relationship between the redshift and the crystal structure would in this connection, be of great help.

In this chapter, the determination of the accommodating sites in A I B II PO 4 :Eu 2+ will be investigated with the help of the EF model. Calculations of the environmental factors he (X(i)) and Fc (X(i)) allow us to calculate the energy of the first spin and dipole allowed 4f 7 → 4f 6 5d 1 transition of Eu 2+ for each possible cation site in 25 different ABPO 4 hosts. This energy will be assimilated to the photoluminescence excitation edge defined above. By comparing the calculated energy of this transition for each cation site with that of the experimental data on the excitation edge positions we can identify the doping sites in each host. At the end, this will allow us to conclude on the potential interest of the method for the predictive design of Eu 2+ -doped A I B II PO 4 hosts usable for LED-based lighting. In addition to this phenomenological approach, we have prepared the compound NaZnPO 4 :Eu 2+ and investigated its luminescence properties. The choice of this lattice is motivated by the fact that only one kind of site (Na(1)) is possible for Eu 2+ . For this reason, as it will be detailed in the following, this compound will be considered as a model system. 2. At 650°C to 710°C, the compound seems to consist of NaZnPO 4 monoclinic phase. At 860°C < T < 920 °C, the compound presents at least two phases (monoclinic and orthorhombic)

Synthesis and structural/microstructural characterizations of

with additional features that are not identified by any of the two phases marked by (*). At T ≥ 950 °C, the main features start to disappear until we have a destruction of the compound at T= 1010 °C. One main peak remains at ≈ 40° which is related to platinium originating from the heating strip where the powder was installed during the X-ray experiments. As we were looking for a well-crystallized monoclinic compound, the sample was treated at 850 °C for 6h. The flowing atmosphere was a mixture of argon and hydrogen (95/5) in order to reduce Eu 3+ to Eu 2+ . The corresponding X-ray diffraction pattern recorded at ambient temperature is shown in Figure 3.

It is consistent with the monoclinic polymorph of NaZnPO 4 (JCPDS standard 01-079-0217), except a weak reflection (*) which belongs to NaZnP 3 O 9 (JCPDS 96-703-5628). This very small amount of extra phase was not considered as significant enough to affect the luminescence properties. * marks a reflection that is not indexed using the JCPDS standard 01-079-0217 (see text).

The monoclinic crystal structure of NaZnPO 4 (ICSD 87557) is represented in Figure 4. polyhedron is calculated as (19) :

𝐷. 𝐼. = 1 𝑁 𝑙 𝑖 -𝑙 𝑎𝑣 𝑙 𝑎𝑣 𝑁 𝑖=1 (1)
where l i is the distance from the central atom to the i th coordinating atom, l av is the average bond length and N is the coordination number. This data gives information on the degree of irregularity of a given polyhedron. In the case of NaZnPO 4 , the D.I. value of (NaO 6 ) octahedra amounts ≈ 10%. The very small Zn 2+ sites being tetrahedrally coordinated in the lattice, are not expected to accommodate Eu 2+ .There is therefore no doubt that the Eu 2+ ion occupy the Na + sites in the compound.

The sample was also studied by electron scanning microscopy using a JSM-5910 LV microscope from Jeol operated at 15 kV. A representative image is shown in Figure 5. The microstructure of NaZnPO 4 :Eu 2+ consists of a large granular distribution covering the range ≈ 10 -100 µm. The corresponding microprobe images shown in Figure 6 illustrate the highlighted distribution of each atom showing the elemental concentrations in the powder. The green color is related to phosphorous, the blue to sodium, the brown to zinc and red to oxygen. These images confirm homogenous distribution of the ions within the material. Eu 2+ could not be observed due to the small doping rate. The steady state excitation and emission spectra were collected using a setup of devices designed by Horiba Jobin Yvon company. A Xenon lamp of 400 W was used as the light source (Figure 7) and two Jobin-Yvon/Horiba TRIAX 180 and TRIAX 550 monochromators were used for the selection of excitation and emission wavelengths. The emission of the sample is focalised on a bundle of fiber optics connected to triax 550 and detected using a CCD SYMPHONY detector of 1024x256 pixels cooled by liquid nitrogen.The excitation spectra were detected by a hamamatsu R928 photomultiplier. The emission signal of NaZnPO 4 :Eu 2+ being too weak to be accurately measured at room temperature, the powder was cooled down to 77 K after being conditionned in sealed ultrapure quartz tube and immersed in liquid nitrogen. The excitation spectra were spectrally corrected using sodium salicylate. At 77 K, the compound emits in the blue spectral range. The corresponding excitation shows a broad spectrum from 270 to 450 nm containing three bands ascribable to the splitting of the lowest-lying 4f 6 5d 1 (t 2g ) manifold as a result of the C 1 distortion of the octahedral Eu 2+ site (Figure 8). The Stokes shift is ≈2000 cm -1 and the excitation edge is estimated at 437 nm. We did not observe any extra luminescence ascribable to Eu 3+ impurities.

Calculation of the environmental factors in A I B II PO4 lattices

As stated in Chapter 1, it is possible to connect he(X(i)) and Fc(X(i)) respectively to the centroid energy Ec(X(i)) and the crystal field splitting 10Dq(X(i)) for a given site X(i) occupied by Eu 2+ . We show in Figure 9 the energy levels of the Eu 2+ as a free ion and when incorporated in a solid with cubic symmetry.

If the position of centroid energy E c (X(i)) and the magnitude of the crystal field splitting 10 Dq (X(i)) are known for the cation site X(i), then the energy E fd (X(i)) corresponding to the first allowed 4f 7 →4f 6 5d 1 transition can be calculated using the following Equation: E c (X(i)) and Dq(X(i)) can be obtained from the Equations ( 25) and ( 26) of Chapter one respectively, which can be written for the purpose of this work as:

𝐸 𝑓𝑑 𝑋 𝑖 𝐸 𝑐 𝑋 𝑖 -𝐷𝑞 𝑋 𝑖 (2)
𝐸 𝑐 𝑋 𝑖 𝑐𝑚 - 𝑒 - 𝑒 𝑋 𝑖 (3)
𝐷𝑞 𝑋 𝑖 𝑐𝑚 - 𝐹𝑐 𝑋 𝑖

Knowing that Equations (3) and ( 4) have errors of ± 360 cm -1 and ± 470 cm -1 respectively, then we can estimate the error of Equation ( 2) at ± 550 cm -1 .

At this point, several concerns have to be taken into account:

1) First, Equation (3) was initially established for hosts with cubic symmetry (20) . In the ABPO 4 family, most of the lattices do not have cubic symmetry, which raises a question of uncertainty of applying the Equation on these lattices.

2) Equation ( 4) was found for cubic halides (21) . This means that we should consider the influence of the type of anion. However, this influence was assumed to be very low if we compare for example, the crystal field splitting of Ce 3+ in LiYF 4 and YPO 4 (22) that are very close.

3) None of the ABPO 4 lattices are cubic, they present distortions in their polyhedra relative to the ideal octahedral, cubal, dodecahedral or cuboctahedral polyhedra. This will be figured out by using Equation (1). Further, the incorporation of Eu 2+ induces even more distortions especially when substituted in smaller B II sites (16), (23) . This causes a stress for anions around the center resulting in changing the shape of the polyhedron. On the other hand, the substitution in the A I sites demands charge compensation that may also affect the shape of the coordination polyhedron in an unknown manner.

The above arguments lead to conclude that the applicability of the model on ABPO 4 :Eu 2+ could result in large errors that basically need to be corrected by reformulating Equation (2). Unfortunately, this is in practice very difficult to maintain. We have therefore decided to apply the model on our compounds and tentatively estimate the importance of these errors. This is the reason why we have prepared NaZnPO 4 :Eu 2+ for which we are sure that Eu 2+ is incorporated in the single Na + Wyckoff position. Assuming that the octahedron is the geometry for which the CFS of Eu 2+ is the strongest in the A I B II PO 4 lattices, we estimate that the difference between experimental and calculated excitation edges induced by the local site distortion in NaZnPO 4 -Eu 2+ should be representative of the largest committed error with respect to an equation in which we would have implemented a correction from non-cubic crystal fields.

The results of calculations of he(X(i)), Fc(X(i) and D.I.(X(i)) are all compiled in Table 1. The calculations of he(X(i)) and Fc(X(i)) were carried out for each cation site X(i) that is considered as a possible doping site in a number of ABPO 4 lattices using Equations ( 9) and (10) of Chapter 1. Therefore, the four and five-coordinated sites were not included in the calculations because it is nearly impossible for these to act as accommodating sites for Eu 2+ ions. Furthermore, such low-coordinated sites are not reproduced by Equation (2). In practice, this concerns the Mg 2+ sites in CsMgPO 4 and α-KMgPO 4 (ICSD 72401), the Li + sites in LiCaPO 4 and LiSrPO 4 and all the Zn 2+ , Be 2+ and P 5+ sites. The useful structural data were obtained from the ICSD database and treated using VESTA (24) . For lattices that present more than one polymorph like for instance KZnPO 4 , the calculations were made for all the different structures found in the ICSD database. However, for the compounds reported in the literature and for which the crystal structure of the synthesized polymorph is known with certainty, the calculations were made only for the corresponding polymorph. For the lattices LiBaPO 4 , NaSrPO 4 and RbSrPO 4 that have been investigated from a luminescence viewpoint when doped with Eu 2+ , the calculations could not be achieved because the structure is not available in the ICSD database. We also could not proceed for the calculations of KCaPO 4 (ICSD 27958) for which the available crystal data present errors and is not reliable (the PO 4 tetrahedron could not be observed with VESTA). In NaBaPO 4 (ICSD 15253), the structure has a cation site with wyckoff position 4i that has an occupancy of half Na + and half Ba 2+ . Here, the charge of the cation (Na + ,Ba 2+ ) required for the calculations of the environmental factors, was taken as 1.5

((charge of Na + + charge of Ba 2+ )/2). Lattices based on transition metals (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ )

were not considered in the present work.

Doping sites in A I B II PO4:Eu 2+

We have compiled in Ec(X(i)), Dq(X(i)) and E fd (X(i)) as calculated from Equations ( 2) -( 4), using the environmental factors compiled in Table 1. Lattice (space group) E exp (cm -1 )

X(i) Ec (X(i)) (cm -1 ) Dq (X(i)) (cm -1 ) E fd (X(i)) (cm -1 ) NaZnPO 4 (P2 1 /n) 22880* Na + (1) 27190 1005 23170
CsMgPO 4 (Pnma) 25315 (17) 21740 (7) Cs + (1) 28737 611 26293

LiMgPO 4 (Pnma) 25315 (16) Li + (1) (14) 22440 (12) Ca 2+ (1) 28737 1625 22237

Mg 2+ (
LiSrPO 4 (P6 5 ) 23750 (13) 23585 (25) Sr 2+ (1) 27492 1212 22643

NaMgPO 4 (P2 1 2 1 2 1 ) 26525 (18) Na + (1)

Na + (2) 24630 (17) 23925 (10) Na + (2) NaBaPO 4 (C2/m) 25445 (14) 26670 (26) Na + (1) (17) 25250 (8) 26315 (27) K + (1)

Na + (3) 31340
Na + (
Ba
Sr 2+ (1) 28680 29129 1006 1137 KBaPO 4 (Pnma) 26740 (17) 25250 (8) 26110 (26) 25775 (27) K + (1) The results show that for a number of sites, the experimental energy of the excitation edge matches that of the calculated E fd with an error of ±1000 cm -1 . However, in few cases the error is as large as ± 6682 cm -1 like the case of Mg 2+ (1) in NaMgPO 4 . We also point out that for a given A I B II PO 4 -Eu 2+ lattice, the experimental edge position is not defined with an accuracy better than ± 750 cm -1 (e.g. KSrPO 4 and KBaPO 4 ), depending on the synthesis condition, Eu 2+ concentration, temperature at which the spectra are collected, relevancy of the spectral corrections, etc…In β-NaCaPO 4 :Eu 2+ or CsMgPO 4 :Eu 2+ , the accuracy is even worse (>± 1000 cm -1 ). As a matter of comparison, the energy difference between E exp and E fd (Na(1)) in NaZnPO 4 -Eu 2+ is 290 cm -1 , which is much below the above uncertainties. In these circumstances, we come to the conclusion that the refinement of Equation ( 2) to take into consideration a non-cubic CFS would, in fact, not lead to a significant improvement of the accuracy of our analysis.

Ba 2+ (1
We can see from Table 2 that the experimental energy may match only one value of E fd corresponding to one cation site (e.g. in LiMgPO 4 , the E exp being 25315 cm -1 , is in agreement with the calculated value 26719 cm -1 corresponding to site Li + but very far from that of site Mg 2+ , 19286 cm -1 ). In other lattices, the experimental data can match the calculated energy of more than one site like in KMgPO 4 for which both K + (1) and K + (2) sites (28016 and 28003 cm -1 respectively) are in agreement with E exp (27930 cm -1 ). This is interesting for the purpose of identifying the doping sites in each lattice. We will assume here that the sites for which the difference in absolute value between E fd and E exp is less than 1000 cm -1 can be identified as possible doping sites for Eu 2+ .

To have a complete vision of the situation, we have plotted the scheme of Figure 10 showing the difference E fd (X(i))-E exp for each cation site in each lattice. For sake of clarity, only the average value of the different experimental excitation energies listed in Table 2 have been considered for each given A I B II PO 4 -Eu 2+ lattice (when necessary), except for CsMgPO 4 :Eu 2+ . In this case, it is clear from Table 2 that the Cs + site cannot account for an excitation edge positioned as low as 21740 cm -1 . This suggests that the corresponding transition is not of f-d character but more probably of excitonic origin as argued in ref. (28) or is due to Eu 2+ ions in close interaction with cation vacancies, as argued in ref. (7) sites of RbBaPO 4 . The Li + sites of LiMgPO 4 and the K + /Sr 2+ sites of KSrPO 4 are also located very close to this interval. In KSrPO 4 , we note that the cation sites would be located within the ± 1000 cm -1 interval by considering an experimental excitation edge of 25250 cm -1 for this lattice (see Table 2) instead of taking the average value of the available experimental data. The sites for which E fd (X(i) > E exp by more than 1000 cm -1 are related to higher lying excitation levels and are of lesser interest in the context of the present work. The sites for which E fd (X(i)) -E exp < 1000 cm -1 could not be considered as doping sites because they do not contribute to the experimental excitation band.

Designing A I B II PO4-Eu 2+ phosphors for solid state LED-based lighting

Using the EF model we have shown that it is possible to determine the nature of the doping sites in A I B II PO 4 lattices that contribute to the photoluminescence excitation edge of Eu 2+ within an accuracy of ± 1000 cm -1 . The method will now tentatively be used to assist the design of A I B II PO 4 -Eu 2+ phosphors for an application in solid state LED-based lighting.

For this, we show in Figure 11 a plot of he(X(i)) versus Fc(X(i)). We can see that no correlation is found between the two factors. Knowing that he(X(i)) is related to centroid energy and that Fc(X(i)) is related to the crystal field splitting, then Figure 11 is in agreement with the well-admitted fact that the centroid energy and the crystal field splitting behave independently of each other (29) . The three solid curves in this plot indicate the combinations of (he(X(i)),Fc(X(i))) that give an excitation edge located at 400 nm (25000 cm -1 ), 450 nm (22220 cm -1 ) and 500 nm (20000 cm -1 ) according to Equation (2). The dots in this figure belong to the different cation sites X(i) of the ABPO 4 lattices. Here, we have incorporated all the lattices listed in Table 1 including those that are not reported in the literature as doped with Eu 2+ .

Figure 11.Fc(X(i)) vs he(X(i)) plot for the different cation sites in A I B II PO 4 lattices. The solid lines indicate the (Fc(X(i)), he(X(i)))

values corresponding to excitation edges located at 400, 450 and 500 nm in A I B II PO 4 :Eu 2+ .

We find that up to 14 cation sites potentially available for Eu 2+ have a predicted excitation edge located in the 400 -450 nm range. This means that the corresponding sites, if occupied by Eu 2+ , have a chance to be excited using a LED source emitting in the 400 -450 nm range. We note further that 3 cation sites are close to the 500 nm line. This is the case of the Cd 2+ sites in Pna2 1 polymorphs of LiCdPO 4

and NaCdPO 4 and of the Mg 2+ (2) sites in NaMgPO 4 . In this connection, we point out that the excitation edge reported at 500 nm in ref. (11) for NaMgPO 4 :Eu 2+ could be well explained by the incorporation of some amount of Eu 2+ in the Mg 2+ (2) sites.

In complement to Figure 11, we provide in Figure 12 an abacus allowing an easy estimation of the excitation edge wavelengths (λ edge ) that can be reached in A I B II PO 4 -Eu 2+ lattices for a selection of (Fc(X(i)), he(X(i))) couples. The horizontal dotted lines in this graph mark the emission wavelength of three excitation sources emitting respectively at 375, 400 or 450 nm. This could, for instance, be LEDs.

The intersection between the abacus and one given dotted line gives immediately the sets of Fc(X(i)) and edge matches the emission of the considered LED and, therefore, is excited.

Figure 12.Calculated excitation edge wavelengths λ edge for a selection of (Fc(X(i)), he(X(i))) couples in A I B II PO 4 : Eu 2+ lattices. The horizontal dotted lines mark the emission wavelength of a 375, 400 and 450 nm emitting LED. See text for further details.

