Jonathan Amodeo

Fabio Pietrucci

Julien Lam

Out-of-equilibrium Polymorph Selection in Nanoparticle Freezing

The ability to design synthesis processes that are out of equilibrium has opened the possibility of creating nanomaterials with remarkable physico-chemical properties, choosing from a much richer palette of possible atomic architectures compared to equilibrium processes in extended systems. In this work, we employ atomistic simulations to demonstrate how to control polymorph selection via cooling rate during nanoparticle freezing in the case of Ni3Al, a material with a rich structural landscape. State-of-the-art free-energy calculations allow to rationalize the complex nucleation process, discovering a switch between two kinetic pathways, yielding the equilibrium structure at room temperature and an alternative metastable one at higher temperature. Our findings address the key challenge in the synthesis of nano-alloys for technological applications, i.e., rationally exploiting the competition between kinetics and thermodynamics by designing a treatment history that forces the system into desirable metastable states.

Crystallization is a complex and incompletely understood phenomenon that has long fascinated scientists from various research fields including environmental science [START_REF] Campbell | Nucleation-and Emergence-Limited Growth of Ice from Pores[END_REF][START_REF] Myint | Nanosecond Freezing of Water at High Pressures: Nucleation and Growth near the Metastability Limit[END_REF][START_REF] Tabazadeh | Surface crystallization of supercooled water in clouds[END_REF][START_REF] Pipolo | Navigating at Will on the Water Phase Diagram[END_REF], pharmaceutics [START_REF] Ma | Tuning crystallization pathways through sequence engineering of biomimetic polymers[END_REF][START_REF] Varongchayakul | Direct Observation of Amyloid Nucleation under Nanomechanical Stretching[END_REF] and materials engineering [START_REF] Tidhar | Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications[END_REF][START_REF] Shen | Nanosecond homogeneous nucleation and crystal growth in shockcompressed SiO2[END_REF][START_REF] Fei | Observable Two-Step Nucleation Mechanism in Solid-State Formation of Tungsten Carbide[END_REF]. One of the key challenges of crystallization remains the ability to control the polymorph selection since properties of most materials among which mechanical, optical and chemical depend drastically on the crystal structure. This intricate relationship between structure and properties is particularly important in nanomaterials where novel crystal phases can be stabilized thus enabling for unique technological applications [START_REF] Nguyen | Engineered Hybrid Nanoparticles for On-Demand Diagnostics and Therapeutics[END_REF][START_REF] Sharma | Novel development of nanoparticles to bimetallic nanoparticles and their composites: A review[END_REF].

Although the understanding of polymorph selection at the bulk scale has been the subject of numerous works [START_REF] Desgranges | Insights into the Molecular Mechanism Underlying Polymorph Selection[END_REF][START_REF] Desgranges | Polymorph selection during the crystallization of Yukawa systems[END_REF][START_REF] Desgranges | Molecular Mechanism for the Cross-Nucleation between Polymorphs[END_REF][START_REF] Russo | The microscopic pathway to crystallization in supercooled liquids[END_REF][START_REF] Russo | Selection mechanism of polymorphs in the crystal nucleation of the Gaussian core model[END_REF], it remains unclear to what extent the specific features of nanoparticles, including finite-size effects and surface preponderance, can affect the resulting structure. As far as nanoparticles are concerned, many simulations studies have focused on finding minimum energy configurations [START_REF] Laurens | Comparison of aluminum oxide empirical potentials from cluster to nanoparticle[END_REF][START_REF] Hong | Geometric and electronic structure and magnetic properties of Fe-Au nanoalloys: insights from ab initio calculations[END_REF][START_REF] Morales-García | Understanding the interplay between size, morphology and energy gap in photoactive TiO2 nanoparticles[END_REF][START_REF] Lam | Atomistic Mechanisms for the Nucleation of Aluminum Oxide Nanoparticles[END_REF][START_REF] Macià Escatllar | Efficient preparation of TiO2 nanoparticle models using interatomic potentials[END_REF][START_REF] Viñes | Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk[END_REF] and on studying the nucleation path near the melting temperature [START_REF] Desgranges | Molecular Simulation of the Nucleation and Growth of Gold Nanoparticles[END_REF][START_REF] Quigley | Free energy and structure of calcium carbonate nanoparticles during early stages of crystallization[END_REF][START_REF] Quigley | Sampling the structure of calcium carbonate nanoparticles with metadynamics[END_REF][START_REF] Ngale | Nucleation and growth of C60 nanoparticles from the supersaturated vapor and from the undercooled liquid: A molecular simulation study[END_REF][START_REF] Ou | Kinetic pathways of crystallization at the nanoscale[END_REF][START_REF] Pavan | Metallic nanoparticles meet metadynamics[END_REF].

However, from the experimental point of view, nanoparticles are usually obtained through out of equilibrium processes especially when using physical routes of synthesis such as laser ablation [START_REF] Ayyub | Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures[END_REF][START_REF] Semaltianos | Nanoparticles by Laser Ablation[END_REF][START_REF] Zhang | Laser Synthesis and Processing of Colloids: Fundamentals and Applications[END_REF][START_REF] Kim | Synthesis of Nanoparticles by Laser Ablation: A Review[END_REF], flame pyrolysis [START_REF] Rosner | Flame Synthesis of Valuable Nanoparticles: Recent Progress Current Needs in Areas of Rate Laws, Population Dynamics, and Characterization[END_REF][START_REF] Teoh | Flame spray pyrolysis: An enabling technology for nanoparticles design and fabrication[END_REF][START_REF] Li | Flame aerosol synthesis of nanostructured materials and functional devices: Processing, modeling, and diagnostics[END_REF], and magnetron sputtering [START_REF] Ayyub | Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures[END_REF][START_REF] Kelly | Magnetron sputtering: a review of recent developments and applications[END_REF][START_REF] Alexeeva | Application of the magnetron sputtering for nanostructured electrocatalysts synthesis[END_REF]. In all these cases, as the temperature is decreased, the system goes from the gas phase to a liquid droplet whose subsequent freezing lead to the formation of a nanocrystal [START_REF] Kesälä | Molecular dynamics simulation of pressure dependence of cluster growth in inert gas condensation[END_REF][START_REF] Zhao | Crystallization of silicon nanoclusters with inert gas temperature control[END_REF][START_REF] Förster | Janus and core-shell nanoparticles: numerical modeling of their nucleation and growth in physical synthesis[END_REF]. While being crucial to reach a rational design of nanoparticles, the highly complex nucleation pathways and the resulting polymorph selection remain difficult to understand in this out of equilibrium context. In particular, numerous works investigated the freezing mechanisms in nanoparticles [START_REF] Nam | Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters: Surface-Induced Mechanism[END_REF][START_REF] Qi | Freezing of silver cluster and nanowire: A comparison study by molecular dynamics simulation[END_REF][START_REF] Lewis | freezing, and coalescence of gold nanoclusters[END_REF] and how the size and the * julien.lam.pro@gmail.com chemical composition of the liquid droplet can influence the final structure [START_REF] Chushak | Freezing of Ni-Al Bimetallic Nanoclusters in Computer Simulations[END_REF]. However, the cooling rate -an important tuning parameter in synthesis experimentshas been scarcely studied [START_REF] Kesälä | Molecular dynamics simulation of pressure dependence of cluster growth in inert gas condensation[END_REF][START_REF] Förster | Janus and core-shell nanoparticles: numerical modeling of their nucleation and growth in physical synthesis[END_REF][START_REF] Qi | Freezing of silver cluster and nanowire: A comparison study by molecular dynamics simulation[END_REF][START_REF] Rossi | Thermodynamics of CuPt nanoalloys[END_REF] even though it provides a primary route to study the interplay between kinetic and thermodynamical pathways.

