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The heat of formation of ye'elimite Ca4Al6O12.SO4 was calculated using density functional theory (DFT). The crystallographic ground state of ye'elimite is the orthorhombic Pcc2 modification in agreement with experimental information. Three different many-body interaction functionals were used for DFT calculations: the generalised gradient approximation (GGA), the Armiento/Mattsson 2005 functional (AM05) and the strongly conditioned and appropriately normed functional (Scan). The best results were obtained with the Scan functional and a heat of formation at 298 K of −35·5 kJ/mol referred to pure calcium oxide (CaO), aluminium oxide (Al2O3) and calcium sulfate (CaSO4) was found. All other reported modifications of ye'elimite (cubic, tetragonal, monoclinic) are unstable with endothermic heat of transformations ranging from +18 to +78 kJ/mol.

International audience; The question of material stability through the determination of the thermodynamic properties is of fundamental and technological importance to any analysis of system properties in many applications. For Al 4 C 3 , its experimental heat of formation varies widely, from −0.187 to −0.363 eV/atom, which makes it difficult to use such experimental information for any reactivity assessment. Here, we demonstrate that density functional theory (DFT), with the recently developed strongly constrained and appropriately normed (SCAN) functional, is especially powerful in critically assessing these experimental data. In order to have a more complete description of thermodynamic properties of Al 4 C 3 , we also determine the temperature dependence of its heat capacity using both the harmonic and quasi-harmonic approximation. In addition, and to select the most efficient approximation , we come back to the experimental background by using differential scanning calorimetry (DSC) to measure this quantity at constant pressure up to high temperatures, namely 873 K.

We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory.

T2-Al2MgC2 was synthesized from the elements in a Mg-Al melt at 1000 °C using sealed Ta crucibles. Single crystals of T2-Al2MgC2 were extracted by evaporating the Mg-Al matrix. The crystal structure of T2-Al2MgC2 was refined for the first time on the basis of single-crystal X-ray diffraction. The crystal is trigonal (space group P-3m1, Z = 1) with lattice parameters of a = 3.3767(11) A, c = 5.807(2) A and V = 57.34(5) A3. Based on the refined crystal structure, DFT calculations were conducted to evaluate the thermodynamic properties and the electronic structure of the phase. The heat of formation of T2-Al2MgC2 was calculated to be −23.6 kJ/mol of atoms at 298 K. The heat capacity of T2-Al2MgC2 was measured by DSC from 300 to 871 K and calculated by DFT from 0 to 1000 K. Based on the calculated heat capacity, the entropy of formation of the phase at 298 K was determined to be 70.0 J/mol/K. The band structure and the electronic density of state of T2-Al2MgC2 was calculated leading to an indirect band gap value of 1.73 eV.

International audience; A complete thermodynamic description of the Ca-O system is proposed including 3 rd generation modelling of crystalline CaO, liquid and amorphous CaO, and crystalline CaO2. Compared to previous modellings, a more robust description of the thermodynamic properties of the binary phases is achieved using data recently obtained by density functional theory calculations and molecular dynamics simulations. The heat capacity of crystalline CaO is reassessed, leading to a noticeably higher value below the melting point compared to previous modellings and resulting in a slightly higher standard entropy. It is highlighted that the parameters given in terms of polynomial functions of temperature that were employed so far in 3 rd generation models to describe anharmonic contributions in the heat capacity of compounds were not suited to satisfactorily describe the thermodynamic properties of crystalline CaO. It is suggested that this observation can be generalized to most refractory oxides. Alternative terms are proposed in the Gibbs energy function that give more flexibility in fitting the experimental data and lead to more numerically reasonable values for the parameters. The liquid and amorphous CaO phase is described using the two-state model, leading to a significant improvement in the description of its heat capacity. The description of crystalline CaO2 is also improved as only estimates of the thermodynamic properties of the compound were available in previous modellings of the system. Finally, phase equilibria data on the Ca-CaO liquidus is reviewed, and it is highlighted that slight discrepancies in the relatively low temperature measurements can lead to significantly different descriptions of the liquid phase. 2