At this stage we are able to predict the possible doping sites in any ABPO 4 :Eu 2+ phosphate phosphor and study the possibility of these phosphates to be designed for LED lighting applications by predicting the location of the excitation edge.

We will now investigate the compounds of the A I B II PO 4 family that are not studied in the literature and for which the excitation edge is yet unknown. For discussion, we should take into account the overlap between the excitation edge of the A I B II PO 4 -Eu 2+ phosphor and the emission spectral distribution of a given LED. Figure 13 shows the typical emission spectrum of a blue commercial LED (InGaN). The FWHM (full width at half maximum) indicates the range of energy in which the emission of the LED is efficient. The excitation edge of the phosphor should then overlap with the emission of the LED source inside this range to maintain the best luminescent outputs. This will be considered here in addition to the uncertainty of the prediction of the excitation edge by the EF model to propose the set of phosphors of the ABPO 4 family that are suitable for the design of LED lighting. Typically, the FWHM is about 10 nm for 375 nm (near UV) LED, 12 nm for 400 nm (violet) LED and 20 nm for 450 nm (blue) LED. These are average values that also depend on the current/power at which the LED is operated. The larger these values, the larger the FWHM. These three ranges are represented in Figure 14 by the grey columns for the 375 nm, 400 nm and 450 nm LED sources, in correspondence with the calculated excitation edges (± 1000 cm -1 ) represented in horizontal bars of all cation sites potentially available for Eu 2+ in a selection 13 A I B II PO 4 lattices in which the luminescence properties have not yet been described in the literature. The overlaps between the grey zones in this graph indicate the nature of the Eu 2+ doping sites in A I B II PO 4 that have the best chance to be excited using a blue, a violet or a Near-UV LED. Using a blue InGaN LED as excitation source, we find that only the Pna2 1 polymorph of NaCdPO 4 -Eu 2+ and the Pnma polymorph of LiCdPO 4 -Eu 2+ have realistic chance to be excited, while all the other compounds should be excitable using a 375 nm LED.

Conclusion

The nature of the doping sites X(i) that correspond to the photoluminescence excitation edge in Eu 2+ -doped A I B II PO 4 lattices (A = Li + , Na + , K + , Rb + , Cs + , Ag + , NH 4 + ; B = Be 2+ , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Zn 2+ ) is determined from the knowledge of two-crystal structure-related environmental factors he(X(i)) and Fc(X(i)) and by the application of empirical equations allowing the calculation of excitation edge energies. With an accuracy of ± 1000 cm -1 , the method appears to be usable as a predictive tool for the design of A I B II PO 4 -Eu 2+ phosphors. It has however to be mentioned that our method does not give information on the Stokes shift and related emitted color and quantum efficiency of the phosphors.

These issues require further developments.

Chapter 3: The origin of luminescence in Bi 3+ -doped closed transition metal oxides

Introduction

The luminescence of Bi 3+ has gained a great attention during the last years owing to its broad and intense band emission in the UV and visible region ( 1)- (3) . Bi 3+ is especially reported as a good sensitizer for trivalent lanthanides especially Eu 3+ , enhancing its red emission in a number of compounds like for instance, CaMoO 4 :Eu 3+ :Bi 3+ (4) and Y 4 MgSi 3 O 13 :Bi 3+ : Eu 3+ (5) . A number of transition metal oxides containing transition metal ions M n+ with d 0 electron configuration (e.g. CaTiO 3 (3) , YNbO 4 (6) , YVO 4 (7) ,…) doped with Bi 3+ has recently been the subject of extensive work. As stated in Chapter 1, the self luminescence in these undoped materials gives rise to emission in the UV or visible region resulting from a ligand-tometal charge transfer. The incorporation of Bi 3+ in these complex oxides induces new emission and excitation features that are generally red-shifted with respect to the optical signals of the undoped host lattices. These new features are ascribed either to a metal-to-meal charge transfer from Bi 3+ to a nearby M n+ (and then recognized as D band), are associated to the formation of Bi 3+ clusters or are simply attributed to regular A transitions (i.e. internal 1 S 0 → 3 P 0,1 transitions of isolated Bi 3+ ion). It appears therefore that the origin of the luminescence in these compounds is not yet totally fixed and is still subject of controversy.

In this chapter, a number of Bi 3+ -doped transition metal oxides will be investigated. The EF model as well as the MMCT model will be used to discuss the origin of luminescence in these materials and help predicting the locations of the A and D states at the light of the three possible configurations introduced in Chapter 1 (Figure 16). Calculations of the A band transition excitation energy, E A , will be carried out using the EF model. The MMCT model will be used to calculate MMCT energies (Bi 3+ -M n+ ). Analysis of the results will be made by comparing the calculated energies to the experimental data collected from the literature and from the present work. For this purpose, the following compounds doped with Bi 3+ will be prepared: GdVO 4 , LaVO 4 , CaMoO 4 , LiNbO 3 , NaNbO 3 , ANbO 4 (A=Y, Gd, La), CaNb 2 O 6 , M'YTaO 4 , LiNbO 3 -Bi 3+

and NaNbO 3 -Bi 3 . In addition, the Stokes shift will be used to introduce a criterion from which the origin of the luminescence signals in these materials can be ascribed.

The two models will also be used to study the yellow emission of YVO 4 :Bi 3+ as a special case. This compound has only one accommodating site available for Bi 3+ which makes it a good example to study the Bi 3+ luminescent properties. It is known for long to glow brightly in the yellow region with an emission band peaking at 570 nm in correspondence with a Stokes shift of ≈ 11500 cm -1 . The corresponding internal quantum efficiency has recently been measured at 0.93 (7) . This interesting optical behaviour makes this compound a possible rare-earth free candidate for replacing the Ce 3+ in the YAG yellow phosphor in LED lighting. Despite this attractive luminescent action, not much is known on the exact nature of the emitting state (A or D) in this phosphor. In the present work, we will use the action of high pressure (up to 20 GPa) to get better insight in the nature of this emitting state expecting that pressure could lift in some way the degeneracy of the involved A and D levels. In other luminescent materials, the effect of pressure was found to be very important in altering the energy level locations and the degree of the different couplings which led to important change in the optical behaviour like in the case of CaWO 4 :Pr 3+ and CaMoO 4 :Pr 3+ ( 8), (9) However, the effect of pressure on the luminescence of YVO 4 :Bi 3+ has not been investigated so far. This part of the work will be carried out through collaboration with Dr Sebastian Mahlik from the Institute of Experimental Physics of the University of Gdaosk, Poland.

Synthesis and photoluminescence of closed transition metal oxides doped with Bi 3+

Several transition metal oxides doped with Bi 3+ were prepared in the context of this work. The raw chemicals and experimental conditions (temperature/duration) used for the syntheses are given in Table1. The nominal doping rate was 5 mol %. This content was chosen not too high to limit the formation of Bi-related dimers and not too low to insure incorporation of Bi 3+ within the host lattices in spite of the evaporation of Bi 2 O 3 at high temperature. The thermal treatments were carried out in air to limit the possible formation of Bi 2+ centers, whose luminescence signatures could interfere with the emission and excitation spectra of Bi 3+ , thus making the spectral analysis possibly difficult. All the compounds were checked by X-ray diffraction and were concluded to be single phased. The photoluminescence emission and excitation spectra were collected at 300 K using the same setup as in Chapter 2. We show in Figure 1 the photoluminescence spectra of the Bi 3+ -doped lattices listed in Table 1. As anticipated, we find no trace of Bi 2+ ions in the luminescence spectra. This specie, when present, should show narrow band-like emission from the reddish-orange to the NIR region (10), (11) in correspondence with excitation signals in the UV ( 2 P 1/2 → 2 S 1/2 transition) and in the visible ( 2 P 1/2 → 2 P 3/2 transition). Due to the action of the crystal field, the 3 P 3/2 level is splitted into two components, which gives rise, in fact, to two transitions, the highest-lying band being located usually in the violet-blue region and the lowest-lying one beyond 500 nm. These characteristic signals are absent from our spectral data. Further, as already demonstrated by Peng et. al (12) , Bi 2+ is preferably stabilized in Sr 2+ or Ba 2+ sites, which are not present in our selection of compounds. The Bi 3+ -doped vanadates, the molybdate CaMoO 4 -Bi 3+ and the two niobates LiNbO 3 -Bi 3+ and NaNbO 3 -Bi 3+ emit in the yellow region, like CaTiO 3 -Bi 3+ (3) or YVO 4 -Bi 3+ (7) . In contrast, the other niobates and the tantalate glow in the blue region.

The emission of M'-YTaO 4 -Bi 3+ was ascribed to regular A-like transition on the basis of the data reported in 1970 by Blasse and Bril (excitation at 290 nm and emission at 430 nm) (13) . This assignment will be revised at the light of the spectra displayed in Figure 1. All the materials show luminescence that differ from that of the undoped lattices. Dealing for instance with LiNbO 3 -Bi 3+ and NaNbO 3 -Bi 3+ , we know that at low temperature, LiNbO 3 has an emission at 440 nm with a corresponding excitation peaking at 260 nm while NaNbO 3 shows two emission bands at 480 and 540 nm correlated with excitation maxima at 300 and 320 nm, respectively (14) .

Calculations of MMCT and EA energies in Bi 3+ -doped transition metal oxides

In this part, the obtained results of our calculations of the energies of the A and D bands using the empirical models EF and MMCT will be compared to the experimental data which helps assigning the origin of emission in the investigated compounds. The experimental data represent the energy of the maximum of the lowest-lying excitation band in the excitation spectrum. These features are supposed to be related to Bi 3+ . They are collected from the literature and from the present work and denoted by E exp .

The energies E A (X(i)) are calculated from Equation ( 27) of Chapter 1 for each cation site X(i) of the host lattice. The MMCT energies are calculated using Equation ( 30) of Chapter 1. The results of calculations for 19 lattices are compiled in Table 2. The values of the calculated environmental factor he(X(i)) for each cation site X(i) in each host lattice, the values of the electronegativity χ CN' (M n+ ) and the shortest Bi 3+ -M n+ interatomic distances corrected for the doping effect d corr are also given in this Table 2.

The necessary values of χ CN (Bi 3+ ) can be found in Tables 9 and10 of Chapter 1. Some lattices may contain different nature of M n+ cations, for instance Ti 4+ (3d 0 ) and Y 3+ (4d 0 ) in Y 2 Ti 2 O 7 , raising therefore two possible charge transfer transitions: Bi 3+ -Y 3+ →Bi 4+ -Y 2+ and Bi 3+ -Ti 4+ →Bi 4+ -Ti 3+ . In this situation, calculations are made for both Bi 3+ → Ti 4+ and Bi 3+ →Y 3+ charge transfer transitions and the considered metal cation is specified in the third column of the table. The factor he(X(i)) is calculated only for cation sites that can reasonably be considered as doping sites for Bi 3+ . In practice, this excludes all the M n+ sites like Ti 4+ , Sn 4+ , Zr 4+ , V 5+ , Nb 5+ , Ta 5+ , Mo 6+ and W 6+ . Lattices presenting more than a single excitation signal are individuated by letters (a), (b), etc...in correspondence with the considered signal. In the case of PbWO 4 -Bi 3+ , only the lowest-lying emission at 563 nm identified recently as an excitonic (or MMCT)-like transition (15) , has been compiled in Table 2.

X (CN) M n+ (CN') d corr (Å) he (X(i)) χ CN' (M n+ ) E exp (Bi 3+ , cm -1 ) Stokes shift (cm-1 ) E A (X(i),cm -1 ) MMCT (Bi 3+ ,cm -1 ) CaTiO 3 (96258) Ca 2+ (8+4) Ti 4+ (6)
3.19 1.31 1.73 27000 (16) 9140 (16) (17) 12660 (17) 32120 CaSnO 3 (193658) Ca 2+ (8+4) Sn 4+ (6) 3.23 1.20 1.71 32825 (18) 10725 (18) (19) 34860 (b) (19) 5600 (a) (19) 14000 (b) (19) 32270 CaZrO 3 (37264) Ca 2+ (8+4) Zr 4+ (6) 3.30 1.19 1.61 32245 (18) 6690 (18) (20) 40800 (b) (20) 8900 (20) 21370 (20) 30525 LiNbO 3 (84578) (21) 31850 (b) (22) 30800 (c) * 10800 (a) (21) 9330 (b) (22) Page (21) 10000 (21) 28735 PbWO4 (56110) Pb 2+ (8) W 6+ (4) 3.79 0.96 2.59 32260 (15) 14515 (15) 32735

The results indicate that both models account for the experimental data within an accuracy of ≈ ± 4000 cm -1 , except for LiNbO 3 -Bi 3+ , NaNbO 3 -Bi 3+ and La 2 Zr 2 O 7 -Bi 3+ (b), which are much better reproduced by the MMCT model. It is worth noting that part of the inaccuracy of the methods is due to fluctuating values of experimental positions of the excitation bands as exemplified in Table 2 for YNbO 4 -Bi 3+ . In this particular case, the 3 input values (a-c) were kept to evaluate the impact of this fluctuation on the spectral assignment. Concerning YTaO 4 -Bi 3+ , we note that the calculated data match better the E exp (Bi 3+ , cm -1 ) value given in Table 2 than the value (34500 cm -1 ) reported by Blasse (21) . In this case, the error is so large (> 5000 cm -1 ) that the determination of the optical character of the involved transition cannot be achieved. It comes from this analysis that it is difficult to determine with certainty the origin of the luminescence in such compounds. To overcome this difficulty, we have investigated the Stokes shifts associated with the emissions. In the archival literature, the Stokes shift is sometimes used as a criterion to assign the origin of the emission in Bi 3+ -activated systems, the larger values being generally ascribed to D-related signals or Bi-related clusters (21) . Note that the question of Bi 3+ clusters is not considered in the present chapter. For some lattices (e.g. YNbO 4 ), several results are given in correspondence with each input value of the experimental energy. We point out that the MMCT energies calculated for Sc 3+ , Y 3+ and La 3+ give rise to high-lying excitation levels that are out of the scale displayed in Figure 2. These energies fall essentially in the region of C transitions as calculated using Equation ( 28) in Chapter 1.

Assigning the origin of luminescence in closed shell metallic oxides doped with Bi 3+ with the help of Stokes shift values

Owing to Figure 2, the large Stokes shift found for La 2 Zr 2 O 7 -Bi 3+ (b), NaNbO 3 -Bi 3+ , LaVO 4 -Bi 3+ , ScVO 4 -Bi 3+ , and LiNbO 3 -Bi 3+ is in good agreement with an emission of MMCT origin and this is confirmed by a calculated MMCT energy very close to E exp (Bi 3+ , cm -1 ) compared to E A (X(i), cm -1 ). This means that these compounds should be classified without ambiguity as D-emitting phosphors. For La 2 Zr 2 O 7 -Bi 3+ (b), LaVO 4 -Bi 3+ and ScVO 4 -Bi 3+ , this conclusion is in line with previous reports (6), (20) . The situation is more ambiguous for the other compounds, since the calculated MMCT(Bi 3+ ,cm -1 ) and E A (X(i), cm -1 ) energies are rather close. For instance, we find compounds with moderate Stokes shifts like CaZrO 3 -Bi 3+ (recognized as a regular A-emitting phosphor) (18) and CaTiO 3 -Bi 3+ (recognized as a D-like phosphor) (3) for which our calculations would lead to an opposite conclusion since the MMCT(Bi 3+ ,cm -1 ) energy is very close to E exp (Bi 3+ , cm -1 ) in the zirconate and E A (X, cm -1 ) very close to E exp (Bi 3+ , cm -1 ) in the titanate.

The same ambiguity occurs for Y 2 Ti 2 O 7 -Bi 3+ , which is recognized as a D-like phosphor (17) and for Y 2 Sn 2 O 7 -Bi 3+ (a), which is recognized as regular A-emitter (19) . In La 2 Zr 2 O 7 -Bi 3+ , the lower-lying excitation state is of A character (20) , as denoted by us as La 2 Zr 2 O 7 -Bi 3+ (a) in the present work. This assignment was based on the relatively small Stokes shift compared to La 2 Zr 2 O 7 -Bi 3+ (b) and is convincing. In this specific case, however, the error committed by both calculations exceeds ± 4000 cm -1 with respect to E exp (Bi 3+ , cm -1 ) (see Figure 2), which prevents confirming this assignment without ambiguity.

To help rationalizing the situation we have calculated the energy difference MMCT(Bi 3+ )-E A (X) separating the calculated MMCT and A states on an energy level diagram. This raises 3 predictable situations that are depicted on Figure 3 and that were previously introduced in Chapter 1: we individuate lattices for which the D (or MMCT) state is located below the A state (scheme 3(a)), lattices for which the D state is located above the A state (scheme 3(c)) and lattices for which A and D state are almost degenerated (scheme 3(b)). This is for instance the case of YVO 4 -Bi 3+ that will be investigated later in more details. Following the arguments in Chapter 1, these configurations can help knowing which states are responsible for the emission and the possible transitions we expect to see in the three situations.

Correspondingly, we show in Figure 4 a plot of the (he(X), χ CN' (M n+ )/d corr ) values for which the corresponding Bi 3+ -doped lattice is described with scheme 3(b). Here we have made a distinction between lattices in which the transition metal is tetra-coordinated (i. e. CN' = 4) and lattices in which the transition metal has larger coordination (CN' > 4) (see Table 2). Lattices positioned above the curves belong to scheme 3(a) while lattices positioned below belong to scheme 3(c). Lattices like NaNbO 3 -Bi 3+ , LiNbO 3 -Bi 3+ and ScVO 4 -Bi 3+ have their MMCT state much lower in energy than their A state (scheme 3(a))

and should behave as D-like emitters with large Stokes shifts. This is in good agreement with Figure 2.