Our work investigates how the cooling rate controls polymorph selection that occurs during nanoparticle freezing. We take the example of Ni 3 Al which is an intermetallic alloy. At the nanometric scale, this class of materials has recently attracted considerable attentions for their catalytic, [START_REF] Luo | High performing and stable supported nano-alloys for the catalytic hydrogenation of levulinic acid to γ-valerolactone[END_REF][START_REF] Wrasman | Synthesis of Col-loidal Pd/Au Dilute Alloy Nanocrystals and Their Potential for Selective Catalytic Oxidations[END_REF] plasmonic [START_REF] Ramade | Nanofried-eggs: Structural, optical, and magnetic characterization of physically prepared iron-silver nanoparticles[END_REF][START_REF] Ramade | Environmental Plasmonic Spectroscopy of Silver-Iron Nanoparticles: Chemical Ordering under Oxidizing and Reducing Conditions[END_REF] and magnetic [START_REF] Capiod | Elaboration of Nanomagnet Arrays: Organization and Magnetic Properties of Mass-Selected FePt Nanoparticles Deposited on Epitaxially Grown Graphene on Ir(111)[END_REF][START_REF] Ferrando | Chapter 8 -Magnetism in nanoalloys[END_REF] properties but also features a very rich structural landscape including different chemical orderings [START_REF] Förster | Janus and core-shell nanoparticles: numerical modeling of their nucleation and growth in physical synthesis[END_REF] as well as different polymorphs [START_REF] Ko | Impact of asymmetric martensite and austenite nucleation and growth behavior on the phase stability and hysteresis of freestanding shape-memory nanoparticles[END_REF]. In particular, NiAl alloys are known for high-temperature structural applications including the well-known γ -γ' superalloys [START_REF] Reed | The superalloys: fundamentals and applications[END_REF] made of nano-to micro-sized L1 2 Ni 3 Al precipitates that exhibit very unique mechanical properties [START_REF] Maaß | Ultrahigh Strength of Dislocation-Free Ni3Al Nanocubes[END_REF][START_REF] Schloesser | Deformation behaviour of freestanding single-crystalline Ni3Al-based nanoparticles[END_REF][START_REF] Amodeo | Mechanical properties and dislocation nucleation in nanocrystals with blunt edges[END_REF].

Here, we use atomistic simulations to show that increasing the cooling rate favors the emergence of a novel metastable structure. To understand this result, we perform free energy calculations on the liquid to nanocrystal transitions using the potential energy and an entropy surrogate as structure-agnostic order parameters. Our results show that while only the most stable structure (L1 2 ) is found at room temperature, a second free energy basin associated to an alternative structure (DO 3 ) becomes preponderant at higher temperature. Altogether, this work further contributes to the rational control of crystal polymorph in nano-alloys which is crucial for a wide range of technological applications, at the same time enhancing our understanding of out of equilibrium nanoparticle formation through competing nucleation pathways.

We studied nanoparticles made of 1000 atoms. As a comparison, recent studies that focused on different materials AgPt [START_REF] Pavan | Metallic nanoparticles meet metadynamics[END_REF], CuPt [START_REF] Rossi | Thermodynamics of CuPt nanoalloys[END_REF] and AuAg [START_REF] Gould | Controlling Structural Transitions in AuAg Nanoparticles through Precise Compositional Design[END_REF] considered particles of less than half this size. To initialize the calcula- tions, atoms were randomly arranged in a sphere of radius 12.5 Å embedded in a large simulation box (40 Å). A timestep of 0.001 ps was employed thorough the calculations. Then, the system was equilibrated at 2000 K in the NVT ensemble. From there, the temperature was linearly decreased to 300 K (with the same thermostat) during a time τ that was varied from 10 ns to 200 ns, corresponding to cooling rates in the range 8.5 -170 K/ns. These values are faster than typical liquid and gas quenching [START_REF] Koga | Novel Bidecahedral Morphology in Gold Nanoparticles Frozen from Liquid[END_REF] but is on par with chaotic bottom-up synthesis including laser ablation in liquids [START_REF] Amans | Origin of the nano-carbon allotropes in pulsed laser ablation in liquids synthesis[END_REF][START_REF] Shih | The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations[END_REF]. Throughout the study, we employed the EAM interatomic potentials developed by Pun and Mishin which is known to correctly mimic experimental lattice and defect properties of various NiAl systems including L1 2 Ni 3 Al [START_REF] Purja Pun | Development of an interatomic potential for the Ni-Al system[END_REF]. In addition, this potential correctly reproduces ab initio formation energies of binary BCC D0 3 and HCP D0 19 Ni 3 Al. The NVT ensemble is obtained by using the Noose-Hoover thermostat with a damping factor equal to 0.1 ps. Simulations were carried out using LAMMPS [START_REF] Plimpton | Fast parallel algorithms for short-range molecular dynamics[END_REF] patched with PLUMED [START_REF] Bonomi | Promoting transparency and reproducibility in enhanced molecular simulations[END_REF][START_REF] Tribello | PLUMED 2: New feathers for an old bird[END_REF]. Crystal analysis of the structures was performed using common neighbor analysis [START_REF] Honeycutt | Molecular dynamics study of melting and freezing of small Lennard-Jones clusters[END_REF] as implemented in OVITO [START_REF] Stukowski | Visualization and analysis of atomistic simulation data with OVITO-the Open Visualization Tool[END_REF].

For a given cooling rate, we performed N tot simulations using various initial conditions (positions and velocities). In the resulting nanoparticles roughly half the atoms are in the first and second surface layers and are necessarily not identified as crystallized. Therefore, a simulation is considered crystallizing when the system reaches more than 40% of crystal-like atomic environments and the number of crystallizing simulations is denoted N crys . Otherwise, the system remains trapped in an amorphous configuration. N tot and N crys for the different quench durations are reported in Table I. The results show that as the quench duration is increased, the proportion of crystallizing simulations also increases from 0% (τ =10 ns) to more than 90% (τ =200 ns). This is an expected result since longer quenching times allow the system to evolve under quasi-equilibrium conditions.

Before proceeding to discuss our freezing simulations, we first recall the polymorphs that have been found experimentally. In the bulk phase, Ni 3 Al is known to be stable at the binary FCC L1 2 structure [START_REF] Stoloff | Physical and mechanical metallurgy of Ni3Al and its alloys[END_REF]. Ni 3 Al-based BCC structures are less commonly observed, but can be obtained using additional alloying elements such as Fe, Ti or Ru [START_REF] Polvani | High temperature creep in a semi-coherent NiAl-Ni 2 AlTi alloy[END_REF][START_REF] Feng | New Phases in Ruthenium-Coating Single-Crystal Superalloys[END_REF]. In our simulations, several types of nanostructures are observed. They are composed of single-and polycrystals, and are including one or more polymorphs. Both binary FCC L1 2 and BCC D0 3 crystals are identified, as well as planar defects such as complex stacking faults and twin boundaries, both characterized by HCP local environments. Pseudo-twins and few HCP D0 19 nanoprecipitates are also sparingly detected which is consistent with previous experimental [START_REF] Kovarik | Microtwinning and other shearing mechanisms at intermediate temperatures in Ni-based superalloys[END_REF] and numerical [START_REF] Amodeo | Mechanical properties and dislocation nucleation in nanocrystals with blunt edges[END_REF] observations. Typical final configurations for τ = 50, 100 and 200 ns along with the temporal evolution of the fraction of BCC, FCC, and HCP-like environments are shown in Fig. 1 (see Supplemental Materials for all cases). The different nanocrystals can be rationalized in 3 categories (i) FCC rich, (ii) BCC rich and (iii) FCC + BCC, all with or without planar defects.