Bringing a liquid into contact with a solid is known to generally promote crystal nucleation at the freezing temperature. In contrast, it is much more difficult to conceive that a solid surface may hinder nucleation and favor large undercooling effects. Here we report on ab initio and classical molecular dynamic simulations to capture the underlying structural mechanism responsible for this striking effect. We find that the substrate/liquid interactions exert an important influence on in-plane ordering of the adjacent liquid layers in the undercooling regime. In particular, we identify that the presence of atomic arrangements with five-fold symmetry (FFS) on the substrate surface in the form of pentagonal atomic motifs allows the liquid to be undercooled well below its freezing temperature. Our findings clearly demonstrate that this pentagonal-coordinated surface enhances the presence of local arrangements with FFS in the adjacent liquid layers that prevents the crystal nucleation. Finally we suggest new technological developments to attain large undercooling effects.

International audience; In the scope of supporting the development of Mg–Al alloys and related materials, an experimental study coupled with a CALPHAD thermodynamic modeling of the Al–C–Mg system was conducted. The peritectic decomposition of Al2MgC2 to form Al4C3, graphite and a liquid phase was measured at 1559 ± 10 K by DTA. In order to model the Mg solubility in Al4C3, DFT calculations were performed on the end-member phases Mg4C3, Al2Mg2C3 and Mg2Al2C3 and it was shown that Mg substitutes on the Al2 crystallographic site of the carbide structure. Based on recent literature data and a revised Al–C binary, a model description of the Al–C–Mg ternary is proposed for the first time. More specifically, it is used to calculate the liquidus projection and the phase formation sequence during Scheil solidification of a 91Mg–9Al wt% alloy inoculated with carbon. This work provides a convincing argument that Al2MgC2 is the nucleant responsible for the grain refinement of Mg–Al alloys inoculated by C.

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose ‘surface distortion’ as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts. Tuning surface structure is key for electrocatalytic performance and stability of proton-exchange membrane fuel cells. Surface distortion as a structural descriptor can help to clarify the role of surface defects and to design enhanced nanocatalysts.

The breakdown of the Stokes-Einstein (SE) law in fragile glassformers is examined by Molecular-Dynamics simulations of atomic liquids and polymers and consideration of the experimental data concerning the archetypical ortho-terphenyl glassformer. All the four systems comply with the universal scaling between the viscosity (or the structural relaxation) and the Debye-Waller factor < u(2)>, the mean square amplitude of the particle rattling in the cage formed by the surrounding neighbors. It is found that the SE breakdown is scaled in a master curve by a reduced < u(2)>. Two approximated expressions of the latter, with no and one adjustable parameter, respectively, are derived. Published by AIP Publishing.

Thermodynamic properties of liquid Al-Cu and Al-Zn alloys are investigated using the free volume model. We also analyse local atomic arrangements and concentration fluctuations in both liquid binary alloys from the determination of the Warren-Cowley short-range order. For liquid Al-Cu alloys, our findings indicate a strong preference for heteroatomic nearest-neighbors bonds on the Cu-rich side and weaker ones in the Al-rich part while for the liquid Al-Zn alloy, a tendency to the phase separation is observed. Basing on Darken’s thermodynamic relationship, diffusivity has been examined for both liquid alloys.

Physicochemical properties like density, surface tension, and viscosity of liquid binary Al-Li and Li-Zn alloys have been measured using draining crucible method. The experimentally measured surface-tension values have been compared to theoretical results based either on the Butler model or the compound formation model assuming the existence of the most favored A 1 B 2 and A 2 B 3 clusters. Several models for viscosity calculation have been also applied and discussed in confrontation with measured data. Finally, the clustering effects in the liquid Al-Li and Li-Zn alloys have been examined using two microscopic functions, i.e., the concentration fluctuation function in the long-wavelength limit and the Warren-Cowley short-range order parameter.