The other lattices have their MMCT state either higher than the A state (scheme 3(c) or at similar energies (scheme 3(b)) within ± 4000 cm -1 . For these, it remains difficult to assign the origin of the signals. The only remarkable exception is La 2 Zr 2 O 7 -Bi 3+ (a) which has an unambiguous A-like character.

Criterion for the assignment of the luminescence signals in Bi 3+doped transition metal oxides

We show in Figure 5 the energy difference |MMCT(Bi 3+ )-E A (X)-ΔStokes|, expressed in absolute values. This energy difference is proposed as an empirical criterion to assign the character of the excitation signals in closed-shell transition metal oxides doped with Bi 3+ (again, regardless of the possible contribution due to clusters in these compounds). Two dotted lines appear in this diagram. The line at 4000 cm -1 indicates the higher limit for compounds recognized as A-emitters in the literature (La 2 Zr 2 O 7 -Bi 3+ (a) (20) , CaZrO 3 -Bi 3+ (18) , Y 2 Sn 2 O 7 -Bi 3+ (19) ), while the line at 9000 cm -1 marks the lower limit for compounds recognized as D-emitters ( 3),( 6),( 18), (21) . We can see that the fluctuations on the input values of E exp (Bi 3+ , cm -1 ) in YNbO 4 -Bi 3+ have finally no impact on the assignments. For the three signals (a,b,c) in this compound, the same energy configuration can be drawn ( Figure 3 (b)) and the transitions can be assigned to the D band in the three cases. At first sight, it may appear unexpected that compounds obeying scheme 3(c) behave as D-like phosphors at normal temperature. This, actually, is the consequence of the large Stokes shift of the D emission which is concomitant with large Franck-Condon offset and large force constant of the corresponding excited potential energy curve. The situation is represented schematically on the onecoordinate Mott-Seitz configurational diagram of Figure 4. Owing to this diagram, the D state can be thermally populated from the lower-lying A state by overcoming the energy barrier separating the two states (22) . At a given temperature, the efficiency of this process depends on |MMCT(Bi 3+ )-E A (X)|, on the lattice vibrational energy and on the excited states relaxation, which, unfortunately, is usually unknown.

Owing to this diagram, we can however comment the situation for five lattices located at the top of Figure 3 that are characterized by scheme 3(c). For LaVO 4 and LaNbO 4 , the Stokes shift values (Figure 2) and the maximum phonon frequency are rather high (≈ 810 cm -1 for LaNbO 4 (24) and > 900 cm -1 for LaVO 4 (25) ). Then 3 to 4 phonons are required to bridge the energy gap separating A and D states in these lattices and certainly much less to bridge the gap ΔE separating A state to its cross-over with D state (see Figure 4, right hand). Once the energy barrier from A to D is overcomed, D-like emission is observed for these compounds at room temperature. For the three perovskites CaM'O 3 (M' = Zr, Ti, Sn), the maximum phonon frequency is significantly lower (< 550 cm -1 ). More than 5 phonons are required to bridge the A-to-D gap in CaZrO 3 -Bi 3+ but less than 4 in CaTiO 3 -Bi 3+ and CaSnO 3 -Bi 3+ . This accounts for the observation of A-like emission in the former (18) and D-like emission for the latters (18), (16) . We note however that in the case of CaTiO 3 -Bi 3+ , we have |E exp (Bi 3+ )-E A (X)| < |E exp (Bi 3+ )-MMCT(Bi 3+ )| and 4000 < |MMCT(Bi 3+ )-E A (X)-ΔStokes| < 9000 cm -1 , meaning that some contribution from the A state (16) cannot be totally excluded in this compound. Further spectroscopic investigations have to be undertaken here to clarify the issue.

It is interesting, at this point, to challenge the model by investigating the complex case of Bi 26)- (31) .

We have compiled the necessary data in Table 3 for discussion. E exp (Bi 3+ , cm -1 ) 35200 (26) 35090 (27) 32800 (28) 36500 (26) 37420 (29) 36860 d (30) 37340 e ( 30), (31) Stokes shift (cm-1 ) 3950 (26) 3340 (27) 12030 (28) 3900 (26) 5160 (29) 4680 d (30) 14650 d (30) 4435 e ( 30), (31) 16130 e ( 30) , (31) E A (X,cm -1 ) 33395 33225 33922 MMCT (Bi 3+ ,cm -1 ) 39635 a 34460 b 41020 c 39305 37875

Letters a-e mark connected data.

As for Table 2, there is reasonably good agreement between calculated and experimental data. In the case of Y 3 Ga 5 O 12 -Bi 3+ , the lower lying MMCT state corresponds to a transition between Bi 3+ and Ga 3+ in six-fold coordination. Only this latter is retained in the following, the Bi 3+ →Y 3+ MMCT being located at much higher energy. The application of the criterion |MMCT(Bi 3+ )-E A (X)-ΔStokes| confirms the assignment of the UV emission to the regular A transition in the three garnets. Despite some discrepancies in the energy values, the corresponding Stokes shifts remain below 5160 cm -1 . For the larger Stokes shifts, the calculation of our criterion gives 10965 cm -1 for Y 3 Ga 5 O 12 -Bi 3+ , 8570 cm -1 for Y 3 Al 5 O 12 -Bi 3+ and 12180 cm -1 for Lu 3 Ga 5 O 12 -Bi 3+ . For all, these results confirm the assignment of the emission signals to MMCT (or exciton-like) transitions, in agreement with Refs. ( 28), ( 30) and (31).

At this stage, we conclude that the above criterion, although empirical, has some predictive value for the assignment of the emitting levels in closed shell transition metal oxides doped with Bi 3+ .

However, ambiguities still exist, e. g. in CaTiO 3 :Bi 3+ . In YVO 4 :Bi 3+ , an important luminescent compound, the criterion predicts a prominent D-like luminescence even though the A and D states are very close in energy. We found it interesting at this stage to check whether the action of high hydrostatic pressure could help lifting the degeneracy of the levels in this system and therefore clarify the situation from an experimental viewpoint. The compound CaTiO 3 :Bi 3+ would have been, in this connection, an excellent candidate, but unfortunately, its room temperature luminescence is not high enough to make accurate measurement in the anvil cell that needs to be used to apply the pressure.

Energy Level Structure of Bi 3+ in YVO4 under high hydrostatic pressure

The sample of YVO 4 -Bi 3+ was prepared by standard solid state reaction with a nominal doping rate of 1 mol % in bismuth. The sample was obtained in its zircon I4 1 /amd form (hereafter denoted as Z-YVO 4 -Bi 3+ ). The crystal building consists of chains of alternating edge-sharing VO 4 tetrahedra and YO 8 units extending parallel to the crystallographic c axis. The Y 3+ ions occupy dodecahedrally coordinated sites with nominal D 2d point symmetry. It is known that a zircon-to-scheelite phase transition occurs at 7.5

GPa in YVO 4 . The scheelite polymorph has I4 1 /a space group and is noted as S-YVO4 in the following. The crystal structure of these two polymorphs is shown in Figure 6. The phase transition from zircon to scheelite involves an expansion of the (VO 4 ) polyhedron and shrinkage of the (YO 8 ) polyhedron. This leads to a global decrease of the unit cell volume and of the interatomic distances as illustrated in Figure 7.

The effective charges of the cations are very close to the formal Y 3+ and V 5+ oxidation states in the zircon phase but not in the scheelite where the formal charge of Y is 4.13 and that of V is 4.73 (33) . In the latter case, the values indicate stronger covalency within the (YO 8 ) polyhedron and lower covalency within the (VO 4 ) polyhedron. As is argued in ref. (34) and ref. (35), this could be the consequence of the transfer of some amount of charge from the oxygen atoms. The charge transfer energy between oxygen O2p (nonbonding t 1 orbitals) and vanadium V3d (antibonding e orbitals) is then lowered, which contributes to decrease the bandgap in the scheelite compared to the zircon (34).This effect is well established and confirmed by optical measurements carried out at the absorption edge of YVO 4 with respect to pressure (Figure8) (36) . As it is clear from this figure, the variation of the bandgap is progressive in the zircon phase then very sharp across the zircon-to-scheelite phase transition (32), (36) .

Figure 8.Pressure induced variation of the absorption edge of YVO4 across zircon-to-scheelite phase transition (36) .

At last, it happens that the zircon-to-scheelite phase transition is irreversible. This will allow us the investigation of the scheelite form of YVO 4 -Bi 3+ after decompression to ambient pressure.

We have thereby investigated the evolution of the luminescence properties of YVO 4 -Bi 3+ in the pressure range 0 -20 GPa, at ambient temperature and in the temperature range 10 -500 K, at ambient pressure. Luminescence excitation spectra were acquired using a system consisting of a 150 W Xe lamp, two monochromators SPM2 and two photomultipliers (the first for the luminescence and the second for reference signal detection). Steady-state luminescence measurements were performed with an Andor SR-750-D1 spectrometer equipped with a CCD camera DU420A-OE type. Excitation source was a He-Cd laser using the wavelengths of 325 nm or 442 nm. To follow the luminescence kinetics, we used a system consisting of a PL 2143 A/SS laser and a PG 401/SH parametric optical generator. The emission signal was analyzed using a Bruker Optics 2501S spectrograph and a Hamamatsu Streak Camera model C4334-01 (37) . High hydrostatic pressure was applied in a Merrill Bassett type (38) diamond anvil cell (DAC).

Polydimethylsiloxane oil was used as the pressure-transmitting medium, and pressure was measured by the shift of the luminescence lines of ruby.

The emission was intense enough in the zircon phase (i.e. up to 7.5 GPa) to record the corresponding excitation spectra and decay profiles. The luminescence data collected in the region 7.5 -10 GPa were not retained as they contain contributions from both zircon and scheelite mixed phases.

Pure scheelite polymorph (i. e. S-YVO 4 -Bi 3+ ) was obtained above 10 GPa. In this phase, however, the emission intensity was too low to collect the excitation spectra. The temperature dependence of the emission spectra and decay profiles was recorded in the zircon phase prior application of pressure and in the scheelite phase after decompression from 20 GPa to ambient pressure.

Luminescence of Z-YVO4:Bi 3+ under pressure (P < 8GPa)

We show in Figure 9 the pressure dependence of the emission and excitation spectra of Z-YVO 4 -Bi 3+ .

The excitation spectra look very similar, but looking in more details, we find that the excitation edge (evaluated at the inflection point of the excitation profile) undergoes shifts that are not monotonous with pressure (Figure 10). This complex evolution can be understood by taking into account that the excitation features consist of overlapping transitions of different origins, both having a charge transfer character. The first CT occurs within VO 4 3-molecular units and is ascribed to the spin allowed 1 T 1 ← 1 A 1 absorption (39) . In pure YVO 4 , this transition presents an excitation edge at 29850 cm -1 in correspondence with an excitation maximum at 30770 cm -1 (7) . The energy bandgap Eg 0 has been estimated by Panchal et. al at 30570 cm -1 (3.79 eV) which is consistent with the excitation edge at ambient pressure (i.e. P ≈ 0 GPa). As reported in ref. (36), this bandgap increases under pressure following:

𝐸𝑔 𝑃 𝑍 = 𝐸𝑔 0 + 150.8𝑃 (1)
with pressure P expressed in Gpa and energies in cm -1 . The upper script Z stands for the zircon polymorph. The corresponding variation is plotted in Figure 10 with the label "Bandgap".

The second CT contributing to the excitation spectrum takes place between Bi 3+ and V 5+ and results from transitions between Bi 3+ (6s 2 )-V 5+ (3d 0 ) and Bi 4+ (6s 1 )-V 4+ (3d 1 ) configurations (i. e. Bi→V CT). The energy of this process is given by Equation (30) of Chapter 1.

For YVO 4 at ambient pressure, this Equation simplifies to: . Excitation at 325 nm, ambient pressure. The solid line is the fit of the data using Equation (4). See text for details.

These results account for a thermal quenching by cross-over to a Franck-Condon shifted state (here the Bi-V CT state) and can be reliably reproduced in the frame of the Struck and Fonger model (42) :

𝐼 𝑇 𝐼 𝐴 -∆𝐸 𝐾𝑇 - (4)
where I(T) is the emission intensity at temperature T, I 0 the intensity at the lowest temperature, A is close to 10 7 and ΔE is the activation energy separating the emitting state to its crossover with the quenching state, i. e. the 1 S 0 ground state of Bi 3+ (Figure12). An activation energy of 4560 cm -1 for Z-YVO 4 :Bi 3+ compares well with the value (viz. 4000 cm -1 ) reported previously for GdVO 4 :Bi 3+ (3) . At this stage, still questionable is the exact nature of the emitting state in Z-YVO 4 :Bi 3+ . As stated at the end of Section 3.5., the A and D states are very close from each other in the energy level diagram. To try lifting this ambiguity, we have calculated the pressure dependence of the A state energy for Bi 3+ ions accommodated in the Y 3+ sites (Viz energy E A (Y 3+ )). For completeness, we have also calculated the pressure dependence of the energy E C (Y 3+ ) of C state and of the energy of the Bi 3+ -Y 3+ MMCT. In the latter case, the lower electronegativity of Y 3+ in eightfold coordination (viz.χ 8 (Y 3+ ) = 1.28, see Appendix A) compared to that of four-coordinated V 5+ (viz.χ 4 (V 5+ ) = 2.46) will undoubtedly locate the Bi→Y CT at higher energy relative to the Bi→V CT.

The calculation of the pressure-dependence of E A (Y 3+ ) and E C (Y 3+ ) requires the calculation of the pressure-dependence of he(Y 3+ ), i. e. the calculation of the pressure dependence of all the involved parameters (see Chapter 1). This was achieved by implementing the available crystal data for each pressure in the range 0 -7.5 GPa in VESTA and running our excel macros. The input data and generated results are listed in Table 4.

(Y 3+ ), E C (Y 3+ ), Bi-V and Bi-Y energies. Pressure (Gpa) V cell (Å) Y-O (Å) V-O(Å) d corr (Å) E A (Y 3+ ) (cm -1 ) E C (Y 3+ ) (cm -1 )
Bi-YCT (cm - The pressure dependence of the energy level scheme of Z-YVO 4 :Bi 3+ and the corresponding schematic coordinate diagram plotted in Figure 13 gives a synthetic view of the situation. Especially, we can see that A and Bi-V CT states can hardly be separated by the means of hydrostatic pressure limited to 7.5 GPa, although some trend is starting to appear to the high pressure side. It is interesting to note that C and Bi-Y CT have also very similar energy. In contrast to the lower lying levels, the action of pressure tends to increase the degeneracy of these levels. The schematic one-coordinate energy diagram displayed at right hand in Figure 13 is then proposed for Z-YVO 4 :Bi 3+ . It is almost pressure independent (regardless the curvature of the parabola that may change a bit with pressure).

This situation corresponds to scheme 3(b) with prominent emission from the Bi-V CT owing to the more pronounced electron-phonon relaxation in this state of excitonic character.

Luminescence of S-YVO4:Bi 3+ under pressure (P ≥ 9GPa)

Representative emission spectra of S-YVO 4 :Bi 3+ (at high pressure, P = 20 GPa or at ambient pressure after decompression) are presented in Figure 14 upon excitation at 325 nm. The compound glowing very weakly at room temperature, the data were recorded at 10 K. They show a maximum at ≈ 14750 cm -1 (678 nm) that is nearly pressure independent. This makes a considerable red shift with respect to the yellow emission of Z-YVO 4 :Bi 3+ . The corresponding decays, measured at ambient pressure in the temperature range 10 -250 K, show a non-exponential behavior (Figure 14, inset). Excitation at 325 nm.

At low temperature, the deviation from the exponential behavior is ascribed to defects created by the action of pressure in the scheelite lattice. These lattice defects could be associated with imperfect atomic re-ordering subsequent to the reconstructive mechanism prevailing during the first order zirconto-scheelite phase transition (43) . These defects may induce perturbation in the crystal field around the Bi 3+ and/or V 5+ ions, likely deforming the potential curve describing the CT emitting state and leading to the presence of non-equivalent optical centers with different quenching behaviors.

Average values of the corresponding time constants were calculated using the expression:

𝜏 𝑎𝑣 = 𝑡𝐼 𝑡 𝑑𝑡 𝐼 𝑡 𝑑𝑡 (5)
where I(t) is the luminescence intensity at time t.

We know that the scheelite phase does not luminescence when undoped, even at 10 K (44) . This quenching is essentially due to the low bandgap value that images the low energy of the 1 T 1 ← 1 A 1 and 3 T 1,2 → 1 A 1 CT transitions within the VO 4 3-units. Following the arguments of Blasse and Bril (45) , the low Page | 107 energy position of the otherwise emitting 3 T 1,2 states accounts for the absence of luminescence in this material. Therefore, we ascribe the 14750 cm -1 emission to the Bi 3+ ions.

We show in Figure 15 the temperature dependence of the average time constant and of the emission intensity evaluated as the integrated area below the decay curves for S-YVO 4 -Bi 3+ .