In the simulations, each crystallized nanoparticle is characterized by an initial BCC D0 3 nucleus, even when the final structure is FCC rich. This feature is emphasized by a relentless BCC peak at the beginning of the liquid-to-crystal transformation, as shown for example in Fig. 1c. Then, the BCC nucleus can either grow and fill most of the particle's volume, coexist with FCC crys-tals or more interestingly transform into a binary FCC via a reverse martensitic transformation [START_REF] Krauss | Fine structure of austenite produced by the reverse martensitic transformation[END_REF][START_REF] Kajiwara | Reversibility of crystal orientation in the reverse martensitic transformation in Fe-Ni alloys[END_REF][START_REF] Kajiwara | Characteristic features of shape memory effect and related transformation behavior in Fe-based alloys[END_REF]. This phase transformation is illustrated in Fig. 2. In this simulation, the first liquid-to-solid transition is identified at t = 29.6 ns where a D0 3 BCC crystallite nucleates. Then, at a different location within the nanoparticle, the expected L1 2 FCC structure emerges and at t = 30.6 ns the two phases coexist. Few tens of picoseconds later, the BCC crystal transforms into an FCC one as shown at t = 37.2 ns. One can note that (i) the grains relative orientation and the remaining twin boundaries foster a phase transformation process rather than grain growth, (ii) most of the crystallization process happens within few tens of ns, around T = 1000 K. Therefore, one can speculate that most of the twin boundaries retrieved from L1 2 nanocrystals in the simulation originate from a local BCC-to-FCC phase transformation.

While it is the first time the binary BCC D0 3 phase of Ni 3 Al is characterized from a liquid-to-solid transformation, the phase has already been observed in MD simulations of Ni 3 Al nanowires under tensile tests using a using a slightly different interatomic potential [START_REF] Mishin | Atomistic modeling of the γ and γ'-phases of the Ni-Al system[END_REF][START_REF] Wang | MAT : Size-dependent transition of deformation mechanism, and nonlinear elasticity in Ni 3 Al nanowires[END_REF]. In the latter studies, the authors observed the aforementioned transformation for particularly low diameters (< 5 nm) only, where the surface over volume ratio is particularly elevated, demonstrating the key-role of harmonic elasticity at the nanoscale, under extremely high stress (in the GPa range). These results are in good agreement with our simulations and a similar argument may apply here as (i) the crystal embryo is particularly distorted at the liquid-to-solid transition and (ii) only small nanodroplets are considered. Furthermore, the presence of a binary BCC embryo at the early stage of nucleation is also consistent with results previously obtained in bulk with two simple systems made of Lennard-Jones atoms [START_REF] Ten Wolde | Numerical Evidence for bcc Ordering at the Surface of a Critical fcc Nucleus[END_REF][START_REF] Desgranges | Controlling Polymorphism during the Crystallization of an Atomic Fluid[END_REF][START_REF] Lam | Lattice induced crystallization of nanodroplets: the role of finite-size effects and substrate properties in controlling polymorphism[END_REF] and hard-spheres [START_REF] Auer | Prediction of absolute crystalnucleation rate in hard-sphere colloids[END_REF]. In our case, we are studying a more complex system which involves (i) nanostructures, (ii) many-body interactions, (iii) multi-component systems and (iv) quenching temperature with time but still manage to observe the emergence of the BCC structure which indicates that this process may be ubiquitous in liquid to solid transitions.

Our results reveal the competition between the L1 2 FCC and the D0 3 BCC crystal structures of Ni 3 Al. These can be observed for all of the investigated values of τ , but in various statistical proportions. Fig. 3.a shows the proportion of each crystal structure as a function of the quench duration. In general, the FCC structure is the most likely to stabilize at the end of the simulation for all the investigated values of τ . However, its preponderance is decreased for the lowest quench duration while the BCC structure has the opposite behavior. In addition, for each of the simulations, we recorded the time and temperature for which an ordered cluster of 10 atoms is formed [See Fig. 3.b]. It appears that the freezing temperature is located at approximately 1000 K and slightly decreases with the quench duration which is typical of a thermodynamic hysteresis [START_REF] Rossi | Thermodynamics of CuPt nanoalloys[END_REF]. Finally, in Fig. 3.(c,d,e), the temporal evolution averaged over all of the crystallizing simulations confirms that (i) BCC structures emerge at the early stages of nucleation and that (ii) decreasing the quench duration drives the system towards further stabilization of the BCC structure. We note that this averaging hides the fact that BCC can also be the most preponderant structure at the end of some of the freezing simulations [See Fig. 1.a]. In the following, the presence of the BCC crystallites will be investigated using metadynamics simulations.

In order to better understand the complex mechanisms associated with the formation of the Ni 3 Al nanoparticles, we estimated the crystallization free energy landscapes using metadynamics simulations [START_REF] Laio | Escaping free-energy minima[END_REF]. In this kind of simulations, the choice of the collective variable (CV) is crucial [START_REF] Tribello | Analyzing and Driving Cluster Formation in Atomistic Simulations[END_REF]. In general, traditional CVs include the size of the biggest crystalline cluster [START_REF] Cheng | Classical nucleation theory predicts the shape of the nucleus in homogeneous solidification[END_REF] and Steinhardt's parameters [START_REF] Desgranges | Can Ordered Precursors Promote the Nucleation of Solid Solutions?[END_REF], which tend to bias the system towards a specific crystal structure. As an alternative, CVs defined from the structure of the initial and final states of the system were proposed, based on permutation invariant vectors [START_REF] Pipolo | Navigating at Will on the Water Phase Diagram[END_REF] or on specific peaks of the X-ray diffractogram [START_REF] Pavan | Metallic nanoparticles meet metadynamics[END_REF][START_REF] Zhang | Improving collective variables: The case of crystallization[END_REF]. In the case of polymorph selection, it becomes convenient to employ structurally agnostic collective variables to avoid any constraint on the crystal structure. For that purpose, we use the effective entropy (denoted S), as recently developed by Piaggi et al. [START_REF] Piaggi | Predicting polymorphism in molecular crystals using orientational entropy[END_REF][START_REF] Piaggi | Entropy based fingerprint for local crystalline order[END_REF][START_REF] Piaggi | Enhancing Entropy and Enthalpy Fluctuations to Drive Crystallization in Atomistic Simulations[END_REF], together with the potential energy (denoted E). While the latter is directly computed from the force field, S is evaluated using an approximation based on liquid state theory:

S = -2πρk B ∞ 0 [g(r) ln g(r) -g(r) + 1]r 2 dr ( 1
)
where ρ is the density of the system, k B is the Boltzman constant and g(r) is the two-body correlation function that can be numerically computed at each simulation step [START_REF] Piaggi | Enhancing Entropy and Enthalpy Fluctuations to Drive Crystallization in Atomistic Simulations[END_REF] within a cutoff of 5 Å.

For the metadynamics simulations, the width of the deposited Gaussians along CVs S and E are chosen as twice the standard deviation of their distribution in the liquid regime, giving δS = 0.125 kJ/mol/K and δE = 0.50 kJ/mol. We adopted a Gaussian height of 5 kJ/mol and a deposition interval of 0.1 ps. The free energy sampling is carried out during 80 ns which is similar to what was done with Ag 147 Pt 147 [START_REF] Pavan | Metallic nanoparticles meet metadynamics[END_REF]. Our setup allows us to observe several reversible transitions and to achieve a satisfactory convergence of the free energy landscapes [see Supplemental Material for quantitative details].

Fig. 4.a shows the free energy profile at 300 K along with the FCC and BCC structure fraction projected onto the (S,E) space. It appears that -as expected for moderate temperatures -the system adopts the L1 2 FCC structure. No other minimum is detected within a free-energy difference of 150 k B T . Interestingly, at 650 K a second region associated with the D0 3 BCC structure emerges, while remaining less stable than the L1 2 region [See Fig. 4.b]. Finally, for the largest temperature of 1000 K -corresponding to the phase change temperature observed in the freezing MD simulations -the free energy landscape becomes more complex. Indeed, a liquid region emerges in the high-energy (E = -390 kJ/mol) and high-effective entropy domain (S = -17.5 k B ). In addition, the crystalline region is divided in two, with the BCC region now the more stable and the FCC region metastable.