Understanding how dynamic properties depend on the structure and thermodynamics in liquids is a long-standing open problem in condensed matter physics. A very simple approach is based on the Dzugutov contribution developed on model fluids in which a universal (i.e. species-independent) connection relates the pair excess entropy of a liquid to its reduced diffusion coefficient. However its application to “real” liquids still remains uncertain due to the ability of a hard sphere (HS) reference fluid used in reducing parameters to describe complex interactions that occur in these liquids. Here we use ab initio molecular dynamics simulations to calculate both structural and dynamic properties at different temperatures for a wide series of liquid metals including Al, Au, Cu, Li, Ni, Ta, Ti, Zn as well as liquid Si and B. From this analysis, we demonstrate that the Dzugutov scheme can be applied successfully if a self-consistent method to determine the packing fraction of the hard sphere reference fluid is used as well as the Carnahan-Starling approach to express the excess entropy.

Dynamical properties of pure silicon in deeply undercooled states, and Cu–Zr bulk metallic glass are calculated from combined classical and ab initio molecular dynamics techniques. An analysis of the results indicates that a boson peak occurs in both systems but its origin is found to be different in each case.

An overview of a recent series of ab initio molecular dynamics (AIMD) simulations for the liquid, undercooled states of materials forming metallic glasses is presented. Here we use a combination of state-of-the-art computational techniques to resolve the atomic-level structure of such disordered systems. The dynamic properties, such as the self-diffusion coefficients and viscosity, are also studied as a function of temperature in the undercooled regime. By analyzing two model systems that involve different chemistry, i.e. Cu-Zr and Au-Si systems, we identify the local icosahedral ordering as a fundamental process underlying structural relaxation. Our findings have also strong implications for understanding the nature of the glass forming ability and properties of metallic glasses.

The Fe-Ni system is a key subsystem for several alloys with important applications. It has been thermodynamically assessed according to the CALPHAD methodology and using 0 K enthalpies of formation of the ordered phases resulting from ab-initio atomistic calculations. This allowed us to model both stable and metastable fcc-based ordered phases (L12 Fe3Ni, L10 FeNi and L12 FeNi3) in the framework of the compound energy formalism (CEF) by using a 4-sublattice model. The combined ab-initio and CALPHAD approach enabled us to predict low temperature stable equilibria which are experimentally not accessible due to an extremely sluggish kinetics. A similar 4-sublattice model has also been used for the bcc-based ordered phases (D03 Fe3Ni, B2 and B32 FeNi, D03 FeNi3), which are metastable in the Fe-Ni system, but need to be reliably modelled in order to enable extrapolations to higher order systems such as Al-Fe-Ni. Magnetic ordering, which is particularly important in this system, has also been thermodynamically described and the influence of magnetism on phase equilibria evidenced.

International audience; We present a brief overview of recent ab initio based developments to designing bulk thermodynamic properties and phase equilibria for alloys. We also discuss the potential links that exist between ab initio methodologies and the Calphad phenomenological approach, in order to build an efficient and predictive tool for the calculation of phase equilibria of complex, multi-component often found in industrial alloys.

As approaching the glass transition, particle motion in liquids becomes highly heterogeneous and regions with virtually no mobility coexist with liquid-like domains. This complex dynamics is believed to be responsible for different phenomena including non-exponential relaxation and the breakdown of Stokes-Einstein relation. Understanding the relationships between dynamical heterogeneities and local structure in metallic liquids and glasses is a major scientific challenge. Here we use classical molecular dynamics simulations to study the atomic dynamics and microscopic structure of $\mbox{Cu}_{50}\mbox{Zr}_{50}$ alloy in the supercooling regime. Dynamical heterogeneities are identified via an isoconfigurational analysis. As deeper supercooling is achieved a transition from isolated to clustering low mobility atoms is reported. These slow clusters, whose size grow upon cooling, are also associated to concentration fluctuations, characterized by a Zr-enriched phase, with a composition $\mbox{CuZr}_2$. In addition, a structural analysis of slow clusters based on Voronoi tessellation evidences an increase with respect of the bulk system of the fraction of Cu atoms having a local icosahedral order. These results are in agreement with the consolidated scenario of the relevant role played by icosahedral order in the dynamic slowing-down in supercooled metal alloys.