Figure 15.Temperature dependence of the emission intensity and average emission time-constant in S-YVO 4 -Bi

3+ . Excitation at 325 nm, ambient pressure. These data cannot be reproduced by the only mean of Equation ( 4). This indicates that several processes contribute to the luminescence quenching in the scheelite phase. Going further in the interpretation of the above spectroscopic behaviors requires again the knowledge of the emitting level in this compound. We have therefore applied the same method as for Z-YVO 4 :Bi 3+ but now in the pressure range 9-20 GPa. In this range, the pressure dependence of the bandgap value (in cm -1 ) is given by (32) : 𝐸𝑔 𝑃>9 = 𝐸𝑔 9 -50.8(𝑃 -9). Concomitantly, the shortest Y-V distance in the scheelite polymorph varies according to: 𝑑 𝑃>9 = 3.39 -0.0046(𝑃 -9), as deduced from Figure 6. The value of d corr at ambient pressure was obtained by simple linear extrapolation. This allows calculating the pressure dependence of Bi-V and Bi-Y CT. The pressure dependence of he(Y 3+ ) and correlated E A (Y 3+ ) and E C (Y 3+ ) was obtained from the structural data listed in Table 5. The corresponding results are plotted in Figure 16.

(Gpa) V cell (Å) Y-O (Å) V-O(Å) d corr (Å) Eg P>9 (cm -1 ) E A (Y 3+ ) (cm -1 ) E C (Y 3+ ) (cm -1 ) Bi-Y CT (cm -1 )
Bi-VCT (cm -1 ) YVO 4 :Bi 3+ is now clearly defined by the vanadate groups. Consequently, the 1 S 0 ground state of Bi 3+ is now located in the valence band of the YVO 4 lattice. Dealing with the Bi 3+ -related levels, we observe a clear splitting of the levels under pressure, the A state being now located lower in energy relative to the Bi-V CT. The corresponding situation is depicted on the schematic one-coordinate energy diagram in Figure 16. Following this diagram, absorption at 325 nm takes place essentially in the Bi-V-CT state. This state is then depopulated either by cross-over to the 1 S 0 ground state (channel (1) in Figure 16) or to the lower-lying A state (channel (2) in Figure 16). At 10 K, only the latter is emitting. The lower energy of this state with respect to the Z-YVO 4 :Bi 3+ case (see Figure 13) accounts well for the observed red shift of the emission when moving from the zircon to the scheelite polymorph of YVO 4 . Further, the low-lying position in the diagram also accounts for the strong quenching of this state cross-over to the 1 S 0 ground state (channel (3) in Figure 16) as temperature is raised as clearly evidenced by Figure 15.

Conclusion

We have investigated in this chapter the origin of the emitting state in the Bi 3+ -doped closed shell transition metal oxides GdVO 4 , LaVO 4 , CaMoO 4 , LiNbO 3 , NaNbO 3 , ANbO 4 (A=Y, Gd, La), CaNb 2 O 6 , M'YTaO 4 , LiNbO 3 and NaNbO 3 . Their optical spectra were measured by photoluminescence. Using the EF and MMCT models, the calculations of the A and C bands of Bi 3+ in addition to the MMCT energies from Bi 3+ to the transition metal were made for these compounds and for other compounds described in the literature. We were able to assign the origin of emission in these compounds by comparing the data on the low-lying excitation energies with the calculated energies with the help of a proposed criterion involving the values of the Stokes shifts. We have also studied the effect of high pressure on the luminescence of YVO 4 :Bi 3+ . Using the EF and MMCT models with our experimental measurements we were able to locate the energy states of this compound in its zircon polymorph and scheelite polymorph (at pressure >9 Gpa) and study the origin of luminescence in both phases. The emission spectrum showed a red shift under the action of pressure in the zircon phase where the emitting state is the Bi→V charge transfer state and the excitation edge is governed by the Bi-V CT in the pressure range 0-5 Gpa.

The A and Bi→V CT states are located very close to each other in this polymorph which is the reason we do not observe the A band in the emission spectrum. When going from the zircon to the scheelite polymorph, the emission undergoes a drastic shift from 570 nm to 680 nm and an important splitting of the A and Bi-V CT states was observed with A being the low lying state. This makes it the emitting state in the scheelite phase and the responsible for the red shift at 10 K. This state is otherwise strongly quenched at higher temperatures.

Chapter 4: Luminescence of Sb 3+ in closed-shell transition metal oxides

Introduction

Trivalent antimony has an 5s 2 electron configuration and thereby belongs to the mercury-like family like Bi 3+ . Nevertheless, the luminescence of Sb 3+ has much less been studied than that of Bi 3+ . In this concern, the luminescence of Sb 3+ in closed shell transition metal oxides is, to our knowledge, totally unknown. The most recent literature dealing with the luminescence properties of Sb 3+ essentially date of the mid-eighties of the previous century ( 1)- (11) . These studies have demonstrated similarities of behaviors between Sb 3+ and Bi 3+ : in both cases, the luminescence shows strong dependence with temperature, excitation wavelength and nature of the nearby environment. For both, the optical properties are also affected by the stereochemical activity of the s 2 lone pair, i. e. with the degree of asymmetry of the coordination polyhedron and related Jahn-Teller effects (3)- (13) . At this stage, it should be noted that if Sb 3+ and Bi 3+ present similarities, they also have noticeable differences: (1) the former has a much smaller ionic radius and is not known with coordination numbers larger than 6 (14) . This suggests a very marked off-centered position of Sb 3+ in large crystal sites. This, in fact, accounts for the generally larger Stokes shifts of the Sb 3+ emission with respect to the Bi 3+ emission and its sensitivity to the Jahn-Teller effect. (2) The small Sb 3+ ion may also possibly substitute for transition or post-transition metal cations (M n+ ) contained in the host lattices to create antimonate units, from which luminescence may occur at low temperature. This doping possibility was discarded for Bi 3+ in the previous chapter owing to the much bigger size of this ion. Consequently, we anticipate the luminescence processes of Sb 3+ activated compounds to be even more complex to interpret than those of Bi 3+ . We Note here that we did not consider any possible luminescence activity from Sb 5+ species that could eventually be stabilized in M 5+ (i. e. Nb 5+ , Ta 5+ ) or M 6+ (i. e. Mo 6+ ) sites. To our knowledge, this activity has not yet been unambiguously demonstrated.

Among the Sb 3+ -activated phosphors, the most famous is certainly the doubly doped Sb 3+ , Mn 2+ halophosphate Ca 10 (PO 4 ) 6 (F,Cl) that shows so bright luminescence that it has been used as a commercial phosphor in lamps for decades. Sb 3+ was shown to be responsible for the blue emission and for the sensitization of the yellow emission of Mn 2+ in this material (4) .The Stokes shift of Sb 3+ in this phosphate is as large as 19000 cm -1 , which should normally be concomitant with a low quenching temperature of the corresponding luminescence (15) . To date, it is still unexplained why Sb 3+ shows so intense luminescence at room temperature in this material (16) . The accommodating site of Sb 3+ in the fluoroapatite Ca 10 (PO 4 ) 6 F 2 -Sb 3+ is also still subject of discussion. Two Ca 2+ sites Ca(1) and Ca(2) are basically available for Sb 3+ in this lattice. It was thought that Sb 3+ may substitute preferentially to Ca(2) but other studies say that it is in fact incorporated as (SbO 3 ) 3-units replacing (PO 4 ) 3-( 2), (4) . The question is still open.

Our motivation in the present context is, first, to check if Sb 3+ can show luminescence in closed shell transition metal oxides and, second, check if, by analogy with Bi 3+ , D transitions could be present in these materials. To our knowledge, such transitions have never been identified yet for Sb 3+ . In the frame of the present work, we will limit ourselves to systems in which the transition metal adopts a coordination of at least 6. This means that vanadates, molydates or tungstates with zircon or scheelite crystal structure will not be the subject of deep investigations in this chapter. In this context, the (EF)

and MMCT models will be adapted to help ascribing the origin of the luminescence signals. Hence, the environmental factors he(X) will be calculated using Equation ( 9) of Chapter 1 for all cation sites X considered as possible doping sites for Sb 3+ in all the investigated host lattices.

Two series of antimony doped compounds will therefore be investigated: the first series is composed of materials showing regular electron transitions for Sb 3+ (i. e. A-like transitions), as ascribed in the archival literature. These lattices either do not contain transition metals or contain trivalent d 0 cations for which the electronegativity χ CN' (M 3+ ) is rather small (Sc 3+ , Y 3+ , Lu 3+ and La 3+ , see Appendix A),

i.e. the corresponding MMCT energy is expected to be higher relative to the A-like transitions. In this case, we will look for an empirical equation connecting the energy of the A transition and he(X), the same as done by Wang for Bi 3+ and Pb 2+ (see Chapter 1). The second series will include the closed shell transition metal oxides that are expected to present transition with charge transfer character (i.e. D band) by referring to the supposed analogy between Bi 3+ and Sb 3+ . In this connection, we have prepared and studied the luminescence of the following materials: CaZrO 3 , SrZrO will also be reported, although not discussed in details in this manuscript.

Synthesis of closed shell transition metal oxides doped with Sb 3+

The raw chemicals and experimental conditions (temperature/duration) used for the synthesis of the Sb 3+ -doped transition metal oxides are given in Table 1. The nominal doping rate was 1 mol %. The obtained compounds were checked by XRD (X-ray diffraction) and their corresponding JCPSD numbers are shown in Table 1. Corresponding patterns are shown in Appendix C.

Luminescence of transition metal oxides doped with Sb 3+

The photoluminescence emission and excitation spectra were collected using the same set-up as in our previous studies. The spectra were corrected from the instrument response using sodium salicylate.

Owing to the low efficiency of Sb 3+ luminescence at room temperature, the spectra were collected at 77 K. At this temperature, self-luminescence of the corresponding host lattices is possible. This issue will be carefully checked on the basis of the archival literature.

Luminescence of Sb 3+ -doped Zirconates

Zirconate perovskites like CaZrO 3 and SrZrO 3 are examples of compounds in which the transition metal is tetravalent and octahedrally coordinated. Undoped CaZrO 3 is known to show optical absorption below 245 nm with a regular emission in the blue at 430 nm (17) . The situation is similar in undoped SrZrO 3 (18) . Other works report on a photoluminescence emission at 395 nm (consisting of 3 overlapping contributions) excited at 246 nm in highly crystallized SrZrO 3 (19) . Doping with Pb 2+ induces an emission in the UV (365 nm, excited at 285 nm in CaZrO 3 or 360 nm, excited at 275 nm in SrZrO 3 ) (20), (21) . These features were ascribed to A transitions. Doping with Bi 3+ leads to a UV emission in CaZrO 3 (≈ 390 nm) for a corresponding excitation at ≈ 310 nm. These features also possess an A character (22), (23) . As shown in Figure 1, the emission spectra of CaZrO 3 and SrZrO 3 doped with Sb 3+ consist of a broad green signal covering the 450 -650 nm region. This band consists of at least two overlapping contributions. Its excitation spectrum covers a wide wavelength range from ≈ 340 nm to 250 nm (instrument limit) and probably below. The maximum of the lowest-lying excitation band is ≈ 310 nm for CaZrO 3 -Sb 3+ and ≈ 305 nm for SrZrO 3 -Sb 3+ . These low-lying features are ascribed to Sb 3+ . We note here that the position of the lowest-lying excitation band in CaZrO 3 -Sb 3+ is consistent with that of CaZrO 3 -Bi 3+ . The resulting emission is however strongly red shifted, revealing a much larger Stokes shift in the case of Sb 3+ . The same is anticipated for SrZrO 3 -Sb 3+ relative to SrZrO 3 -Bi 3+ .

Luminescence of Sb 3+ -doped niobates and tantalates

Niobates and M'-type tantalate are examples of lattices in which the transition metal is pentavalent and octahedrally coordinated. In this case, the values of electronegativities χ 6 (Nb 5+ ) and χ 6 (Ta 5+ ) exceed that of χ 6 (Zr 4+ ) (see Appendix A). Like for the zirconates, the luminescence of undoped YNbO 4 , SrNb 2 O 6 and M'-YTaO 4 is of charge transfer character. For YNbO 4 , the excitation band peaks at 265 nm for an emission at 420 nm (24), (25) . We did not find information on the luminescence of undoped SrNb 2 O 6 , but we found that CaNb 2 O 6 shows an emission at 500 nm upon excitation at 275 nm (26) . SrNb 2 O 6 should behave in a comparable way. For M'-YTaO 4 , the excitation peaks at 230 nm for an emission at 340 nm (25) . Doping with Bi 3+ leads to a band-like emission in the blue (≈ 475 nm) in YNbO 4 for a low-lying excitation reported at 310 (27) , 314 (28) or 324 nm (see Chapter 3). M'-YTaO 4 -Bi 3+ glows in the green region (515 nm) for an excitation peaking at 343 nm. To our knowledge, the luminescence of SrNb 2 O 6 -Bi 3+ has not been reported yet, but CaNb 2 O 6 -Bi 3+ has been shown in Chapter 3 to emit at 490 nm for an excitation peaking at 325 nm. Doping with Sb 3+ yields broad emissions that appear to be red shifted with respect to Bi 3+ (Figure 2).

In the niobates, these bands consist of at least two overlapping contributions, as evidenced by the corresponding excitation spectra (* and ¤). The features located in the red spectral region (¤) are excited at ≈ 335 nm. They are ascribed to Sb 3+ . The other feature in the green region (*) are excited at ≈ 280 nm and are presumably host-related. In YTaO 4 -Sb 3+ , the red band observed in Figure 2 and its corresponding excitation are ascribed to Sb 3+ .

Luminescence of Sb 3+ -doped molybdates

Molybdates SrMoO 4 and CaMoO 4 are examples of compounds in which the transition metal is octavalent and tetrahedrally coordinated. Undoped CaMoO 4 shows an emission at 534 nm when excited with 260 nm and another one at 546 nm when excited with 320 nm (29) . Undoped SrMoO 4 shows an emission peaked at 429 nm when excited with 288 nm. Both compounds when doped with Sb 3+ show very broad emission bands covering the blue, green and red regions from 450 to 750 nm. In SrMoO 4 :Sb 3+ , the emission band is peaking at ≈560 nm upon excitation at 300 nm (Figure 3). This could be host-related. The situation is different when the exciting wavelength is increased. At 330 nm excitation, the band has its maximum at ≈620 nm, while with 360 nm excitation the maximum emission is centred at ≈690 nm. These features are ascribed to Sb 3+ . The excitation was recorded for the monitored emission wavelength of ≈ 620 nm and shows a doublet structure with a maximum at ≈ 310 nm and a shoulder at 326 nm. This is related to Sb 3+ . This behaviour was observed in other Sb 3+ -doped compounds for which the excitation band shows a doublet or a triplet structure. This was ascribed to the crystal field splitting of the 3 P 1 level in some cases (i. e. in ScBO 3 and LuBO 3 ) and to a Jahn-Teller effect in other cases (i. e. in CaS, MgS, in the elpasolites Cs 2 NaMCl 6 with M = Sc, Y, La and in the zircon-structured phosphate MPO 4 with M = Sc, Lu, Y) (3),( 5), (6) . In CaMoO 4 :Sb 3+ , the emission is maximum at ≈580 nm. The excitation was measured at ≈ 625 nm and concern lattices from CaS to LaBO 3 in Table 2. For these (except AlPO 4 (2) ), the lowest lying excitation is nominally ascribed to A transitions. LaOCl-Sb 3+ shows two emissions, a violet one ascribed to single Sb 3+

ions and a green one ascribed to Sb 3+ dimers (34) . Only the former was included in Table 2.

Establishing an empirical equation for A band transitions

We give in Table 3 the experimental energy values E exp of the lowest-lying excitation bands that are ascribed to A-like transitions in the literature.

Host

Cation site (CN) E exp (cm -1 ) E A (X), cm -1 CaS Ca 2+ (6) 25000 (6) 25160 MgS Mg 2+ (6) 24100 (6) 31250 LaOCl La 3+ (9) 40000 (7) 7875 Cs 2 NaLaBr 6

Cs + (12) Na + (6) La 3+ ( 6) 27300 (11) 6) Y( 4) 3+ ( 6) 42500 (1) LuP 3 O 9 Lu(1) 3+ ( 6) Lu( 2) 3+ ( 6) Lu(3) 3+ ( 6) 40500 (1) GdP 3 O 9

Gd 3+ (8) 41000 (1) LaP 3 O 9 La 3+ (8) 41500 (1) Sr 3 (PO 4 ) 2 Sr(1) 2+ ( 12) Sr( 2) 2+ ( 10) 40000 (8) Ca 10 (PO 4 ) 6 F 2 Ca(1) 2+ ( 9) Ca( 2) 2+ ( 7)

38365 (2) ScBO 3
Sc 3+ (6) 32700 (5) LuBO 3 Lu 3+ (6) 33400 (5) LaBO 3 La 3+ (9) 42000 (5) sh Plotting E exp (Sb 3+ ) against he(X) reveals a linear decreasing variation but with an important dispersion of the data (Figure 4). Several factors may account for that:

1) The relatively low accuracy of the energy position of the lowest excitation band in the considered Sb 3+ -doped systems; this position being collected either at liquid helium, liquid nitrogen or room temperature, depending on the reports.

2) The fact that Sb 3+ is, in most of the cases, incorporated in cation sites of larger size relative to its ionic radius. This possibly induces an off-centering of the Sb 3+ ions and creates uncertainties in the determination of interatomic distances which, in turn, affect the values of he(X).

3) The nominal attribution of the optical transitions could be incorrect.