Altogether, the metadynamics calculations show that the free energy landscape is highly dependent on the temperature with a competition between D0 3 BCC and L1 2 FCC regions. We show that the D0 3 BCC region is the most stable at high temperature while the L1 2 FCC region becomes preponderant at room temperature. Based on these findings, we suggest the following scenario able to rationalize the formation of nanoparticles upon freezing: (1) At high temperature (T > 1000 K), the system remains in the liquid state; (2) At approximately 1000 K, a crystalline region emerges with BCC as its most favorable structure; (3) If the nucleation core appears in this temperature region, the system is more likely to grow by keeping the BCC structure, even if FCC growth is also possible; (4) If the nucleation event missed this temperature range, the systems follows the most stable scenario and grows as FCC; [START_REF] Ma | Tuning crystallization pathways through sequence engineering of biomimetic polymers[END_REF] The two types of nucleation events can occur concurrently and in this case, the system relaxes towards a coexisting structure.

As a perspective, we stress that the very surprising nucleation scenario observed here could probably be witnessed with other bimetallic nanoparticles as well as different types of nanoparticles including oxydes and chalcogenides. To study those, we foresee the use of a similar strategy combining direct freezing simulations with metadynamics calculations using the effective entropy as collective variable, as it allowed for reconstructing the nucleation process of Ni 3 Al nanoparticles along different competing pathways without any a priori on the crystalline structures.

From the experimental point of view, Ni 3 Al nanoparticles have mostly been produced with cooling rates that were slower or quicker than what is investigated in our simulations. Indeed, on the one hand, inert gas phase condensation [START_REF] Meesala | Contrasting magnetic behaviors of Ni3Al nanoparticles with and without NiO shell[END_REF], plasma methods [START_REF] Wang | Synthesis and structural features of Ni-Al nanoparticles by hydrogen plasma-metal reaction[END_REF][START_REF] Xu | Fabrication of Ni-Al nanoparticles via vacuum arc plasma evaporation and their catalytic properties for CO oxidation[END_REF] and mechanical alloying [START_REF] Yu | Synthesis of nanocrystalline Ni3Al by mechanical alloying and its microstructural characterization[END_REF] involved slower cooling rates and led to the formation of the most stable crystal structure ie. FCC L1 2 . On the other hand, pulsed laser deposition with very quick cooling rates (10 4 K/ns) produced amorphous thin films [START_REF] Bysakh | Evolution of metastable microstructures in laser ablation deposited Ni75Al25 thin films[END_REF]. As such, our numerical simulations are consistent with all of these experimental results since we obtained FCC L1 2 and amorphous nanoparticles respectively with the slowest and the quickest simulated cooling rates. Furthermore, our findings indicate that one could stabilize the so-far unseen BCC DO 3 structure by adjusting the cooling rate in between these two regimes, for example by changing experimental conditions in inert gas phase condensation or in laser ablation [START_REF] Shih | The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations[END_REF]. Our work elucidates the critical issue of the competition between thermodynamics and kinetics in the out of equilibrium synthesis of nanoparticles, and it indicates a route towards the rational design and control of experimental growth processes.

FIG. 1 .

 1 FIG. 1. Examples of crystallization kinetics and resulting Ni 3 Al nanocrystals. Atoms color coding: Blue = BCC (D03), Red = HCP (planar defects), Green = FCC (L12) local environments. Non-crystalline atoms are removed for the sake of clarity. (a) BCC rich crystal (τ = 50 ns). Only few atoms in FCC and HCP environments are identified. (b) Mixed FCC-BCC polycrystal (τ = 100 ns). (c) FCC rich crystal (τ = 200 ns). The sample is a twinned bicrystal (twin boundaries formed by one HCP layer). The black dashed-line illustrates the classical mirror relation from both sides of the twin boundaries.

FIG. 2 .

 2 FIG. 2. Emergence of a BCC peak during Ni 3 Al nanoparticle crystallization. Color coding corresponds to the previous figure. Grey dashed lines correspond to the snapshots. The black square emphasized the BCC peak.

FIG. 3 .

 3 FIG. 3. (a,b) Fraction of the different crystal structures and freezing temperature as a function of the quench duration τ . Temporal evolution of the crystal fraction for (c) τ = 50 ns, (d) τ = 100 ns and (e) τ = 200 ns. Results for (a-e) are averaged over all of the simulations.

FIG. 4 .

 4 FIG. 4. Left panel: Free energy landscape obtained with (S,E) as collective variable at three different temperatures:(a1), T=650K (b1) and T=1000K (c1). Right panel: Mapping of the average proportion of BCC and FCC structures along the metadynamic trajectory at the same temperatures. Regions with free energy larger than 150kT were not considered in this curve. The dotted green line correspond to the separation between liquid, FCC and BCC structures.

Acknowledgement

JL acknowledges financial support of the Fonds de la Recherche Scientifique -FNRS. Computational resources have been provided by the Consortium des Equipements de Calcul Intensif (CECI) and by the Fédération Lyonnaise de Modélisation et Sciences Numériques (FLMSN). The authors are also grateful to James F. Lutsko, David Amans, Magali Benoit, Michel Perez, Xavier Boulnat and Joel Bonneville for fruitful discussions. All authors finally thank Pablo M. Piaggi for carefully reading the manuscript and providing insightful comments.


Kazushige Ueda

Takuma Yoshino

Yuhei Shimizu

Tetsuo Honma

Florian Massuyeau

Stéphane Jobic

Site-Dependent Eu 3+ Photoluminescence in Double Perovskite-Type Alkaline Earth Lanthanum Tantalates

Keywords: Eu 3+ emission, site-dependent luminescence, double perovskite, alkaline earth lanthanum tantalates

Highlights

1. Phase-pure Eu 3+ doped Ca2LaTaO6 (CLTO) and Ba2LaTaO6 (BLTO) were prepared.

2. Eu 3+ were confirmed to be at A sites in CLTO and B sites in BLTO by XANES analysis.

3. Eu 3+ site-dependent PL spectra were obtained using the Eu 3+ -site-confirmed samples.

4. Site-dependent PL and the lifetime were examined systematically by CT excitation.

5. High external efficiency of 28% was found in the Eu 3+ doped BLTO double perovskite.

INTRODUCTION

Lanthanide (Ln) ions are frequently doped in various host lattices (HLs) and used as activators in luminescent materials taking advantage of the 4f-4f and 5d-4f electronic transitions. [START_REF]Phosphor Handbook[END_REF][START_REF] Blasse | Luminescent Materials[END_REF][START_REF] Kitai | Luminescent Materials and Applications[END_REF] Because the sizes of the Ln ions are relatively large, they are usually substituted for large cations such as La, Y, Ba, Sr and Ca ions. [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF] In the HLs of simple perovskite-type oxides (ABO3), these large cations are basically located at A sites accompanying smaller cations such as transition metal ions at B sites. [START_REF] Roth | Classification of Perovskite and Other ABO3-Type Compounds[END_REF] Therefore, in many cases, luminescence properties of Ln doped simple perovskite-type oxides reported in literatures are derived from Ln ions located at A sites.

Accordingly, the luminescence properties of Ln ions at B sites have not been investigated sufficiently so far. Even though the study on the B-site luminescence is intended, the doping of large Ln ions at B site is basically difficult in the simple perovskite-type oxides for steric reasons. This can be simply understood from the Goldschmidt's tolerance factor, which gives a criterion of the stability of the perovskite-type structure. [START_REF] Goldschmidt | Die Gesetze der Krystallochemie[END_REF] In contrast to the simple perovskite-type oxides, double perovskite-type oxides, which can be expressed by the formula of (AA')[BB']O6, have a more wide diversity of the combinations of cations in the viewpoints of both their valence and sizes. [START_REF] Vasala | A2B'B"O6 perovskites: A review[END_REF][START_REF] Hossain | An overview of double perovskites A2B'B"O6 with small ions at A site: Synthesis, structure and magnetic properties[END_REF][START_REF] Volonakis | Oxide analogs of halide perovskites and the new semiconductor Ba2AgIO6[END_REF] This is because two ions at each site, A and A' or B and B', can be chosen separately and taken into account for the structural stability.