A new crystalline ground state was discovered in the Au-Si system through first-principles electronic structure calculations. The new structure was found using the experimentally and theoretically determined local atomic structure in the liquid as a guide for the solid state. Local atomic structure in the liquid was matched with that for all known crystal structures as compiled in the Pauling File structural database. The best matching crystalline structures were then explicitly calculated using first-principles methods. Most candidate crystal structures were found to be close, but above the enthalpy of a composition weighted average of the face-centered cubic Au and diamond structure Si terminal phases, but one crystal structure was more stable than the terminal phases by about 10 meV atom(-1) at T = 0 K. As first-principles simulations of local structure are feasible for most liquid alloys, the present methodology is applicable to other alloys lying near a eutectic composition

High-pressure induced phase transitions in a single crystal of gadolinium molybdate, Gd-2(MoO4)(3) were studied by the Raman spectroscopy and ab initio calculations. The amorphization of the sample takes place at about 6 GPa in a mixture of alcohol as a pressure transmitting medium and begins as soon as 3 GPa in argon. In both media, the amorphization is irreversible in the 0-9 GPa investigated pressure range. The joint ab initio and Raman results allowed us to conclude that rotations of MoO4 tetrahedra axe the primary structural changes involved in the first phase transition (at about 2 GPa) explaining the softening of the low frequency modes at about 50 cm(-1). In addition, a progressive distortion of tetrahedra followed by a coordination change (IV-VI) of Mo atoms is observed through the five structural transitions including amorphization. This mechanism based on the steric hindrance of polyhedra is believed to be the most relevant for explaining the amorphization of Gd-2(MoO4)(3).

We show that thermodynamic properties of Pu metal as well as those of Pu (1− x ) M x (M = Al, Ga, In) compounds are poorly described by traditional DFT but rather well modeled when including magnetic interactions. The cluster variation method is used to emphasize that the presence of Ga atoms in the δ-Pu phase leads to an important chemical short-range order which stabilizes this phase as a function of temperature and composition. Our results indicate also that Ga atoms introduce local strain fields in δ phase which relax with increasing distance from the Ga atoms. Finally we show that local relaxations are also essential to predict correct vacancy formation and migration energies in this phase.

International audience; Density functional (DFT) calculations have been used to investigate the stability of point defects in uranium dioxide. Correlation effects are taken into account within the DFT+U approach as implemented in the Vienna Ab initio Simulation Package (VASP). More particularly the formation energies of both intrinsic and extrinsic point defects, i.e. vacancies, interstitials, Frenkel pairs and Schottky trio defects are calculated. Our results are compared with available experimental data and are also compared to previous calculations based on conventional functionals such as the local spin-density approximation and generalized gradient approximations.

The conditions of formation of the Au-induced Si-(6×6) reconstruction, its stability, as well as its atomic structure are studied experimentally using grazing incidence x-ray diffraction techniques. This reconstruction is found to form at 680 K when cooling down eutectic droplets previously obtained by the dewetting of thin gold films deposited at room temperature on a Si(111) substrate. The quality of the reconstruction is found to depend on the annealing temperature. The formation of the (6×6) reconstruction at low temperature after it has been destroyed by ion bombardment, together with the recovery of the Si surface, give evidence of its high stability and highlight the surfactant properties of gold atoms. The determination of the Si(111)-(6 × 6)Au atomic structure is performed using quantitative surface x-ray diffraction with an existing complex model proposed in the literature as a starting structure. The resulting gold structure is found to be 1 ML thick and consists in two domains related by a mirror. They are composed of trimer and pentagonal units with special sites presenting a partial occupancy of 0.5. Our experimental data set does not provide enough accuracy to determine the positions of the Si atoms of the substrate, but ab initio calculations tend to confirm that they are only slightly displaced from the bulk position.