4) The fact that the calculated energies are carried out for all possible cation sites for which Sb 3+ may substitute without any preconceived idea. Obviously, not all the considered sites are effectively occupied by Sb 3+ in the material.

Considering these issues, we have decided to perform the fitting of the data within an accuracy of ± 2500 cm -1 which looks, in these circumstances, acceptable in a first approximation. Sites located out of this interval are therefore considered as not currently occupied by Sb 3+ in the host lattice. Taking this assumption, the best fitting was achieved using the following empirical linear equation (in cm -1 ):

Establishing an empirical equation for MMCT (or D-like) transitions

By reference to Equation (30) in Chapter 1, we tentatively introduce the following empirical equation to account for the Sb 3+ →M n+ MMCT energy in closed shell transition metal oxides:

𝑀𝑀𝐶𝑇 𝑆𝑏 3+ -𝑀 𝑛+ = 𝑘 𝐶𝑁′ 𝜒 𝑆𝑏 3+ -𝛼 𝐶𝑁′ 𝐶𝑁 𝜒 𝐶𝑁 ′ 𝑀 𝑛 + 𝑑 𝑐𝑜𝑟𝑟 (𝑋) (2)
where the parameters herein have the same meaning as in Chapter 1. In the present work, this equation will be applied to compounds in which CN' ≥ 6, i. e. the transition metal cations occupy 6 (or more)coordinated sites. We are then dealing with zirconates, niobates and lattices containing Sc 3+ , Y 3+ , Lu 3+

and La 3+ , which are also d 0 elements. As stated before, we anticipate that the D transitions in these materials should be located at higher energy than A transitions.

The crystal radius of Sb 3+ used for the calculation of d corr is known for coordination numbers lower or equal to 6 only. For this reason, this value (0.90 Å) (14) will be used each time we have CN ≥ 6 or CN' = 6 in case of an insertion of Sb 3+ in the M n+ sites like Zr 4+ , Nb 5+ or Ta 5+ . For the same reason, we will only retain one value for the electronegativity of Sb 3+ :𝜒(𝑆𝑏 3+ ) = 1.476 (37) , i. e. the value in correspondence with a 6-fold coordination. Therefore, 2 parameters need to be determined to run Equation (1): 𝑘 𝐶𝑁 ′ and 𝛼 𝐶𝑁′ 𝐶𝑁 .

They were obtained by Plotting E exp against

𝜒 𝐶𝑁 ′ 𝑀 𝑛 + 𝑑 𝑐𝑜𝑟𝑟 (𝑋)
for all selected lattices and retaining data located within an interval of uncertainty of ± 2500 cm -1 . Doing so, we ended up on 𝑘 𝐶𝑁 ′ = 36900 cm -1 (with CN'≥6) and 𝛼 𝐶𝑁′ 𝐶𝑁 = 1.22 (whatever CN and CN' ≥ 6), which finally gives the empirical equation:

𝑀𝑀𝐶𝑇 𝑆𝑏 3+ -𝑀 𝑛+ = 54243 -45018 𝜒 (𝑀 𝑛 + ) 𝑑 𝑐𝑜𝑟𝑟 𝑋 (3)
Note that this equation is basically valid for oxides. It will not be used to calculate the MMCT energies in oxygen-free lattices.

Origin of the photoluminescence excitation in Sb 3+ -doped closed shell transition metal oxides

We will now discuss the nature of the absorbing states of Sb 3+ in closed shell transition metal oxides on the basis of the two models described above. To facilitate the discussion, we have plotted in Figure 5 the difference in absolute values between E exp (Sb 3+ ) and E A (X), on one side and between E exp (Sb 3+ ) and MMCT(Sb 3+ ) on the other side.

The vertical dotted lines figurate the uncertainty interval ± 2500 cm -1 . Data beyond these limits should be considered as meaningless. This is typically the case for the Y 3+ site in YNbO 4 and YTaO 4 and for the Zr 4+ site in CaZrO 3 . In other words, our calculations suggest that Sb 3+ is not incorporated in these sites of the corresponding lattices. Some other lattices are borderline with respect to the ± 2500 cm emphasize that the A transition is predicted at 48690 cm -1 from Equation ( 1). This value is in fact very close to the position of the major absorption band in this material (48000 cm -1 ), which suggests correspondence, in good agreement with ref. (38). In this case, we would ascribe the shoulder signal at ≈ 42000 cm -1 to a D band.

In the niobates and tantalates, the low-lying excitation features are undoubtedly ascribed to D transitions for Sb 3+ ions located preferentially in the six-fold coordinated Nb 5+ or Ta 5+ sites. For the zirconates, Sb 3+ seems more probably incorporated in the divalent alkaline-earth sites where it promotes the formation of a low-lying D state. Occupancy of the Zr 4+ site seems excluded in CaZrO 3 but not in SrZrO 3 , which is somewhat puzzling. This particular point requires deeper investigation.

Conclusion

We have started the investigation of the luminescence properties of Sb 3+ in closed shell transition metal oxides and have confirmed that the observed features are related to the dopant. The corresponding low lying excitation signals have tentatively been ascribed on the basis of two empirical equations allowing us to calculate the energy of A-like and D-like transitions from the knowledge of the crystal structure. The accuracy of the method is however not good enough today to make a robust diagnosis. This is certainly due to the tendency of the small Sb 3+ ion to locate in off-centered position with respect to the well-defined crystallographic sites. This led to unknown alterations of the interatomic distances, which in turn alter the accuracy of the calculations. At present, only trends are evidenced. Going deeper in the accuracy requires further refinements of the equations based on a more accurate description of the immediate chemical surrounding of Sb 3+ in the host lattices.

General Conclusion

LED-based lighting technology has witnessed important progressions in the last decades especially in the design of phosphors for warm white light. However, the materials found in the LED market today are strongly dependent on temperature and present some disadvantages like low color rendering index or low efficiency. This requires the development of new broad band emitting materials that can be excited in the UV region and emit in the visible region. For this reason, we have studied friendly and easy to use empirical methods for the design of band-like emitting compounds doped with Eu 2+ which is widely used for LED lighting purposes, or mercury like ions (Bi 3+ and Sb 3+ ). Trivalent bismuth, in addition to being a good sensitizer for trivalent lanthanides, is interesting in an economical point of view with respect to rare earths. Sb 3+ , however is not widely studied as for Bi 3+ but could present important properties to be explored following the analogy with Bi 3+ .

Our results showed that the environmental factor model is reliable for designing A I B II PO 4 -Eu 2+ phosphors where A is a monovalent and B is a divalent cation. Using the model we were able to identify the doping sites in this family of phosphates within an error of ±1000 cm -1 . The model was found efficient to predict the nature of the doping sites that contribute to the excitation edges in a series of glowed in the blue region. The study of high pressure YVO 4 :Bi 3+ showed important red shift in the emission energy from 570 nm at ambient pressure (in the zircon phase) to 680 nm at high pressure (in the scheelite phase). With the help of the models, we showed that the energy bands A and D are very close in the zircon form and undergo important splitting under the action of high pressure. This helped interpreting the pressure induced red shift of Bi 3+ emission to the fact that the emitting level is of A character in the scheelite form, whereas it is of D character in the zircon form.

The luminescence measurements of a group of Sb 3+ -doped closed shell transition metal oxide compounds (CaZrO 3 , SrZrO 3 , YNbO 4 , SrNb 2 O 6 , M'-YTaO 4 , CaMoO 4 and SrMoO 4 ) showed broad bands in the visible region ascribed to Sb 3+ . The identification of the luminescence character in these compounds and in other compounds from the literature were tentatively made using two empirical equations proposed in this work on the basis of the EF and MMCT models. These equations, however, were not considered accurate enough due to the important crystallographic changes when Sb 3+ is introduced in these compounds mainly the off-centered positions usually occupied by this ion. Additional work is then required to clarify the situation. 1 0 0 0 0 1 O2 0 0 3 1 0 0 0 0 1 O3 1 1 0 0 1 0 1 0 0 O4 0 1 0 0 0 1 1 0 0 O5 1 0 1 0 0 0 0 1 O6 0 2 0 1 0 0 0 0 1 O7 0 0 3 3 0 0 1 0 0 1 0 O8 0 0 3 3 0 0 0 1 0 0 0 1 O9 1 0 0 1 0 0 0 1 O10 1 0 0 0 0 1 0 0 0 1 O11 1 1 0 1 0 0 1 0 O12 2 0 1 0 0 1 0 0 O13 0 0 3 0 1 0

1 O14 1 1 1 1 O15 0 2 1 0 1 O16 0 1 3 1 1 K1
1 0 1 0 1 0 0 0 1 1 1 2 1 K2 0 0 1 1 0 2 0 0 0 0 1 0 1 2 1 K3 0 1 1 1 1 0 0 0 2 1 1 1 1 K4 1 1 K5 1 1 0 1 K6 1 1 1 Zn1 1 1 0 0 1 0 0 0 0 0 0 1 Zn2 0 0 0 0 0 1 0 0 1 0 1 0 1 Zn3 0 0 1 0 0 0 1 0 0 0 0 0 1 1 Zn4 0 1 0 1 1 0 0 1 P1 0 0 1 1 0 0 0 0 0 0 0 1 0 1 0 0 P2 0 0 0 0 0 0 1 0 0 0 1 0 1 1 P3 1 1 0 0 1 0 0 0 1 0 0 0 P4

1. 6 . 2 . 3 . 2 . 4 . 2 . 5 .Chapter 3 :

 62324253 Semi-empirical models ..................................................................................................................... 1.6.1. Importance of semi-empirical models ....................................................................................... 1.6.2. Overview of some theoretical models ....................................................................................... 1.6.3. The Environmental factor (EF) model........................................................................................ 1.6.4. MMCT model ............................................................................................................................. Bibliography ............................................................................................................................................................................ Chapter 2 The doping sites in A I B II PO4:Eu 2+ and their consequence on the photoluminescence excitation spectra .................................................................................................................................................................. 2.1. Introduction ..................................................................................................................................... 2.2. Synthesis and structural/microstructural characterizations of NaZnPO4:Eu 2+ ................................ Luminescence properties of NaZnPO 4 :Eu 2+ ..................................................................................... Calculation of the environmental factors in A I B II PO 4 lattices .......................................................... Doping sites in A I B II PO 4 :Eu 2+ ............................................................................................................. 2.6. Designing A I B II PO 4 -Eu 2+ phosphors for solid state LED-based lighting ............................................. 2.7. Conclusion ........................................................................................................................................ Bibliography ............................................................................................................................................................................ The origin of luminescence in Bi 3+ -doped closed transition metal oxides ............................. 3.1. Introduction ..................................................................................................................................... 3.2. Synthesis and photoluminescence of closed transition metal oxides doped with Bi 3+ ...................

Figure 1 .

 1 Figure 1.Absorption and emission of energy. E 1 is the ground state and E 2 is the excited state. hν is the energy absorbed or emitted.

Figure 3 .

 3 Figure 3.Wavefunctions in the vibrational levels of the ground state. It is maximum at the equilibrium position in the first vibrational levels, whereas in higher levels it is maximum at the edges of the parabola.

Figure 4 .

 4 Figure 4.Effect of interactions on the spectral shape, in a) ΔR=0 results in a sharp line, while in b) a broad band appears because ΔR>0.

Figure 6 .

 6 Figure 6 .Illustration of internal relaxation. The red and black arrows indicate the absorption and emission respectively. The blue arrow shows the crossing from the higher excited state e' to the lowest excited state e.

Figure 7 .

 7 Figure 7.Scheme showing the locations of shallow and deep traps of impurities in the band gap. CB is the conduction and VB is the valence band. Black filled circles represent the electrons and white circles represent holes. Shallow traps are located near the CB or VB. Deep traps are located deep inside the band gap.

Figure 8 .

 8 Figure 8.Energy transfer from the activator to the activator leading to luminescence.

Figure 9 .

 9 Figure 9.Crystal field splitting of d levels in a)8-fold b)12-fold c)4-fold d)spherical and e)6-fold symmetries.

Figure 10 .

 10 Figure 10.Ground state 4f n and excited state 4f n-1 5d of divalent lanthanides.

Figure 11 .

 11 Figure 11.Energy splitting of 5d states and the lowering of their barycenter of energy Ec of a lanthanide in solid having cubic symmetry with respect to the free ion.

Figure 12 .

 12 Figure 12.Location of 5d and 4fn states of different divalent lanthanide ions in CaF 2 . The abscissa n indicates the number of electrons in the 4f shell(28)

Figure 13 .

 13 Figure 13.Locations of 5d and 4f n energy levels of divalent and trivalent lanthanides in CaF 2 . The horizontal red line represents the 5d levels of the divalent Ln 2+ (28)

Figure 14 .

 14 Figure 14.Energy levels of s 2 free ions. A, B and C represent the possible optical transitions.

Figure 15 .

 15 Figure 15.The 3 possible transition bands of Bi 3+ indicated by blue arrows A,B and C. The MMCT charge transfer transition from Bi 3+ to the conduction band of the host indicated by the vertical black arrow. The ground state of Bi 2+ in red(40)

Figure 16 . 3 P 1 ,

 1631 Figure 16.Three different configurations for the position of the MMCT state with respect to the 3 P 1,0 states of Bi 3+ in oxides: a)MMCT is lower than 3 P 1,0 b)MMCT and 3 P 1,0 are equal, it is the case where 3 P 1,0 are located near the conduction band c)MMCT is higher than 3 P 1,0 .

Figure 17 .

 17 Figure 17.Luminescence spectra of YPO 4 :Bi3+ at various temperatures(41)

Figure 18 .

 18 Figure 18.Stucture of a LED chip.

Figure 19 .

 19 Figure 19.Different approaches for obtaining white LEDs

Chapter 2 .

 2 To individuate the crystallographic sites (i= 1, 2, ..., n) that may be occupied by the cations and anions in the crystal structure, the compound will be reformulated as 𝐴(𝑖) 𝑎 𝑖 𝐵(𝑖) 𝑏 𝑖 𝑃(𝑖) 𝑝 𝑖 𝑂(𝑖) 𝑜 𝑖 , where 𝑎 𝑖 = 𝑚 𝑖 (𝐴)/𝑍 , 𝑏 𝑖 = 𝑚 𝑖 (𝐵)/𝑍 , 𝑝 𝑖 = 𝑚 𝑖 (𝑃)/𝑍 , 𝑜 𝑖 = 𝑚 𝑖 (𝑂)/𝑍 and

  )P(1)O(1)O(2)2O(3) in the following sum of binary units:

( 15 )

 15 𝑁 (𝐴(𝑖)-𝑂(𝑖)) = 𝛼 𝛼 +𝛽 𝑁 𝐴(𝑖) + 𝛽 𝛼+𝛽 𝑁 𝑂 𝑖 (16)

  compiles another set of input data obtained from VESTA. It includes the distances of all the bonds and the charge of the cation site in each bond in KSrPO 4 . The calculations of all parameters are then done automatically by the program and the two factors he and Fc are obtained for each cation site in the KSrPO 4 crystal (Table

Figure 20 .

 20 Figure 20.Correlation between the environmental factor he and the centroid energy of the 5d levels of a)Ce 3+ and b)Eu 2+ in various cubic lattices

Figure 21 .

 21 Figure 21.Correlation between the environmental factor Fc and the crystal field splitting 10 Dq of a)Ce 3+ and b)Eu 2+ in cubic halides

Figure 22 .

 22 Figure 22.Correlation between the environmental factor he and the energy of the excitation bands (a) A and C of Bi 3+ and of (b) Pb 2+ (A band only) in different types of crystals.

( 60 )

 60 ,(61)

Figure 23 .

 23 Figure 23.Variation of the Bi 3+ → M n+ MMCT energy against the ratio 𝝌 𝑪𝑵 ′ (𝑴 𝒏+ ) 𝒅 𝒄𝒐𝒓𝒓 in Bi 3+ -doped transition metal oxides. Solid lines correspond to Equation (30) and appropriate parameters given by Tables9 and 10.

Figure 1 .

 1 Figure 1.Excitation spectra of ABPO 4 :Eu2+ phosphors(14)

Figure 2 .

 2 Figure 2.High temperature X-ray diffraction patterns of NaZnPO 4 :Eu 2+ . The heating rate was 1°C/min and the dwell time was 2 and half hours.

Figure 3 .

 3 Figure 3.XRD pattern of NaZnPO 4 .Eu 2+ collected at room temperature .

Figure 4 .

 4 Figure 4.Crystal structure of monoclinic NaZnPO 4 (P 2 1 /n)

Figure 5 .

 5 Figure 5.SEM image for NaZnPO 4 :Eu 2+ .

Figure 6 .2. 3 .

 63 Figure 6.SEM image highlighting the distribution of each atom in the obtained powder of NaZnPO 4 2.3. Luminescence properties of NaZnPO4:Eu 2+

Figure 7 .

 7 Figure 7.Experimental set-up for steady-state luminescence measurements.

Figure 8 .

 8 Figure 8.Photoluminescence excitation (λ em = 520 nm, solid line) and emission (λ exc = 370 nm, dotted line) spectra of NaZnPO 4 -Eu 2+ at 77K

Figure 9 .

 9 Figure 9.Energy levels of Eu 2+ in a cubic crystal. E c is the barycenter energy and E fd represents the 4f 7 → 4f 6 5d 1 transition.

Figure 10 .

 10 Figure 10 .Difference between calculated and experimental excitation edge energies for different cation sites X(i) in A I B II PO 4 :Eu 2+ .