Alkaline earth lanthanum tantalates, Ca2LaTaO6 (CLTO) and Ba2LaTaO6 (BLTO), are typical examples; La 3+ ions occupy A sites randomly with Ca 2+ ions in CLTO as expressed by (CaLa)[CaTa]O6, whereas La 3+ ions can inhabit B sites alternately with Ta 5+ ions in BLTO as indicated by Ba2[LaTa]O6. [START_REF] Trunov | The X-ray analysis of the compounds Ca2LnTaO6, Russ[END_REF][START_REF] Doi | Magnetic properties of ordered perovskites Ba2LnTaO6 (Ln = Y, lanthanides)[END_REF][START_REF] Saines | Structures and crystal chemistry of the double perovskites Ba2LnB'O6 (Ln = lanthanide B'= Nb 5+ and Ta 5+ ): Part I. Investigation of Ba2LnTaO6 using synchrotron X-ray and neutron powder diffraction[END_REF] Therefore, each La 3+ ion is coordinated by eight O 2-ions at A site in CLTO and six O 2-ions at B site in BLTO. The crystal structures of CLTO and BLTO are shown in Figure 1.

Eu 3+ ions are widely used as activators for red or red-orange luminescence originating from 5 D0 -7 FJ electronic transitions. In addition, among the Ln ions, Eu 3+ ions are especially known to show clear site-dependent luminescence; the induced electronic dipole (ED) transitions from 5 D0 to 7 F2 states are mainly observed at a site without inversion symmetry, while the magnetic dipole (MD) transitions from 5 D0 to 7 F1 states are predominantly seen at a site with inversion symmetry. [START_REF] Binnemans | Interpretation of europium(III) spectra[END_REF][START_REF] Blasse | On the Eu 3+ fluorescence in mixed metal oxides[END_REF] In the crystal structures of CLTO and BLTO, the site-symmetry for La sites is 1 without inversion symmetry in CLTO and 1 � with inversion symmetry in BLTO. Consequently, distinguishable Eu 3+ emissions are anticipated between Eu 3+ doped CLTO (CLTO:Eu 3+ ) and BLTO (BLTO:Eu 3+ ) if Eu 3+ are substituted at La sites as intended.

Many Eu 3+ doped double perovskite-type oxides have been already reported to show red or redorange luminescence. Nevertheless, data on the site-dependent photoluminescence (PL) of Eu 3+ in these host lattices (HLs) are still limited [START_REF] Yin | Excellent red phosphors of double perovskite Ca2LaMO6:Eu (M=Sb, Nb, Ta) with distorted coordination environment[END_REF][START_REF] Zhu | Structural phase transitions and photoluminescence properties of Eu 3+ doped Ca(2-x)BaxLaNbO6 phosphors[END_REF][START_REF] Sletnes | Photoluminescence of A-and B-site Eu 3+ -substituted (SrxBa1-x)2CaWyMo1-yO6 phosphors[END_REF][START_REF] Wang | Unraveling the distinct luminescence thermal quenching behaviours of A/B-site Eu 3+ ions in double perovskite Sr2CaMoO6:Eu 3+[END_REF] In particular, comprehensive comparisons of Eu 3+ PL characteristics between A and B sites and the fluorescence lifetimes have been hardly discussed systematically. In some reports, Eu 3+ PL properties in several double perovskite-type oxides were studied, but the site-dependence and the lifetime were not primarily concerned. In other contributions, Eu 3+ ions were excited differently either via HL, charge transfer (CT) bands or 4f-4f transitions, which possibly resulted in different PL spectral shapes and lifetimes.

Therefore, the site-dependent Eu 3+ PL properties in the double perovskite-type oxides have not been completely understood yet.

In our previous study, the site-dependent Eu 3+ PL was examined in the simple perovskite-type oxide of LaLuO3 doped with Eu 3+ at A and B sites (LaLuO3:Eu 3+ (A) and LaLuO3:Eu 3+ (B), respectively). [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] Although the differences of the spectral shapes between LaLuO3:Eu 3+ (A) and LaLuO3:Eu 3+ (B) were evidently observed depending on the Eu 3+ sites, some mixed luminescence from both A and B sites were observed, especially in B-site doped samples at high Eu 3+ concentration. In this study, the PL and PLE properties of CLTO:Eu 3+ and BLTO:Eu 3+ were examined in details to observe individual Eu 3+ PL from A or B sites more clearly taking advantage of the structural difference in these double perovskite-type oxides. The location of Eu 3+ ions were surely determined by the X-ray absorption near edge structures (XANES) of the Eu LIII absorption edge. The site-dependent PL spectra and the lifetimes were obtained from Eu 3+ ions, which were commonly excited via CT bands. [START_REF] Dorenbos | The Eu 3+ charge transfer energy and the relation with the band gap of compounds[END_REF]

EXPERIMENTS

The samples of CLTO:Eu 3+ and BLTO:Eu 3+ were prepared by the conventional solid state reaction. The nominal compositions of the samples are (CaLa1-xEux)[CaTa]O6 for CLTO:Eu 3+ and Ba2[La1-xEuxTa]O6 for BLTO:Eu 3+ (x = 0.01, 0.10). Starting materials of CaCO3 or BaCO3, La2O3, Ta2O5, and Eu2O3 (Kojundo Chemical Laboratory Co., Ltd., 99.9%) were weighed in stoichiometric amount and thoroughly mixed with ethanol in an agate mortar. The mixed powders were dried and shaped into pellets, and heated at 1300 °C for 6 h in air. The pellets were then finely ground, powders were pressed into pellets again and heated at 1400 °C for 6 h in air.

Nondoped samples (x = 0.00) were also prepared similar way to determine the absorption edges of the HLs. X-ray diffraction (XRD) measurements were carried out using an X-ray diffractometer with a rotary Cu target (Rigaku, RINT-2500). The powder XRD patterns for the pure crystalline phases were simulated using the RIETAN code, [START_REF] Izumi | Three-dimensional visualization in powder diffraction[END_REF] and the images of their crystal structures were depicted using the VESTA code. [START_REF] Momma | VESTA3 for three dimensional visualization of crystal, volumetric and morphology data[END_REF] The spectra of X-ray absorption near edge structures (XANES) at the Eu LIII edges were measured at the BL14B2 beamline of the SPring-8 synchrotron radiation facility. The XANES spectra of the samples with x = 0.10 were obtained along with those of reference samples of EuAlO3 and Ba2[EuTa]O6. The PL and PLE spectra of the samples were measured at room temperature (RT; 298 K) and liquid nitrogen temperature (LNT; 77 K) using conventional spectrofluorometers (JASCO FP-6500 and Horiba Fluorolog-3).

The decay curves of Eu 3+ PL peaks were examined to estimate the lifetimes for Eu 3+ at A and B sites. The quantum efficiencies of Eu 3+ PL derived from A and B sites were also evaluated at RT using an integrating sphere.

RESULTS AND DISCUSSION

The XRD patterns of the samples, (CaLa1-xEux)[CaTa]O6 and Ba2[La1-xEuxTa]O6 (x = 0.00, 0.01, 0.10), are shown in Figure 2. All samples are phase pure and no secondary phases are detected. Magnified diffraction peaks are shown in insets to clarify peak shifts induced by Eu 3+ substitution at La sites. The peak shifts are clearly seen for the samples of x =0.10 in both CLTO:Eu 3+ and BLTO:Eu 3+ . The peak shifts to higher angles suggest the decrease of the lattice spacing due to the substitution of Eu 3+ for La 3+ . These results are reasonable because the ionic radii of Eu 3+ ions (rVIII = 1.206 Å at A sites, rVI = 1.087 Å at B sites) are smaller than those of La 3+ ions (rVIII = 1.300 Å, rVI = 1.172 Å). [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF] The locations of Eu 3+ ions in the crystal structures were determined by XANES measurements.