Results of first-principles (FP) total energy calculations for 32 different configurations of the μ phase in the binary system Nb–Ni are used in the compound energy formalism (CEF) to model finite-temperature thermodynamic properties. A comparison with Cluster Expansion Hamiltonian–Cluster Variation Method (CEH-CVM) calculations indicates that the CEF describes temperature-dependent site occupancies as well as the CEH-CVM within the temperature range of interest for applications. This suggests that the Bragg–Williams–Gorsky approximation (BWGA) used in the CEF is sufficient to describe site occupancies and thermodynamics of the μ phase. A phase diagram is calculated using the μ phase description derived in the present work together with a previous Calphad description for the other phases of this system. The FP-CEF approach significantly improves the description of the thermodynamic properties as a function of composition compared to the Calphad procedure generally used up to now.

The relative stabilities of L12, D022, D023 and several one dimensional long period structures (1D-LPS's) in the TiAl3 intermetallic compound are investigated employing the Vienna ab initio simulation package (VASP). The energetic effects due to the tetragonal distortion of the D022, D023 structures and other 1D-LPS's are important. The effect of the cell-internal displacements of the atoms in the long period structures has also been studied. After this last stage of relaxation D023 is found to be the ground state. With distortion only, D022 is the most stable structure. 1D-LPS's with one and two bands have energies which are very near to those of D023 and D022. The formation energies of TiAl3 in the L12, D022, and D023, structures are calculated. The computed electronic densities of states show that each structure has a pseudo gap in its density of states distribution. The energetic results are discussed in the framework of the axial nearest neighbor Ising model (ANNNI). Predictions of the energy differences from L12 of series of 1D-LPS's are provided.

International audience; Using density-functional theory in combination with the direct force method and molecular dynamics we investigate the vibrational properties of a binary Cr-Re sigma-phase. In the harmonic approximation, we have computed phonon dispersion curves and density of states, evidencing structural and chemical effects. We found that the s-phase is mechanically unstable in some configurations, for example, when all crystallographic sites are occupied by Re atoms. By using a molecular-dynamics-based method, we have analysed the anharmonicity in the system and found negligible effects (similar to 0.5 kJ/mol) on the Helmholtz energy of the binary Cr-Re sigma-phase up to 2000 K (similar to 0.8T(m)). Finally, we show that the vibrational contribution has significant consequences on the disordering of the sigma-phase at high temperature. (C) 2014 AIP Publishing LLC.

The relative stabilities of L1 2 , D0 22 , and D0 23 structures in ZrAl 3 and HfAl 3 compounds are investigated employing the Vienna ab initio simulation package (VASP). In ZrAl 3 and HfAl 3 compounds, the energetic effects related to the tetragonal distortion are important and lead to the stabilization of the D0 22 and D0 23 structures with respect to the L1 2 structure. The effect of the cell internal displacements of the atoms in the D0 23 structure is studied. In the case of the HfAl 3 compound, the D0 23 structure appears as the most stable structure after this last stage of relaxations. In the case of ZrAl 3 compounds, the D0 23 structure is the most stable structure in both distorted and fully relaxed stages of the relaxation. The results are discussed in the framwork of an antiphase boundary model.

We examined the feasibility of preparing new high-pressure polymorphs of IrTe2 by determining the relative energies and unit cell volumes of known and hypothetical forms of IrTe2 on the basis of first-principles electronic band structure calculations using the Vienna ab initio simulation package (VASP). The IrTe2 polymorphs included in our analysis are three known phases, i.e., the polymeric CdI2-type, pyrite-type, and monoclinic IrTe2 phases, as well as four hypothetical phases, i.e., ramsdelite-type, pyrolusite-type, IrS2-type, and marcasite-type phases. The charge balances of these IrTe2 phases were analyzed by carrying out extended Huckel tight-binding electronic band structure calculations for the crystal structures optimized by VASP calculations.

The relative stabilities of L1 2 , D0 22 , and D0 23 structures in the ZrAl 3 intermetallic compound have been investigated employing the Vienna ab initio simulation package (VASP). The effects due to the tetragonal distortion of the D0 22 and D0 23 structures are important. The effect of the cell-internal displacements of the atoms in the D0 23 structure is studied. The more stable structure is D0 23 followed by D0 22 and by L1 2 . The calculations show that at high pressure L1 2 becomes more stable than D0 22 . The energies of formation of ZrAl 3 in the L1 2 , D0 22 , and D0 23 structures are calculated. The computed electronic densities of states show that each structure has a pseudo gap in the density of states distribution. The preferred crystal structure is the one in which the Fermi level lies in the pseudo gap and for which the density of states at the Fermi level is the lowest. The energetic results are discussed in the framework of the ANNNI model.