Figure 13 .

 13 Figure 13.Emission spectrum of a blue LED.

Figure 14 .

 14 Figure 14.Calculated position of the excitation edge of different cation sites in A I B II PO 4 lattices. The horizontal bars mark the ± 1000 cm -1 uncertainty on the values. The vertical bars mark the spectral output of blue, violet and near UV LEDs.

Figure 1 .

 1 Figure 1.Photoluminescence excitation (left) and emission (right) spectra of some closed-shell transition metal oxides doped with Bi 3 .

Figure 2

 2 Figure 2 shows the difference, in absolute scale, between experimental and calculated energies of E A and MMCT given as |E xp (Bi 3+ )-E A (X)| and |E xp (Bi 3+ )-MMCT(Bi 3+ )| for each investigated compound. The compounds in this figure are ordered along with increasing values of the Stokes shifts of their emission.

Figure 2 .

 2 Figure 2.Left hand: difference, in absolute values, between experimental (E exp (Bi 3+ )) and calculated (MMCT(Bi 3+ ) or E A (X)) energies in some closed-shell transition metal oxides doped with Bi 3+ . Right hand: corresponding Stokes shift values.

Figure 3 .

 3 Figure 3.Difference between calculated (MMCT(Bi 3+ ) and E A (X)) energies in some closed-shell transition metal oxides doped with Bi 3+ .

Figure 4 .

 4 Figure 4.Left hand: he(X) and χ CN' (M n+ )/d corr values for which the corresponding Bi 3+ -doped closed shell transition metal oxides are described with scheme 3(b). Right hand: schematic one-coordinate configuration diagram corresponding to scheme 3(c).

Figure 5 .

 5 Figure 5.The proposed criterion for the assignment of the optical transitions (see text for details).

Figure 6 .

 6 Figure 6.Crystal structure of YVO 4 in the (a) zircon and (b) scheelite polymorphs.

Figure 7 .

 7 Figure 7.Pressure dependence of the unit cell volume (left hand side) and of the interatomic distances (right hand side) in Z-YVO4 and S-YVO4

Figure 9 .

 9 Figure 9.Pressure dependence of the emission and excitation spectra of Z-YVO 4 -Bi 3+ . Pressures are mentioned on the graph. Excitation at 325 nm.

Figure 10 .

 10 Figure 10.Energy position of the bandgap, the Bi-V CT, the excitation edge and the emission maximum in Z-YVO 4 -Bi3+ . Excitation at 325 nm

Figure 11 .

 11 Figure 11.Temperature dependence of the emission time-constant and integrated emission area in Z-YVO 4 -Bi3+ . Excitation at 325 nm, ambient pressure. The solid line is the fit of the data using Equation(4). See text for details.

Figure 12 .

 12 Figure 12.A schemetic coordinate diagram representing the activation energy separating the emitting state with its crossover to the quenching ground state 1 S 0 of Bi 3+ in Z-YVO 4 .

Figure 13 .

 13 Figure 13.Left hand: Pressure dependence of the calculated energies Bi 3+ -Y 3+ MMCT, Bi 3+ -V 5+ MMCT, E A (Y 3+ ) and E C (Y 3+ ). Right hand: Corresponding schemetic diagram of the position of the different states of Bi 3+ in Z-YVO 4 .

Figure 14 .

 14 Figure 14.Emission spectrum (T = 10 K) and temperature dependence of the emission decays of S-YVO 4 -Bi3+ .

Figure 1 .

 1 Figure 1.Excitation (left) and emission (right) spectra of the Sb 3+ -doped zirconates at 77 K. Symbols * indicate the monitored wavelengths for the excitation spectra. Emission was collected upon excitation at 325 nm.

Figure 2 .

 2 Figure 2.Excitation (left) and emission spectra (right) of the Sb 3+ -doped niobates and tantalate at 77 K. Symbols * and ¤ indicate the monitored wavelengths for the excitation spectra. Emission was collected upon excitation at 330 nm.

Figure 3 .

 3 Figure 3.Excitation (left) and emission (right) of Sb 3+ -doped molybdates at 77 K. The symbol * indicate the monitored wavelength for the excitation spectra. Emission corresponding to, dashed line, solid line and dotted line were recorded upon excitation at 300, 330 and 360 nm respectively in SrMoO 4 :Sb 3+ .

Figure 4 .

 4 Figure 4.Plot of the structural parameter he versus the experimental energies of Sb 3+ doped-materials, E exp , identified as A bands in the literature.

Figure 5 .

 5 Figure 5.Difference in absolute values between the experimental energies E exp (Sb 3+ ) and the calculated E A (X) or MMCT(Sb 3+ ) for a set of Sb3+ -doped compounds.

Figure 24

 24 Figure 24 XRD pattern of SrZrO 3 -Sb 3+ with JCPDS standard 01-076-9442

Figure 26 XRD

 26 Figure 26 XRD pattern of SrNb 2 O 6 -Sb 3+ with JCPDS standard 45-0277

Figure 28 Figure 29

 2829 Figure 28 XRD pattern of CaMoO 4 -Sb 3+ with JCPDS standard 7-0212

Table of Contents

 of General Introduction ............................................................................................................................................................ Chapter 1: State of the art .................................................................................................................................................. 1.1. Introduction to luminescence and its applications .......................................................................... 1.2. Basic principles of luminescence ..................................................................................................... 1.3. Effect of crystal structure on luminescence .................................................................................... 1.4. Spectroscopy of important luminescent activators ......................................................................... 1.4.1. 4f elements ................................................................................................................................

1.4.2. Transition metal ions ................................................................................................................. 1.4.3. Mercury like ions (s 2 ) ................................................................................................................. 1.5. LED-based lighting materials ...........................................................................................................

Table 2

 2 Stokes shift of Bi 3+ in different hosts.

	(1)

Table 4

 4 Coordination matrix of KSrPO 4

		K(1)	Sr(1)	P(1)	O(1)	O(2)	O(3)
	O(1)	3	2	1
	O(2)	6	4	2
	O(3)	1	3	1
	K(1)				1	2	1
	Sr(1)				2	2	2
	P(1)				1	1	1
	Total	10	9	4	4	5	4

Table 7

 7 Input distances, V cell and Q A for each bond in the KSrPO 4 crystal.

Table 8

 8 Obtained set of parameters involved in the calculation of the environmental factors he and Fc of KSrPO 4 by the excel program. Only Sr and K sites were considered here.

NaZnPO4:Eu 2+ NaZnPO

  4 :Eu 2+ was prepared by standard solid state method using Na 2 CO 3 (ACROS,>99.6%) as the alkaline source, NH 4 H 2 PO 4 (Sigma-ALDRICH, >98.5%) as the phosphate source, Zn(NO 3 ) 2 6H 2 O (ACROS, 98%) as the zinc source and Eu 2 O 3 (PURATREM, 99.99%) as the dopant source. The nominal doping rate was 1 mol%. The reagents were homogeneously mixed in stoichiometric proportions with the presence of ethanol. The mixtures were dried at 80°C and heated at 300 °C in air for 15h. An amount of the sample was used to obtain the high temperatures X-ray diffraction (HTK) patterns by using an X-pert Pro diffractometer (Panalytical) with Cu anode. The patterns collected in the temperature 650 °C to 1010 °C are reproduced in Figure

Table 1

 1 Structural characteristics and corresponding environmental factors for a selection of A I BII PO 4 lattices. N represents the coordination number of the considered cation site. D.I. is the distortion index of the cation polyhedron.

	ABPO 4	ICSD	Space group	Cation site	N	Point	D.I.%	Fc(X(i))	he(X(i))
				X(i)		group	(X(i))
	CsMgPO 4	260423 Pnma	Cs + (1)	11	Cs	4.8	0.18	1.28
	LiMgPO 4	201138 Pnma	Li + (1)	6	Ci	1.4	0.36	0.74
				Mg 2+ (1)	6	Cs	2.4	1.06	1.13
	LiCaPO 4	66387	P3 1 c	Ca 2+ (1)	8	C1	6	0.60	1.28
	LiSrPO 4	Ref. (25) P6 5	Sr 2+ (1)	8	C1	4.6	0.38	2.10
	α-NaMgPO 4 59249	P2 1 2 1 2 1	Na + (1)	8	C1	8.4	0.22	0.75
				Na + (2)	8	C1	9.2	0.21	0.58
				Na + (3)	8	C1	9	0.16	0.98
				Mg 2+ (1)	6	C1	11.5	0.88	1.39
				Mg 2+ (2)	6	C1	5.4	0.88	1.25
				Mg 2+ (3)	6	C1	10.3	0.70	0.80
	NaCaPO 4	35629	Pnma	Na + (1)	8	C1	7.4	0.21	0.84

calculated for the Na + sites of NaZnPO 4 is representative of the largest distortion among all the investigated ABPO 4 lattices.

  Table 2 the experimental values of the energies E exp corresponding to the photoluminescence excitation edge in the members A I B II PO 4 :Eu 2+ whose optical properties have been already investigated in the literature or in the present work (i.e. NaZnPO 4 :Eu 2+ ). For each cation site X(i) that can potentially accommodate Eu 2+ in the considered A I B II PO 4 lattice, we give the energy values

Table 2

 2 Experimental and calculated spectroscopic properties of Eu 2+ -doped ABPO 4 lattices.

  This concerns the Ca 2+ sites of NaCaPO 4 and LiCaPO 4 , the Na + sites of NaZnPO 4 , NaBaPO 4 and NaMgPO 4 (glaserite form), the K + sites of KMgPO 4 and KBaPO 4 , the Cs + site of CsMgPO 4 , both Li + and Sr 2+ sites in LiSrPO 4 and both Rb + and Ba 2+

. Such kinds of interactions cannot reliably be reproduced with Equation

(2)

. The two vertical dotted lines represent the error of ±1000 cm -1 . This accuracy represents approximately the energy of one lattice phonon and is in first approximation acceptable. The cation sites contained inside these two lines are well reproduced by the model and therefore considered as possible doping sites for Eu 2+ .

Table 1

 1 Raw materials and experimental conditions used for the synthesis of several closed-shell transition metal oxides doped with Bi 3+ .

	Host lattice Raw chemicals (analytical grade) Temperature (°C) / duration (h)	JCPDS standard
	GdVO 4	Gd 2 O 3 , V 2 O 5	650 / 6	17-0260
	LaVO 4	La(OH) 3 ,V 2 O 5	1100 / 24	23-0324
	CaMoO 4	CaCO 3 , Mo 2 O 3	780 / 3	29-0351
	LiNbO 3	Li 2 CO 3 , Nb 2 O 5	1200 / 4	20-631
	NaNbO 3	Na 2 CO 3 , Nb 2 O 5	1200 / 4	33-1270
	YNbO 4	Y 2 O 3 , Nb 2 O 5	1200 / 4	23-1486
	GdNbO 4	Gd 2 O 3 , Nb 2 O 5	1200 / 4	22-1104
	LaNbO 4	La(OH) 3 , Nb 2 O 5	1200 / 4	22-1125
	CaNb 2 O 6	CaCO 3 , Nb 2 O 5	1200 / 4	39-1392
	M'-YTaO 4	Y 2 O 3 , Ta 2 O 5	1200 / 12	72-2018

Table 2

 2 Results of the calculations of E A and MMCT energies. X is the doping site for Bi 3+ in coordination CN. * this work

	Host lattice
	(ICSD)

  3+ -doped garnets. Host lattices like Y 3 Ga 5 O 12 , Y 3 Al 5 O 12 and Lu 3 Al 5 O 12 show both UV and visible emissions whendoped with Bi 3+ . If the UV emission is always ascribed to regular A-like transition, the assignment of the visible emission is still subject of discussion; it is ascribed either to dimers or to bound excitons(

Table 3

 3 Experimental (E exp (Bi Some useful parameters required for the calculations are also provided.

		Y 3 Ga 5 O 12	Y 3 Al 5 O 12	Lu 3 Al 5 O 12
	X (CN)	Y 3+ (8)	Y 3+ (8)	Lu 3+ (8)
	M n+ (CN')	Y 3+ (8) a	Y 3+ (8)	Lu 3+ (8)
		Ga 3+ (6) b
		Ga 3+ (4) c
	d corr (Å)	3.50 b 3.82 a	3.74	3.73
		3.14 c
	he (X)	0.92	0.93	0.89
	χ CN' (M n+ )	1.58 b 1.29 a	1.29	1.37
		1.75 c

3+

) and Stokes shift) and calculated (MMCT(Bi 3+ ) or E A (X)) energies in garnets doped with Bi 3+ .

Table 4

 4 Pressure dependence of the unit cell volume V and of distances Y-O and V-O in Z-YVO 4 :Bi 3+ and corresponding dependence of E A

Table 5

 5 Pressure dependence of the unit cell volume V and of distances Y-O and V-O in S-YVO 4 :Bi

	3+ and corresponding

  3, YNbO 4, CaNb 2 O 6 , SrNb 2 O 6 andYTaO 4 doped with trivalent antimony. Their luminescence will be reported for the first time in this work and an empirical equation will be tentatively introduced on the model of Equation(30) in Chapter 1, to account for the observed data. In addition, the luminescence spectra of CaMoO 4 -Sb 3+ and SrMoO 4 -Sb 3+

Table 1

 1 Raw materials and experimental conditions used for the synthesis of several closed-shell transition metal oxides doped with Sb 3+ and the JCPDS number of the obtained structure.

	Host lattice Raw chemicals (analytical grade) Temperature (°C) / duration (h)	JCPDS standard
	CaZrO 3	CaCO 3 , ZrO 2	1100/12	01-72-7550
	SrZrO 3	SrCO 3 ,ZrO 2	1200/10	01-076-9442
	YNbO 4	Y 2 O 3 , Nb 2 O 5	1500/12	01-072-2077
	SrNb 2 O 6	SrCO 3 , Nb 2 O 5	1200/12	45-0227
	M'-YTaO 4	Y 2 O 3 , Ta 2 O 5	1200/10	9-0341
	CaMoO 4	CaCO 3 , MoO 3	780/6	7-0212
	SrMoO 4	SrCO 3, MoO 3	1200/10	96-153-0911

Table 2

 2 Corrected distance d corr (X) between Sb 3+ and the cation site X, structural parameter he(X) at site X and electronegativity χ CN' (M n+ ) of the metal cation in the lattice. CN is the coordination number at the considered X site. CN' is the coordination number at the closed shell transition metal site M n+ . n.a. means not applicable. MgS, in the elpasolites Cs 2 NaMCl 6 with M = Sc, Y, La and in the zircon-structured phosphate MPO 4 with M = Sc, Lu, Y). In these cases, only the lowest-lying feature is retained. We have also added in Table 2 some useful data (i. e. d corr and χ CN' ) for the calculation of Sb 3+ → M n+ MMCT energies. The necessary χ CN' values were taken from Appendix A. Note that incorporation of Sb 3+ in the very small P 5+ of B 3+ sites was excluded in the present work. This is in line with the results reported in ref.(2) stating that in phosphate lattices, Sb 3+ replaces a host lattice cation and is not substituted to (PO 4 ) 3-groups to form (SbO 3 ) 3-units. Therefore, in lattices like ScPO 4 , YPO 4 , LaP 3 O 9 , ScBO 3 , LaBO 3 , etc…, the only possible doping site is a closed shell transition metal site (Sc 3+ , Y 3+ or La 3+ ). The same goes for Y 2 O 3 in which two sites of point symmetry S 6 and C 2 . When a lattice incorporates more than one type of closed shell transition metal (e.g. Y 3+ and Nb 5+ in YNbO 4 ), more than one MMCT process may occur. Taking YNbO 4 -Sb 3+ , we may have Sb 3+ → Y 3+ and Sb 3+ → Nb 5+ MMCTs, the energy of which depending on the electronegativity of the cations and the shortest distance between them. This, of course, depends on the considered doping site (either Y 3+ or Nb 5+ ) for Sb 3+ . This raises therefore four different situations (for YNbO 4 -Sb 3+ and M'-YTaO 4 -Sb 3+ ) that are detailed in Table 2. For YNbO4, for instance, we have

	Host lattice	X(CN)	d corr (X) (Å)	he(X)	χ CN' (M n+ )
	(ICSD)
	CaS	Ca 2+ (6)	n.a.	1.96	1.16
	(619529)
	MgS	Mg 2+ (6)	n.a.	1.59	1.23
	30241
	LaOCl	La 3+ (9)	3.55	3.01	1.26
	(40297 )
	Cs 2 NaLaBr 6 (426112)	Cs + (12) Na + (6)	4.43 5.63	0.16 0.85	n.a. n.a.
		La 3+ (6)	8.01	1.74	1.33
	Cs 2 NaYBr 6 (65733)	Cs + (12) Na + (6)	4.33 5.65	0.17 0.86	n.a. n.a.
		Y 3+ (6)	7.99	1.39	1.34
	Cs 2 NaLaCl 6 (425945)	Cs + (12) Na + (6)	4.72 5.46	0.15 0.73	n.a. n.a.
		La 3+ (6)	7.72	1.45	1.33.
	Cs 2 NaYCl 6 (65732)	Cs + (12) Na + (6)	4.09 5.37	0.16 0.72	n.a. n.a.
		Y 3+ (6)	7.59	1.21	1.34
	Y 2 O 3 (23811)	Y 3+ (6) -C2 Y 3+ (6) -S6	3.46 3.45	1.61 1.61	1.34 1.34
	ScPO 4	Sc 3+ (8)	3.53	0.97	1.35
	(74483)
	YPO 4	Y 3+ (8)	3.63	1.14	1.29
	(56113)
	LuPO 4	Lu 3+ (8)	3.60	1.08	1.37
	(2505)
	LaPO 4	La 3+ (9)	3.91	1.13	1.26
	(21066)
	AlPO 4	Al 3+ (4)	n.a.	2.50	1.51
	(16542)
	LiLaP 4 O 12 (416877)	Li + (6) La 3+ (8)	n.a. 5.49	2.16 1.32	n.a. 1.28
	YP 3 O 9 (R-3h 98554)	Y(1) 3+ (6) Y(2) 3+ (6)	Y(1)-Y(2)=5.75	1.95 1.88	1.29 1.29
		Y(3) 3+ (6)		1.99	1.29
	YP 3 O 9	Y(1) 3+ (6)	Y(1)-Y(2)=5.28	2.07	1.29

In a few cases, the excitation band shows a doublet or a triplet structure which is ascribed either to a crystal field splitting of the 3 P 1 level (i. e. in ScBO 3 and LuBO 3 , in their calcite form ) or to a Jahn-Teller effect (i. e. in CaS, indeed to consider the four shortest distances Y-Ta, Y-Y, Ta-Y and Ta-Ta, depending on the considered doping sites.