The spectra of XANES at the Eu LIII edges for CLTO: Eu 3+ and BLTO: Eu 3+ are depicted in Figure 3. Perovskite-type EuAlO3 and double perovskite-type Ba2[EuTa]O6 were used as references for Eu 3+ at A site and B site, respectively. [START_REF] Doi | Magnetic properties of ordered perovskites Ba2LnTaO6 (Ln = Y, lanthanides)[END_REF][START_REF] Geller | Crystallographic Studies of Perovskite-like Compounds. II. Rare Earth Aluminates[END_REF] Because the Eu LIII absorption is primarily derived from electronic transitions from 2p to 5d states, the shape of the intense LIII absorption peaks denotes the densities of the empty Eu 3+ 5d states, which enable us to identify the crystal sites occupied by the Eu 3+ ions. [START_REF] Agarwal | X-Ray Spectroscopy[END_REF][START_REF] Bunker | Introduction to XAFS[END_REF] EuAlO3 shows an intense single peak at 6979 eV, whereas Ba2[EuTa]O6 does a main peak at 6981 eV with a shoulder at 6978 eV. The positions of two broad peaks observed at higher energy are also different for the two compounds, located at 6991 and 7010 eV for EuAlO3 and at 6997 and 7019 eV for Ba2[EuTa]O6. The shoulder peak of the main peak observed in Ba2[EuTa]O6 can be regarded as a consequence of the splitting of the empty Eu 3+ 5d states under the influence of the crystal field. This splitting is expected to be significantly larger for Eu 3+ ions at small octahedral B sites than at large cuboctahdral A sites. Namely, an energy splitting as large as ca.

3 eV can be envisioned for Ba2[EuTa]O6 with the average Eu-O bond length of 2.26 Å, which is much shorter than that in EuAlO3 (dEu-O = 2.46 Å). [START_REF] Doi | Magnetic properties of ordered perovskites Ba2LnTaO6 (Ln = Y, lanthanides)[END_REF][START_REF] Geller | Crystallographic Studies of Perovskite-like Compounds. II. Rare Earth Aluminates[END_REF] CLTO:Eu 3+ gives a main single peak at E = 6979 eV and two broad peaks at higher energy, indicating the location of Eu 3+ at A sites as in EuAlO3. BLTO:Eu 3+ provides a spectral shape similar to that of Ba2[EuTa]O6 with a double peak at 6981 and 6978 eV, which endorses Eu 3+ ions at B sites. These observations of Eu 3+ local structures directly revealed the locations of Eu 3+ ions in CLTO and BLTO HLs. Figure 4 shows PL and PLE spectra of CLTO:Eu 3+ 10% and BLTO:Eu 3+ 10% along with the absorption (Abs) spectra of CLTO and BLTO HLs. The spectral features for the Eu concentration of x = 0.01 were almost the same with those for x = 0.10. Going from x = 0.01 to x = 0.10, PL intensities increase simply and BLTO:Eu 3+ systematically exhibited higher PL intensities than CLTO:Eu 3+ . To achieve a systematic understanding of site-dependent Eu 3+ PL, Eu 3+ ions were directly excited through the CT from O 2-to Eu 3+ in both CLTO:Eu 3+ and BLTO:Eu 3+ as previously done in LaLuO3:Eu 3+ . [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] Based on the examination of the CLTO absorption spectrum, the absorption threshold of the CLTO HL is estimated to be at 240 nm. The PLE spectrum of CLTO:Eu 3+ , monitoring the PL peak at 614 nm, shows an intense broad band centered at 298 nm and several sharp peaks in the 360-420 nm range. The former intense band can be unambiguously assigned to the CT transitions, and the latter is to Eu 3+ intraconfigurational 4f-4f ones. Let us notice that the position of the CT excitation band (λex = 298 nm) in CLTO:Eu 3+ is very close to that (λex = 292 nm) in LaLuO3:Eu 3+ (A). [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] This may originate from almost identical average Eu-O bond length at A sites, regarding the Eu-O bond length as La-O one, i.e. 2.61 Å for CLTO and 2.66 Å and for LaLuO3. Formally, CT excitation bands for La perovskite-type HLs with Eu 3+ activators at A sites, such as LaAlO3 (dLa-O = 2.68 Å) and LaGaO3 (dLa-O = 2.65 Å), are observed at analogous wavelengths (310 and 303 nm, respectively). [START_REF] Dorenbos | The Eu 3+ charge transfer energy and the relation with the band gap of compounds[END_REF][START_REF] Geller | Crystallographic Studies of Perovskite-like Compounds. II. Rare Earth Aluminates[END_REF][START_REF] Howard | Neutron powder diffraction study of rhombohedral rare-earth aluminates and the rhombohedral to cubic phase transition[END_REF][START_REF] Slater | High-Temperature Powder Neutron Diffraction Study of the Oxide Ion Conductor La0.9Sr0.1Ga0.8Mg0.2O2.85[END_REF] The PL spectrum of CLTO:Eu 3+ obtained by the CT excitation at 298 nm consists of an intense peak at 614 nm that derives from the 5 D0 -7 F2 transitions. The excitation through a 4f-4f transition at 397 nm did not lead to any significant change in the PL spectral features in comparison with the CT excitation.

The absorption spectrum of BLTO reveals an absorption edge at 250 nm. The PLE spectrum of BLTO:Eu 3+ , monitored at 594 nm, shows an intense broad band at 278 nm. Accordingly, the intense band is associated with the CT transition. The 4f-4f excitation peaks in BLTO: Eu 3+ are negligible and much less intense than those observed in CLTO:Eu 3+ . This stems from the forbidden character of the 4f-4f ED transitions for Eu 3+ at the B sites in contrast with partly permitted 4f-4f ED transitions for Eu 3+ at the A sites. Moreover, the CT excitation band in BLTO:Eu 3+ appears at shorter wavelengths than that in CLTO:Eu 3+ (278 vs. 298 nm). This result is consistent with the one that the CT band for Eu 3+ at B sites is observed at shorter wavelengths than that for Eu 3+ at A sites in Eu 3+ doped LaLuO3. [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] The blue-shift of the CT band for Eu 3+ ions at B sites is attributed to the shorter Eu -O bond lengths at B sites in BLTO:Eu 3+ (dLa(Eu)-O = 2.43 Å) than at A sites in CLTO:Eu 3+ (dLa(Eu)-O = 2.61 Å). Namely, Coulomb repulsion between electrons in Eu 4f orbitals and those in O 2p orbitals becomes larger at B sites resulting in the uplifted energy of the CT states. It is noteworthy that the blue-shift of the CT excitation peak in BLTO:Eu 3+ is not as large as that observed in LaLuO3:Eu 3+ (B) (λex = 230 nm). This is because the Eu -O bond lengths in BLTO:Eu 3+ is not as short as those in LaLuO3:Eu 3+ (B) (dLu(Eu)-O = 2.22 Å). The PL spectrum recorded at λex = 278 nm consists mainly of the peak at 594 nm assigned to the 5 D0 -7 F1 transitions. The PL spectral shapes are largely different between CLTO:Eu 3+ and BLTO:Eu 3+ and the details are discussed below.

The PL peaks in CLTO:Eu 3+ and BLTO:Eu 3+ obtained by the CT excitation at RT and LNT are magnified in Figure 5. In the CLTO:Eu 3+ , Eu 3+ ions at A sites without the inversion symmetry make the PL for the 5 D0 -7 F2 induced ED transitions dominant and the PL for the 5 D0 -7 F4 transitions secondly dominant. The weak single PL peak for the 5 D0 -7 F0 transition and three peaks for the 5 D0 -7 F1 transitions are seen although they are forbidden originally. In the BLTO:Eu 3+ , Eu 3+ ions at B sites with the inversion symmetry drive the PL for the 5 D0 -7 F1 MD transitions predominant and the PL for the 5 D0 -7 F0 transition almost absent. These features are very similar to the Eu 3+ luminescence observed in Ba2MgWO6:Eu 3+ , Ba2La2/3TeO6:Eu 3+ and Ba2GdNbO6:Eu 3+ . [START_REF] Miniajluk | Eu 3+ ions in the highly symmetrical octahedral site in Ba2MgWO6 double perovskite[END_REF][START_REF] Lal | Structural characterization of B-site ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) double perovskites and probing its luminescence as Eu 3+ phosphor hosts[END_REF][START_REF] Yu | Enhanced photoluminescence of Ba2GdNbO6: Eu 3+ /Dy 3+ phosphors by Li + doping[END_REF] The single PL peak for the 5 D0 -7 F0 transition can be recognized at LNT by further magnification. The red-shift or larger splitting for the 5 D0 -7 F4 transition peaks is also observed in BLTO:Eu 3+ , which is probably attributed to the differences in spin-orbital coupling and crystal field between A and B sites.

The single peak of the 5 D0 -7 F0 transition at different wavelength in CLTO:Eu 3+ (580 nm) and BLTO:Eu 3+ (581 nm) confirms the single site of Eu 3+ in each material. [START_REF] Blasse | On the Eu 3+ fluorescence in mixed metal oxides. II. the 5 D0-7 F0 emission[END_REF] The peak splitting for the 5 D0 -7 F1 transition is clearly seen in CLTO:Eu 3+ but not in BLTO:Eu 3+ . Taking the site symmetry into account, the triple splitting in CLTO:Eu 3+ is reasonable for Eu 3+ at A sites with the low site symmetry of 1. [START_REF] Binnemans | Interpretation of europium(III) spectra[END_REF][START_REF] Blasse | On the Eu 3+ fluorescence in mixed metal oxides[END_REF] The site symmetry of B sites in BLTO is 1 � and low enough to give the peak splitting, but it is hardly observed in BLTO:Eu 3+ . Two possibilities are considered to explain the absence of the splitting. One is that the splitting is too small to detect for the spectrometer used in this study. The other is that the local structural relaxation around Eu 3+ ions at B sites occurs and makes the Eu 3+ site symmetry very high because the distortion of the B-site octahedron is originally small in BLTO and Ba2[EuTa]O6. In comparison with the spectral shape of Ba2MgWO6:Eu 3+ , [START_REF] Miniajluk | Eu 3+ ions in the highly symmetrical octahedral site in Ba2MgWO6 double perovskite[END_REF] the latter seems to be more presumable. As for the temperature dependence of spectral shapes, the peaks in the spectra for both materials become sharper at LNT than at RT, but the spectral features are unchanged. This results indicates the doped Eu 3+ ions are distributed homogeneously and they are isolated enough to provide the individual Eu 3+ PL from A or B sites in each HL. Therefore, the mixing of the Eu 3+ PL from A and B sites is not observed at RT and LNT in these double perovskite-type HLs, which contrasts with the simple perovskitetype HL of LaLuO3. 41 6. The main peak at 614 nm for CLTO:Eu 3+ derived from the induced ED transitions and the intense peak at 594 nm for BLTO:Eu 3+ derived from the MD transitions were examined at RT and LNT. It was found obviously that the Eu 3+ PL from A sites in CLTO:Eu 3+ shows much faster decay than that from B sites in BLTO:Eu 3+ . This result is very similar to that observed in the simple perovskite-type oxide of LaLuO3 doped with Eu 3+ at A and B sites. [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] The faster decay at A sites than at B sites is basically attributed to that the probability of the induced ED transitions is usually higher than that of the MD transitions. The transition probability will be dependent on not only the site symmetry but also the local structures such as bond lengths and coordination numbers.

The decay curves for CLTO:Eu 3+ are not largely dependent on the temperatures between RT and LNT, whereas those for BLTO:Eu 3+ showed conspicuous temperature dependence implying that the Eu 3+ PL from B sites is largely influenced by the lattice vibrations. Although these results are similar to the Tb 3+ doped samples reported previously, [START_REF] Ueda | Site Dependence of Tb 3+ Luminescence in Double Perovskite-Type Alkaline Earth Lanthanum Tantalates[END_REF] the temperature dependence for the present Eu 3+ doped samples is notably larger than the one for the Tb 3+ doped samples.

The B-O bonds in the perovskite-type ABO3 are much shorter and generally have more covalent character than the A-O bonds. [START_REF] Woodward | Octahedral tilting in perovskites. II. structure stabilizing forces[END_REF] In addition, the BO6 octahedra are three-dimensionally connected sharing corners in simple or double perovskite-type oxides. Therefore, the larger influence of the lattice vibration on the B-site luminescence seems to be reasonable. The valence states of Eu ions become to be divalent just after the CT excitation and their ionic sizes increase, whereas the valence states of Tb ions are the same after the 4f-5d excitation. The differences in the excited states between Eu and Tb ions are probably attributed to the larger temperature dependence for the Eu ions at B sites.

The lifetimes (τ ) for the Eu 3+ PL derived from the 5 D0 -7 F1 and 5 D0 -7 F2 transitions were estimated by the plots of ln(I/I0) vs. t as shown in the inset presuming that the decay curves follow a single-exponential function. The slopes in the inset provide the lifetimes τ = 0.69 ms for the 5 D0 -7 F2 transitions both at RT and LNT in CLTO:Eu 3+ andτ = 3.7 ms for the 5 D0 -7 F1 transitions at RT and τ = 5.1 ms at LNT in BLTO:Eu 3+ . These values were obtained by the direct excitation of Eu 3+ through the CT transitions. The evaluation of the lifetimes was also carried out by the direct Eu 3+ excitation through 4f-4f transitions at 395 or 397 nm. The results were almost the same with those by the CT excitation, indicating the relaxation from the excited CT states (Eu 2+ states) to the excited Eu 3+ 5 D0 states is far much faster than the relaxation from the 5 D0 to [START_REF] Vasala | A2B'B"O6 perovskites: A review[END_REF] FJ states.