Table 3

 3 Experimental E exp and calculated E A energies in a selection of Sb

3+

-doped compounds. sh:shoulder.

  -1 accuracy limits, namely Y 2 O 3 (C 2 site), LaPO 4 , LuP 3 O 9 , LuBO 3 and LaBO 3 . In the case of Y 2 O 3 (C 2 site), our results suggest an insertion of Sb 3+ in the S 6 site with a low-lying excitation ascribed to A transition. For the four other lattices, it is difficult to draw out reliable conclusions from our data owing to some possible off-centering position of Sb 3+ in the much larger La 3+ and Lu 3+ sites or even formation of Sb 3+ pairs. Nevertheless, dealing with the MP 3 O 9 series (M = Y, Lu, La) and LiLaP 4 O 12 (La 3+ site only), our data strongly suggest that the low-lying excitation is of D character. In contrast, in the MPO 4 series (M = Sc, Lu), the low-lying excitation is more probably of A character. The case of YPO 4 is still ambiguous, since both A and D transitions fit the accuracy of ± 2500 cm -1 . Presumably, the two states are located close to each other in the energy level diagram, as for YVO 4 :Bi 3+ (see Chapter 3). At last in LaBO 3 :Sb 3+ , we

  A I B II PO 4 :Eu 2+ phosphors that are not explored yet. This helped to suggest the members of this family that are good candidates for the design of blue-based LED materials. It was found that NaCdPO 4 :Eu 2+ (Pna2 1 polymorph) and LiCdPO 4 :Eu 2+ (Pnma polymorph) are the best in this connection. Our attempts to prepare these two compounds unfortunately failed. Further chemistry work is required in this connection.Concerning trivalent bismuth, the empirical models were found useful for the prediction of the character of luminescence in Bi 3+ -doped closed shell transition metal oxides. This was accomplished by using the EF and MMCT models with a criterion involving the Stokes shift values. Depending on this method, the low-lying excitation signals in these compounds were identified as A ( 1 S 0 → 3 P 1 ) or D (Bi 3+ →M n+ charge transfer) bands, where M n+ is a transition metal cation with d 0 electron configuration.The set of closed shell transition metal oxides doped with Bi 3+ prepared in this work showed yellow emission for the vanadates (GdVO 4 and LaVO 4 ), the molybdate CaMoO 4 and the two niobates LiNbO 3 -Bi 3+ and NaNbO 3 -Bi 3+ . The other niobates (YNbO 4 , GdNbO 4 , LaNbO 4 CaNb 2 O 6 ) and the tantalate M'-YTaO 4

Table 2

 2 EN of cations with the most common CN of main group elements.

Table 3

 3 EN of cations with the most common CN for group IB-VB metals.

Table 4

 4 EN of cations with the most common CN for group VIB-VIIIB metals.

Table 18 NH 4 ZnPO 4 (P6 3 )

 18

		NH4(1) NH4(2) Zn1 Zn2 P1 P2	O1	O2	O3	O4	O5	O6
	O1	0	6	1	0	1	0
	O2	1	3	1	0	1	0
	O3	1	0	1	0	1	0
	O4	2	0	1	0	0	3
	O5	3	0	0	3	1	0
	O6	1	0	0	1	0	1
	NH4(1)								1	1	2	3
	NH4(2)							2	1	0	0	0
	Zn1							1	1	1	1	0
	Zn2							0		0	1	1
	P1							1	1	1	0	1
	P2							0	0	0	1	0
	Total	8	9	4	4	4	4	4	4	3	5	5
	multiplicity	6	2	6	2	6	2	6	6	6	6	6

Table 19 LiCaPO 4 (P3 1 c)

 19

		Li	Ca	P1	P2	P3	O1	O2	O3	O4	O5	O6
	O1	1	0	1	0	0
	O2	0	1	0	1	0
	O3	0	1	0	0	1
	O4	1	2	3	0
	O5	1	2	0	3
	O6	1	2	0	0	3
	Li						3		0	1	1
	Ca						0	3	3	2	2
	P1						1	0	0	1	0
	P2						0	1	0	0	1
	P3								1
	Total	4	8	4	4	4	4	4	4	4	4
	multiplicity	6	6	2	2	2	2	2	2	6	6

Table 20 NaMgPO 4 (P2 1 2 1 2 1 )

 20

Table 24 KZnPO 4 (P6 3 )

 24

							Z	Z	Z	Z														O	O	O	O	O	O	O
	K	K	K	K	K	K	n	n	n	n	P	P	P	P	O	O	O	O	O	O	O	O	O	1	1	1	1	1	1	1
	1	2		4	5	6	1	2	3	4	1	2	3	4	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6
	O1 1 0 6

Table 25 CaTiO 3 (P b n m)

 25 The formula is : 𝐶𝑎 1 1 𝑇𝑖 1 1 𝑂 1 1 𝑂 2 1 . The compound is decomposed as:

						Ca	Ti	O1	O2
				O1		4	2
				O2		8	4
				Ca				2	2
				Ti				2	2
				Total		10	6	4	4
			multiplicity	4	4	4	8
	Z=4. 𝐶𝑎 1	4×1 12 𝑂 1	2×1 4 + 𝐶𝑎 1	8×1 12 𝑂 2	2×1 4 + 𝑇𝑖 1	1×1 6 𝑂 1	2×1 4 + 𝑇𝑖 1	4×1 6 𝑂 2	2×1 4

Table 26 Y 2 Ti 2 O 7 (Fd-3m)

 26 The formula is : 𝑌 1 2 𝑇𝑖 1 2 𝑂 1 1 𝑂 2 6 . The compound is decomposed as:

						Y	Ti	O1	O2
				O1		2	0	0	0
				O2		6	6	0	0
				Y		0	0	4	2
				Ti		0	0	0	2
				Total		8	6	4	4
				multiplicity	16	16	8	48
	Z=8. 𝑌 1	2×2 8 𝑂 1	4×1 4 + 𝑌 1	6×2 8 𝑂 2	2×6 4 + 𝑇𝑖 1	6×2 6 𝑂 2	2×6 4

Table 27 PbWO 4 (I 41/a)

 27 The formula is : 𝑃𝑏 1 1 𝑊 1 1 𝑂 1 4 . The compound is decomposed as: 𝑃𝑏 1

				Pb	W	O1
			O1	8	4
			Pb		2
			W		1
			Total	8	4	3
			multiplicity	4	4	16
	Z=8. 8×1 8 𝑂 1	2×4 3 +
	𝑊 1	4×2 4 𝑂 1	1×4 3

Table 28 Lu 3 Al 5 O 12 (I a -3 d)

 28 The formula is : 𝐿𝑢 1 3 𝐴𝑙 1 2 𝐴𝑙(2) 3 𝑂 1 12 . The compound is decomposed as:

								Lu	Al1	Al2	O
						O			8	6		4
						Lu							2
						Al1						1
						Al2						1
						Total		8	6		4	4
					multiplicity	24	16		24	96
	Z=8. 𝐿𝑢 1	8×3 8	𝑂 1	2×12 4	+ 𝐴𝑙 1	6×2 6	𝑂 1	1×12 4	+ 𝐴𝑙 2	4×3 4	𝑂(1)	1×12 4

Table 29 Y 3 Al 5 O 12 (I a -3 d)

 29 The formula is : 𝑌 1 3 𝐴𝑙 1 2 𝐴𝑙(2) 3 𝑂 1 12 . The compound is decomposed as:

									Y	Al1	Al2	O
						O		8	6		4
							Y						2
						Al1						1
						Al2						1
						Total		8	6		4	4
					multiplicity	24	16	24	96
	Z=8. 𝑌 1	8×3 8	𝑂 1	2×12 4	+ 𝐴𝑙 1	6×2 6	𝑂 1	1×12 4	+ 𝐴𝑙 2	4×3 4	𝑂(1)	1×12 4

Table 30 Y 3 Ga 5 O 12 (I a -3 d)

 30 The formula is : 𝑌 1 3 𝐺𝑎 1 2 𝐺𝑎(2) 3 𝑂 1 12 . The compound is decomposed as:

		Y	Ga1	Ga2	O
	O	8	6	4	0
	Y	0	0		2
	Ga1	0	0		1
	Ga2				1
	Total	8	6	4	4
	multiplicity	24	16	24	96
	Z=8.

Table 31 ScVO 4 (I 41/a m d)

 31 The formula is : 𝑆𝑐 1 1 𝑉 1 1 𝑂 1 4 . The compound is decomposed as:

						Sc	V	O1
				O1	8	4
				Sc		2
				V		1
				Total	8	4	3
				multiplicity	4	4	16
	Z=4. 𝑆𝑐 1	8×1 8 𝑂 1	2×4 3 + 𝑉 1	4×1 4 𝑂 1	1×4 3

Table 32 YVO 4 (I 41/a m d)

 32 The formula is : 𝑌 1 1 𝑉 1 1 𝑂 1 4 . The compound is decomposed as:

						Y	V	O1
				O1	8	4
					Y	2
					V	1
				Total	8	4	3
				multiplicity	4	4	16
	Z=4. 𝑌 1	8×1 8 𝑂 1	2×4 3 + 𝑉 1	4×1 4 𝑂 1	1×4 3

Table 33 GdVO 4 (I 41/a m d)

 33 The formula is : 𝐺𝑑 1 1 𝑉 1 1 𝑂 1 4 . The compound is decomposed as:

		Gd	V	O1
	O1	8	4	0
	Gd	0	0	2
	V	0	0	1
	Total	8	4	3
	multiplicity	4	4	16
	Z=4.

Table 34 LiNbO 3 (R 3 c)

 34 The formula is : 𝐿𝑖 1 1 𝑁𝑏 1 1 𝑂 1 3 . The compound is decomposed as:

						Li	Nb	O1
				O1		6	6
				Li		2
				Nb		2
				Total	6	6	4
				multiplicity	6	6	18
	Z=6. 𝐿𝑖 1	6×1 6 𝑂 1	2×3 4 + 𝑁𝑏 1	6×1 6 𝑂 1	2×4 3

Table 35 YNbO 4 (C2/c)

 35 The formula is : 𝑌 1 1 𝑁𝑏 1 1 𝑂 1 2 𝑂 2 2 . The compound is decomposed as:

						Y	Nb	O1	O2
				O1		4	4
				O2		4	2
				Y				2	2
				Nb				2	1
				Total		8	6	4	3
				multiplicity	4	4	8	8
	Z=4. 𝑌 1	4×1 8 𝑂 1	2×2 4 + 𝑌 1	4×1 8 𝑂 2	2×2 3 + 𝑁𝑏 1	4×1 6 𝑂 1	2×2 4 + 𝑁𝑏(1)	2×1 6 𝑂(2)	2×2 3

Table 36 YTaO 4 (I2/a)

 36

		Y	Ta	O1	O2
	O1	4	2
	O2	4	4
	Y			2	2
	Ta			1	2
	Total	8	6	3	4
	multiplicity	4	4	8	8

Table 39 La 2 Zr 2 O 7 (Fd-3m)

 39 The formula is : 𝐿𝑎 1 2 𝑍𝑟 1 2 𝑂 1 6 𝑂 2 1 . The compound is decomposed as:

					La	Zr	O1	O2
				O1	6	6
				O2	2	0
				La			2	4
				Zr			2	0
				Total	8	6	4	4
			multiplicity	16	16	48	8
	Z=8. 𝐿𝑎 1	6×2 8 𝑂 1	2×6 4 + 𝐿𝑎 1	2×2 8 𝑂 2	4×1 4 + 𝑍𝑟 1	6×2 6 𝑂 1	2×6 4

Table 40 Y 2 Sn 2 O 7 (Fd-3m)

 40

						Y	Sn	O1	O2
				O1		6	6
				O2		2	0
				Y				2	4
				Sn				2	0
				Total		8	6	4	4
				multiplicity	16	16	48	8
	Z=8. The formula is : 𝑌 1 2 𝑆𝑛 1 2 𝑂 1 6 𝑂 2 1
	The compound is decomposed as:
	Y 1	6×2 8 𝑂 1	2×6 4 + 𝑌 1	2×2 8 𝑂 2	4×1 4 + 𝑆𝑛 1	6×2 6 𝑂 1	2×6 4

Table 41 GdNbO 4 (I2/a)

 41 The formula is : 𝐺𝑑 1 1 𝑁𝑏 1 1 𝑂 1 2 𝑂 2 2 . The compound is decomposed as:

		Gd	Nb	O1	O2
	O1	4	2
	O2	4	4
	Gd			2	2
	Nb			2	2
	Total	8	6	4	4
	multiplicity	4	4	8	8
	Z=4.

Table 42 CaZrO 3 (P c m n)

 42 The formula is : 𝐶𝑎 1 1 𝑍𝑟 1 1 𝑂 1 1 𝑂 2 2 . The compound is decomposed as:

					Ca	Zr	O1	O2
				O1	2	2	0		0
				O2	6	4	0		0
				Ca	0	0	1		3
				Zr	0	0	2		2
				Total	8	6	3		5
			multiplicity	4	4	4		8
	Z=4. 𝐶𝑎 1	2×1 8 𝑂 1	1×1 3 + 𝐶𝑎 1	6×1 8 𝑂 2	3×2 5 + 𝑍𝑟 1	2×1 6 𝑂 1	2×1 3 + 𝑍𝑟(1)	4×1 6 𝑂(2)	2×2 5

Table 43 CaNb 2 O 6 (P b c n)

 43 The formula is : 𝐶𝑎 1 1 𝑁𝑏 1 2 𝑂 1 2 𝑂 2 2 𝑂(3) 2 . The compound is decomposed as:

					Ca	Nb	O1	O2	O3
				O1	4	1
				O2	4	2
				O3	0	3
				Ca			2	2	0
				Nb			1	2	3
			Total	8	6	3	4	3
			multiplicity	4	8	8	8	8
	Z=4. 𝐶𝑎 1	4×1 8 𝑂 1	2×2 3 + 𝐶𝑎 1	4×1 8 𝑂 2	2×2 4 + 𝑁𝑏 1	1×2 6 𝑂 1	1×2 3 + 𝑁𝑏 1	2×2 6 𝑂 2	2×2 4 +
	𝑁𝑏 1	3×2 6 𝑂(3)	3×2 3

Table 46 CaMoO 4 (I 41/a)

 46 The formula is : 𝐶𝑎 1 1 𝑀𝑜 1 1 𝑂 1 4 . The compound is decomposed as:

						Ca	Mo	O1
				O1		8	4
				Ca		2
				Mo		1
				Total	8	4	3
				multiplicity	4	4	16
	Z=4. 𝐶𝑎 1	8×1 8 𝑂 1	2×4 3 + 𝑀𝑜 1	4×1 4 𝑂 1	1×4 3

Table 47 CaS(Fm-3m)

 47 The formula is : 𝐶𝑎 1 1 𝑆 1 1 . The compound is decomposed as: 𝐶𝑎 1

		Ca	S
	S	6
	Ca		6
	Total	6	6
	multiplicity	4	4
	Z=4. 6×1 6 𝑆 1	6×1 6

Table 48 MgS(Fm-3m)

 48 The formula is : 𝑀𝑔 1 1 𝑆 1 1 . The compound is decomposed as: 𝑀𝑔 1

		Mg	S
	S	6	0
	Ca	0	6
	Total	6	6
	multiplicity	4	4
	Z=4. 6×1 6 𝑆 1	6×1 6

Table 49 Y 2 O 3 (Ia-3)

 49

		Y1	Y2	O1
	O1	6	6
	Y1			1
	Y2			3
	Total	6	6	4
	multiplicity	8	24	48

Table 50 SrMoO 4 (I 41/a)

 50 The formula is : 𝑆𝑟 1 1 𝑀𝑜 1 1 𝑂 1 4 . The compound is decomposed as:

						Sr	Mo	O1
				O1		8	4	0
				Sr		0	0	2
				Mo		0	0	1
				Total		8	4	3
				multiplicity	4	4	16
	Z=4. 𝑆𝑟 1	8×1 8 𝑂 1	2×4 3 + 𝑀𝑜 1	4×1 4 𝑂 1	1×4 3

Table 51 Cs 2 NaLaBr 6 (Fm-3m)