Although the lifetimes of Eu 3+ PL in the double perovskite-type oxides have been reported in several papers, [START_REF] Tsai | A novel single-phased white light emitting phosphor of Eu 3+ ions-doped Ca2LaTaO6[END_REF][START_REF] Sun | Double perovskite Ca2LuTaO6:Eu 3+ red-emitting phosphors: Synthesis, structure and photoluminescence characteristics[END_REF][START_REF] Lee | Luminescent properties of Eu 3+ -activated Gd2ZnTiO6 double perovskite red-emitting phosphors for white light-emitting diodes and field emission displays[END_REF][START_REF] Miniajluk | Eu 3+ ions in the highly symmetrical octahedral site in Ba2MgWO6 double perovskite[END_REF][START_REF] Lal | Structural characterization of B-site ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) double perovskites and probing its luminescence as Eu 3+ phosphor hosts[END_REF][START_REF] Yu | Enhanced photoluminescence of Ba2GdNbO6: Eu 3+ /Dy 3+ phosphors by Li + doping[END_REF][START_REF] Yu | Structure and luminescence properties of Eu 3+doped trigonal double-perovskite Ba2Lu0.667WO6[END_REF][START_REF] Li | Double perovskite LiLaMgWO6:Eu 3+ novel red-emitting phosphors for solid state lighting: synthesis, structure and photoluminescent properties[END_REF][START_REF] Fu | A novel double perovskite La2ZnTiO6:Eu 3+ red phosphor for solid-state lighting: Synthesis and optimum luminescence[END_REF][START_REF] Liu | Enhanced luminescence properties of double perovskite (Ba, Sr)LaMgSbO6:Eu 3+ phosphors based on composition modulation[END_REF][START_REF] Zhong | Novel red-emitting Sr2LaSbO6:Eu 3+ phosphor with enhanced 5 D0→ 7 F4 transition for warm white light-emitting diodes[END_REF][START_REF] Ran | Crystal structure, electronic structure and photoluminescence properties of KLaMgWO6:Eu 3+ phosphors[END_REF][START_REF] Wei | Synthesis and luminescence properties of Eu 3+ -doped a novel double perovskite Sr2YTaO6 phosphor[END_REF][START_REF] Fan | Thermally stable double-perovskite Ca3TeO6:Eu 3+ red-emitting phosphors with high color purity[END_REF][START_REF] Wei | Synthesis and photoluminescence properties of Eu 3+ -Activated Double Perovskite Ba2YTaO6 Red Phosphor[END_REF][START_REF] Liang | Synthesis and photoluminescence properties of a novel high-efficiency red-emitting Ca2LuSbO6:Eu 3+ phosphor for WLEDs[END_REF][START_REF] Liu | Highly efficient red-emitting Ca2YSbO6:Eu 3+ double perovskite phosphors for warm WLEDs[END_REF] Eu 3+ sites and the excitation wavelengths have not been examined systematically for the understanding of the site-dependent Eu 3+ luminescence. Therefore, it is limited to compare the lifetimes among the double perovskite-type oxides. The present results were roughly the same with τ = 0.5 ms in Ca2LaTaO6:Eu 3+ , τ = 0.69 ms in Ca2LuTaO6:Eu 3+ and τ = 0.61 ms in Gd2ZnTiO6:Eu 3+ for Eu 3+ ions at A sites, and τ = 4.23 ms in Ba2MgWO6:Eu 3+ and τ = 3.55 ms in Ba2La2/3Te1/3O6:Eu 3+ for those at B sites. [START_REF] Tsai | A novel single-phased white light emitting phosphor of Eu 3+ ions-doped Ca2LaTaO6[END_REF][START_REF] Sun | Double perovskite Ca2LuTaO6:Eu 3+ red-emitting phosphors: Synthesis, structure and photoluminescence characteristics[END_REF][START_REF] Lee | Luminescent properties of Eu 3+ -activated Gd2ZnTiO6 double perovskite red-emitting phosphors for white light-emitting diodes and field emission displays[END_REF][START_REF] Miniajluk | Eu 3+ ions in the highly symmetrical octahedral site in Ba2MgWO6 double perovskite[END_REF][START_REF] Lal | Structural characterization of B-site ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) double perovskites and probing its luminescence as Eu 3+ phosphor hosts[END_REF] The quantum efficiencies (η ) of the Eu 3+ PL were evaluated at RT for excitation via the CT peaks, namely, at 298 nm for CLTO:Eu 3+ and at 278 nm for BLTO:Eu 3+ , using all emission peaks in the visible region. The internal quantum efficiency (η in) and external quantum efficiency (η ex) for CLTO:Eu 3+ (x=0.10) were 8 and 5 %, respectively, which were lower than those for LaLuO3:Eu 3+ (A). In contrast, those values for BLTO:Eu 3+ (x=0.10) were η in = 39 and η ex = 28 %, which were higher than those for LaLuO3:Eu 3+ (B) (η in = 34 and η ex = 27 %). [START_REF] Ueda | Site-Selective Doping and Site-Sensitive Photoluminescence of Eu 3+ and Tb 3+ in Perovskite-Type LaLuO3[END_REF] Between the two double perovskite-type materials, Eu 3+ at B sites in BLTO:Eu 3+ provided higher efficiency than that at A sites in CLTO:Eu 3+ . The dominant Eu 3+ luminescence at B sites is also observed in the double perovskite-type Sr2CaMo1-yWyO6 doped with Eu 3+ at A or B sites. [START_REF] Sletnes | Photoluminescence of A-and B-site Eu 3+ -substituted (SrxBa1-x)2CaWyMo1-yO6 phosphors[END_REF] On the other hand, the efficiency is comparable between A and B sites in the simple perovskite-type LaLuO3. 41 Because the η ex values are dependent on the HLs, excitation wavelengths, Eu 3+ concentrations, grain sizes and so on, it is not simple to compare them in details. In the other double perovskite-type oxide hosts for the application to white light emitting diodes, several η values have been reported; η = 17.8 % (λex = 395 nm), 27.7% (λex = 534 nm) for Ca2La0.6Eu0.4NbO6, η = 20.4 % (λex = 395 nm), 25.4% (λex = 465 nm) for Sr2Ca(Mo0.5W0.5)O6:Eu0.04 and η = 17 % (λex = 390 nm) for Sr2Ca0.7Eu0.15Li0.15W0.5Mo0.5O6. [START_REF] Yin | Excellent red phosphors of double perovskite Ca2LaMO6:Eu (M=Sb, Nb, Ta) with distorted coordination environment[END_REF][START_REF] Sletnes | Photoluminescence of A-and B-site Eu 3+ -substituted (SrxBa1-x)2CaWyMo1-yO6 phosphors[END_REF][START_REF] Zhang | Structure evolution and tunable luminescence of (Sr0.98-mBamEu0.02)2Ca(Mo1-nWn)O6 phosphor with ultraviolet excitation for white LEDs[END_REF]

CONCLUSION

The Eu 3+ luminescence of B-site-ordered double perovskite-type (CaLa1-xEux)[CaTa]O6 and Ba2[La1-xEuxTa]O6 (x = 0.01, 0.10) were investigated based on the experimental examination of Eu 3+ locations. XANES measurements confirmed that Eu 3+ ions are located at A sites in CLTO:Eu 3+ and at B sites in BLTO:Eu 3+ as expected in the nominal compositions. In the PLE spectra, broad CT excitation bands were observed in both CLTO:Eu 3+ and BLTO:Eu 3+ revealing the blue shift of the CT band for BLTO:Eu 3+ compared with CLTO:Eu 3+ due to the larger crystal field at B sites. Although both materials showed red PL, the PL spectral shapes were largely different depending on the Eu 3+ occupation sites accompanying the difference of the PL lifetimes. Eu 3+ ions at A sites without the inversion symmetry in CLTO:Eu 3+ provide the intense PL peak derived from the 5 D0 -7 F2 transition, whereas those at B sites with the inversion symmetry in BLTO:Eu 3+ give the sharp PL peak derived from the 5 D0 -7 F1 transition. As a result, evident site-dependent Eu 3+ PL and the lifetimes were obtained in these double perovskite-type oxides clarifying the spectral shape of each Eu 3+ PL from A or B site.

ACKNOWLEDGMENT

This work was partly supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant JP16K06724 and JP19K05004). The synchrotron radiation experiments were

Figure 1 .

 1 Figure 1. Crystal structures of (a) Ca2LaTaO6 and (b) Ba2LaTaO6 with double perovskite structure. La ions are located at A sites in (CaLa)[CaTa]O6 and at B sites in Ba2[LaTa]O6.

Figure 2 .

 2 Figure 2. XRD patterns of (top) (CaLa1-xEux)[CaTa]O6 and (bottom) Ba2[La1-xEuxTa]O6 (x = 0.00, 0.01, 0.10) along with those simulated from the structural data. Magnified diffraction peaks at high 2θ angles are shown in insets.

Figure 3 .

 3 Figure 3. XANES spectra of Eu LIII edges for (CaLa0.90Eu0.10)[CaTa]O6 and Ba2[La0.90Eu0.10Ta]O6 samples along with those for EuAlO3 and Ba2[EuTa]O6 references.

Figure 4 .

 4 Figure 4. PL and PLE spectra of (CaLa0.90Eu0.10)[CaTa]O6 (red lines) and those of Ba2[La0.90Eu0.10Ta]O6 (blue lines) samples along with absorption spectra of pure HL samples. PL, PLE, and Abs spectra are drawn with thin lines, thick lines and dashed lines, respectively.

Figure 5 .

 5 Figure 5. Magnified PL spectra of (CaLa0.90Eu0.10)[CaTa]O6 (red lines) and those of Ba2[La0.90Eu0.10Ta]O6 (blue lines) obtained by the CT excitation at RT (thin lines) and LNT (thick lines).

Figure 6 .

 6 Figure 6. Decay curves of PL peak intensities for (CaLa0.90Eu0.10)[CaTa]O6 (red circle symbols) and those for Ba2[La0.90Eu0.10Ta]O6 (blue square symbols) observed under CT excitation at RT (open symbols) and LNT (filled symbols).

The decay curves of the PL peaks for the 5 D0 -7 F2 transitions in CLTO:Eu 3+ and those for the 5 D0 -7 F1 transitions in BLTO:Eu 3+ obtained by excitation via the CT bands are shown in Figure

performed at beamline BL14B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal 2017B1576 and 2018B1573).