 51 The formula is : 𝐶𝑠 1 1 𝐿𝑎 1 1 𝑁𝑎(1) 1 𝐵𝑟 1 6 . The compound is decomposed as:

					Cs	La	Na	Br
				Br	12	6	6	0
				Cs	0	0		4
				La	0	0		1
				Na				1
				Total	12	6	6	6
			multiplicity	8	4	4	24
	Z=4. 𝐶𝑠 1	12×2 12 𝐵𝑟 1	4×6 6 + 𝐿𝑎 1	6×1 6 𝐵𝑟 1	1×6 6 + 𝑁𝑎 1	6×1 6 𝐵𝑟(1)	1×6 6

Table 52 Cs 2 NaYBr 6 (Fm-3m)

 52

		Cs	Y	Na	Br
	Br	12	6	6
	Cs				4
	Y				1
	Na				1
	Total	12	6	6	6
	multiplicity	8	4	4	24

Table 53 Cs 2 NaYCl 6 (Fm-3m)

 53 The formula is : 𝐶𝑠 1 1 𝑌 1 1 𝑁𝑎(1) 1 𝐶𝑙 1 6 . The compound is decomposed as:

						Cs	Y	Na	Cl
				Cl		12	6	6
				Cs				4
				Y				1
				Na				1
				Total		12	6	6	6
				multiplicity	8	4	4	24
	Z=4. 𝐶𝑠 1	12×2 12 𝐶𝑙 1	4×6 6 + 𝑌 1	6×1 6 𝐶𝑙 1	1×6 6 + 𝑁𝑎 1	6×1 6 𝐶𝑙(1)	1×6 6

Table 54 Cs 2 NaLaCl 6 (Fm-3m)

 54 The formula is : 𝐶𝑠 1 1 𝐿𝑎 1 1 𝑁𝑎(1) 1 𝐶𝑙 1 6 . The compound is decomposed as:

					Cs	La	Na	Cl
				Cl	12	6	6
				Cs				4
				La				1
				Na				1
				Total	12	6	6	6
			multiplicity	8	4	4	24
	Z=4. 𝐶𝑠 1	12×2 12 𝐶𝑙 1	4×6 6 + 𝐿𝑎 1	6×1 6 𝐶𝑙 1	1×6 6 + 𝑁𝑎 1	6×1 6 𝐶𝑙(1)	1×6 6

Table 55 YPO 4

 554 The formula is : 𝑌 1 𝑃 1 1 𝑂 1 4 . The compound is decomposed as:

			(I 41/a m d)
		Y	P	O1
	O1	8	4
	Y			2
	P			1
	Total	8	4	3
	multiplicity	4	4	16

Table 56 ScPO 4 (I 41/a m d)

 56 The formula is : 𝑆𝑐 1 1 𝑃 1 1 𝑂 1 4 . The compound is decomposed as:

						Sc	P	O1
				O1	8	4
				Sc	2
				P		1
				Total	8	4	3
				multiplicity	4	4	16
	Z=4. 𝑆𝑐 1	8×1 8 𝑂 1	2×4 3 + 𝑃 1	4×1 4 𝑂 1	1×4 3

Table 57 LuPO 4 (I 41/a m d)

 57 The formula is : 𝐿𝑢 1 𝑃 1 1 𝑂 1 4 . The compound is decomposed as:

						Lu	P	O1
				O1	8	4
				Lu		2
				P		1
				Total	8	4	3
				multiplicity	4	4	16
	Z=4. 𝐿𝑢 1	8×1 8 𝑂 1	2×4 3 + 𝑃	4×1 4 𝑂 1	1×4 3

Table 58 LaPO 4 (P2 1 /a)

 58 The formula is :𝐿𝑎 1 1 𝑃 1 1 𝑂 1 1 𝑂 2 1 𝑂 3 1 𝑂(4) 1.The compound is decomposed as:

								La		P		O1		O2	O3	O4
					O1				2		1		0		0	0
					O2				3		1		0		0	0
					O3				3		1		0		0	0
					O4				2		1		0		0	0
					La				0		0		2		3	2
					P				0		0		1		1	1
					Total			10		4		3		4	3
				multiplicity		4		4		4		4	4
	Z=4. 𝐿𝑎 1	3×1 10	𝑂 1	2×1 4	+ 𝐿𝑎 1	2×1 6	𝑂 2	2×1 4	+ 𝐿𝑎 1	1×1 6	𝑂 3	1×1 4	+ 𝐿𝑎 1	1×1 6	𝑂 4	1×1 4	+
	𝑃 1	1×1 4 𝑂 1	1×1 4 + 𝑃 1	2×1 4 𝑂 2	1×1 4 + 𝑃 1	1×1 4 𝑂 3	1×1 4 + 𝑃 1	1×1 4 + 𝑂(4)	1×1 4

Table 59 LaOCl (P 4/n m m)

 59 The formula is : 𝐿𝑎 1 1 𝑂 1 1 𝐶𝑙(1) 1 . The compound is decomposed as:

						La	O	Cl
				O		4	0
				Cl		6	0
				La		1	2
				Total		10	1	2
				multiplicity	2	2	2
	Z=2. 𝐿𝑎 1	4×1 10 𝑂 1	1×1 1 + 𝐿𝑎 1	6×1 10 𝐶𝑙 1	2×1 2

Table 60 Sr 3 (PO 4 ) 2 (R-3m)

 60 The formula is : 𝑆𝑟 2 𝑆𝑟(2) 1 𝑃 1 2 𝑂 1 2 𝑂 2 6. The compound is decomposed as:

		Sr1	Sr2	P	O1	O2
	O1	6	1	1
	O2	6	9	3
	Sr1				3	1
	Sr2				1	3
	P				1	1
	Total	12	10	4	5	5
	multiplicity	6	3	6	6

Table 61 GdP 3 O 9 (C 2 2 2 1)

 61 The formula is :𝐺𝑑 1 1 𝑃 1 1 𝑃(2) 2 𝑂 1 1 𝑂 2 2 𝑂 3 2 𝑂(4) 2 𝑂(5) 2.The compound is decomposed as:

							Gd	P1	P2		O1	O2	O3	O4	O5
				O1				0	1
				O2				2	0
				O3				0	1
				O4				0	1
				O5				2	1
				Gd							2	1	1	0
				P1							1	0	0	1
				P2							2	0	1	1	1
				Total			4	4		2	3	2	2	2
			multiplicity			4	8		4	8	8	8	8
	Z=4. 𝐺𝑑 1	4×1 8	𝑂 2	2×2 3 + 𝐺𝑑 1	2×1 8 𝑂 3	2×2 2 + 𝐺𝑑 1	4×1 8 𝑂 4	1×2 2 + 𝑃 1	2×1 4 𝑂 2	1×2 3 +
	𝑃 1	2×1 4 𝑂 5	1×2 2 + 𝑃	1×2 4 𝑂 1	2×1 2 + 𝑃 2	1×2 4 𝑂 3	1×2 2 + 𝑃 2	1×2 4 𝑂 4	1×2 2 + 𝑃 2	1×2 4 𝑂(5)	1×2 2

Table 67 AlPO 4 (P 31 2 1)

 67 The formula is : 𝐴𝑙 1 1 𝑃 1 1 𝑂 1 2 𝑂(2) 2 . The compound is decomposed as:

						Al	P	O1	O2
				O1		2		2
				O2		2		2
				Al					1	1
				P					1	1
				Total		4		4	2	2
				multiplicity	3		3	6	6
	Z=4. 𝐴𝑙 1	2×1 4 𝑂 1	2×2 2 + 𝐴𝑙 1	2×1 4 𝑂 2	1×2 2 𝑃 1	2×1 4 𝑂 1	1×2 2 + 𝑃 1	2×1 4 𝑂(2)	1×2 2

Table 68 ScBO 3 (R-3c)

 68 The formula is : 𝑆𝑐 1 1 𝐵 1 1 𝑂 1 3 . The compound is decomposed as:

						Sc	B	O1
				O1	6	3
				Sc		2
				B		1
				Total	6	3	3
				multiplicity	6	6	18
	Z=6. 𝑆𝑐 1	6×1 6 𝑂 1	1×3 3 + 𝐵 1	3×1 3 𝑂 1	2×1 3

Table 69 LuBO 3 (R-3c)

 69 The formula is : 𝐿𝑢 1 1 𝐵 1 1 𝑂 1 3 . The compound is decomposed as:

						Lu	B	O1
				O1	6	3
				Lu		2
				B		1
				Total	6	3	3
				multiplicity	6	6	18
	Z=6. 𝐿𝑢 1	6×1 6 𝑂 1	1×3 3 + 𝐵 1	3×1 3 𝑂 1	2×1 3

P states are located in the conduction band(41) . Luminescence from the A band is quenched and the

Acknowledgements

Figure 16.Pressure dependence of the energy level structure of Bi 3+ in S-YVO 4 (left) and one-coordinate configuration diagram (right). See text for further details.

This situation differs notably from the previous case. Here, it is clear that in Bi 3+ -related levels are located by more than 10000 cm -1 higher than the bandgap. This means that the absorption edge in S-

𝑀𝑀𝐶𝑇 𝐵𝑖 3+ , 𝑐𝑚 -1 = 70000 -52000 𝜒 4 (𝑉 5+ ) 𝑑 𝑐𝑜𝑟𝑟

(2)

Using χ 4 (V 5+ ) = 2.46 (40) (see Appendix A) and d corr = 3.21 Å, we locate the Bi-V CT maximum at 30150 cm -1 , i. e. very close to the VO 4 3-excitation at ambient pressure. We therefore estimate that both CT processes contribute to the edge of the excitation spectrum recorded at ambient pressure. Following the arguments developed recently in ref. (41), we estimate the action of pressure on the Bi→V CT energy as follows:

𝑀𝑀𝐶𝑇 𝐵𝑖 3+ , 𝑐𝑚 -1 = 70000 -52000

where d P expresses the pressure dependence of the Bi-V distance in YVO 4 -Bi 3+ . Owing to Figure 7, we can write 𝑑 𝑃 = 𝑑 𝑐𝑜𝑟𝑟 -0.0055𝑃 for the zircon polymorph, with d corr = 3.21 Å. The corresponding pressure-dependence of the Bi-V CT is plotted in Figure 10. In the zircon phase, it comes that the pressure action induces an upward shift of the bandgap and a downshift of the Bi-V CT. In the pressure range 0 -5 Gpa, the excitation edge is governed by the Bi-V CT process, as attested by the similar slope of the corresponding curves. The blue shift of the excitation edge for pressures exceeding 5 Gpa reveals prevailing contributions from higher-lying states whose origin has not been clearly identified in the context of this work.

Dealing with emission, we find that the band-like CT emission of YVO 4 -Bi 3+ presents a complex, unresolved, structure consisting of at least 2 contributions that have previously been identified in ref. (7). There is no trace of emission from the vanadate units. This is due to VO 4 3-→ Bi 3+ energy transfer, the efficiency of which is very high at room temperature (7) . The application of pressure tends to favor the low energy contributions relative to the high energy ones, resulting in a progressive red shift of the band maximum under pressure. This variation is shown in Figure 10 with the label "emission max". In the pressure range 0 -7.5 GPa, the emission decays were found exponential with a time-constant varying progressively from 4.3 to 4.7 µs. There is no significant variation of the emission intensity in this pressure range.

We show in Figure 11 the dependence of the emission time-constant in the temperature range 10 -500 K, at ambient pressure. The integrated emission area follows the same trend.

shows a band with max at ≈ 315 nm. When doped with Bi 3+ , the compound shows yellow emission at 586 nm when excited at 320 nm (29) . This means that the emission of Sb 3+ is similar to that of Bi 3+ in CaMoO 4 with a small red shift and larger stokes shift.

Application of the EF and MMCT models to Sb 3+ -doped compounds

Calculation of environmental factors he(X(i))

We have compiled in Table 2 the values of the environmental factors he(X) for all possible sites X in all the compounds of interest whose crystal structure is available from ICSD database (30) . For lattices having several types of crystal structures, we have calculated the he(X) values using the structure of the polymorph described in the references from where we found the luminescence characteristics. For compounds whose structure was not described in the references, we have carried out the calculations for all types of structures that could be found for the lattice. For the compounds prepared in this work, the calculations were made only for the type of crystal structure that was confirmed by XRD. As previously mentioned, doping of Sb 3+ in the transition metal sites was considered as a possible option.

We have further noted that the vaterite structure of YBO 3 was shown to have either the space group P6 3 /m which led to Y 3+ sites with 8-fold coordination (31) , or the space group P6 3 /mmc with Y 3+ having the 6 and 6+6 fold coordination (5) . Owing to this structural uncertainty, we have decided to discard the lattice YBO 3 from our list. For the same reason, only the high temperature calcite polymorph of LuBO 3 is retained. The phosphates M(PO 3 ) 3 -Sb 3+ , with M = La, Gd, Y, Lu, Sc are known to possess many polymorphs, which creates uncertainties regarding their exact crystal structure. La(PO 3 ) 3 has the crystal structure of Nd(PO 3 ) 3 (space group C222 1 ) while the others (M = Gd, Y, Lu, Sc) have the crystal structure of Yb(PO 3 ) 3 (space group P2 1 /c) (1) . These attributions were based on the work of Hong dated from the mid-seventies (32) . More recently, however, a new space group (R-3h) has been proposed for Yb(PO 3 ) 3 (33) , which therefore raises perplexities. As a matter of fact, we found that Sc(PO 3 ) 3 is described in none of the above space groups, Gd(PO 3 ) 3 is described in the space group of Nd(PO 3 ) 3 instead of that of Yb(PO 3 ) 3 , Lu(PO 3 ) 3 is described in space group R-3h and Y(PO 3 ) 3 exists with both space groups R-3h and P2 1 /c. In this confusing context, we have decided to discard Sc(PO 3 ) 3 from our list and retain Gd(PO 3 ) 3 (C222 1 ), Lu(PO 3 ) 3 (R-3h), Y(PO 3 ) 3 (C222 1 and R-3h) and La(PO 3 ) 3 (R-3h) as plausible lattices.

We also give in

The corresponding values are listed in Table 3 (14) , this suggests that the interatomic distances may change when Sb 3+ is incorporated in Ca 2+ due to the off center position (the ionic radius of Sb 3+ being equal to 0.76 Å, closer to that of Mg 2+ ). The case of Cs 2 NaLaBr 6 :Sb 3+ and Cs 2 NaYBr 6 :Sb 3+ is also similar. Further, if we assume that antimony substitutes in Y 3+

and La 3+ , then we have to consider the interatomic distances and charges (bond valence sum) of both Y 3+

and La 3+ that account to the difference in the values of he(X) and the calculated energy E A . This means that the nature of the cation is very important in Sb 3+ compounds and the aspect that Sb 3+ goes off center in some of these compounds can be responsible for limiting the efficiency of the EF model. For other lattices like Y 2 O 3 , Sr 3 (PO 4 ) 2 and Ca 10 (PO 4 ) 6 F 2, only the Y 3+ (S 6 ), Sr(1) and Ca(2) sites, respectively, are well reproduced. This suggests that these sites are preferably occupied by Sb 3+ in the corresponding lattices. Dealing with the fluoro-apatite, our result is in good agreement with the studies that predict incorporation of Sb 3+ in the Ca(2) sites instead of the Ca(1) sites (4) . Dealing with AlPO 4 , studies showed that Sb 3+ is more probably located in interstitial sites than in the regular tetrahedrally-coordinated Al 3+ sites (2) . This may explain why we cannot reproduce E exp (Sb 3+ ) in this material by means of Equation ( 1).

The case of LaPO 4 , LiLaP 4 O 12 , MP 3 O 9 with M = Y, Lu, Gd, La and LaBO 3 will be further discussed in Section 4.5.

Appendix A: Electronegativities of different cations

In this appendix we show the tables of electronegativity (EN) values of different cations in different coordinations useful for the calculations of MMCT energies. The following tables are taken from the work of Keyan Li and Dongfeng Xue. In Table 1, the EN of six-coordinated cations are given denoted by χ 6 . The numbers in parantheses represent the oxidation states. L means low spin and H means high spin. In Table 2, the EN of elements in valence states with the most common coordination numbers (CN) for main group elements are listed. EN of cations with the most common CN for group IB-VB and VIB-VIIIB metals are gathered in Tables 3 and4. In Table 5, the EN values of trivalent lanthanides with CN ranging from 6 to 12 are given.

Appendix B Coordination Matrices

In this appendix, the coordination matrices useful for the calculations of he(X(i)) and Fc(X(i)) for all the studied lattices in this work are given in addition to the formula and decomposition into binary crystals for each lattice. Li( 1) Mg( 1) P( 1) Z=4. The formula is :

The compound is decomposed as: Z=8. The formula is :

The compound is decomposed as:

+ 𝐾 1

A g1

Ag 2

Ag 3

Be 1

Be 2

1 0 0 1 0 0 1 0 0 1 0 0 P2 0 1 0 0 1 0 0 1 0 0 1 0 P3 0 0 1 0 0 1 0 0

Appendix C: XRD patterns of Sb 3+doped compounds

In this appendix we show the XRD patterns of Sb 3+ -doped closed shell transition metal oxides that were prepared in this work with the corresponding JCPDS reference spectrum (in red) of the obtained structure. The patterns were in general considered acceptable despite the presence of few weak reflections represented by (*) on some spectra that do not correspond to the obtained phase. These

were not considered as important enough to affect the luminescent properties.