The total energies of formation of Au–Ni superstructures based on a fcc lattice have been calculated using the linear muffin-tin-orbital (LMTO) method in the full potential approach. Both unrelaxed and relaxed structures have been included in the calculations. The energy of formation is decomposed into three terms, a volume deformation contribution, a chemical contribution and a relaxation contribution. The enthalpy of mixing of the disordered solid solution has been calculated using an Ising-like cluster expansion for both the chemical and relaxation effects. In the Gibbs energy of mixing, a configurational entropy of mixing has been calculated with the cluster variation method (CVM) and the thermal excitations due to electronic and vibrational effects have been taken into account. The miscibility gap displayed in the Au–Ni system has been calculated. The results are discussed in comparison with the experimental data.

The energetics of the surface region of metallic alloys and the surface segregation are studied in terms of an effective Ising Hamiltonian, whose parameters (often called effective cluster interactions (ECIs)) are determined from ab initio electronic structure calculations using the force theorem. The tight-binding linear muffin-tin orbital method is used in combination with the coherent potential approximation and the formalism of surface Green functions. The band term is treated within the generalized perturbation method (GPM) and the contributions to the on-site effective interactions arising from the core states, the double-counting and the Madelung terms are included. The surface concentration profile and its dependence on the temperature are studied by methods of statistical mechanics, in particular, using a Monte Carlo (MC) simulation. The electronic structure and the Ising Hamiltonian parameters are then recalculated for the new profile, and this procedure is repeated until self-consistency between the electronic structure and the segregation profile is achieved. As an illustration, we review the results for various alloys and as a case study we present and discuss in detail the Pt–Rh alloy system.

Following our previous studies on Pu 1 −x M x (M = Al, Ga and In) compounds [G. Robert, A. Pasturel, B. Siberchicot, Phys. Rev. B 68 (2003) 75109] we present ab initio calculations on Pu 1 −x M x (M = Ge, Zn). The formation energies of the systems are calculated in the framework of GGA-ferromagnetic approximation. The densities of states are analyzed and a correlation between the five systems is considered in relationship with the stability of the δ phase solid solution.

The enthalpies of formation of the various intermetallic and amorphous phases of the Ti-Cu binary system were measured by solution calorimetry in molten aluminum. The experimental results were compared with the values predicted by Miedema's model.

International audience; We present a study of dynamic properties of liquid aluminum using density-functional theory within the local-density (LDA) and generalized gradient (GGA) approximations. We determine the temperature dependence of the self-diffusion coefficient as well the viscosity using direct methods. Comparisons with experimental data favor the LDA approximation to compute dynamic properties of liquid aluminum. We show that the GGA approximation induce more important backscattering effects due to an enhancement of the icosahedral short range order (ISRO) that impact directly dynamic properties like the self-diffusion coefficient. All these results are then used to test the Stokes-Einstein relation and the universal scaling law relating the diffusion coefficient and the excess entropy of a liquid.

An ab-initio total energy and electronic structure calculation has been performed for Al13Ru4 compound with the Al13Fe4 structure type. It is found to be more stable than the earlier predicted DO22 structure for the exact 3:1 stoichiometric ruthenium trialuminide Al3Ru. Using a ‘frozen-potential’ method the relative structural stability of the Al3Fe4, DO22 and L12 structure types is studied as a function of electron concentration. In particular, it is shown that the Al13Fe4 structure is predicted correctly for the group 8 transition metals whereas there is a strong stability of the DO22 structure for group 5. The trends in structural stability are interpreted directly in terms of the shape of the densities of states.

International audience; In the present work the structural properties of low silica content calcium aluminosilicate melts with concentration ratio CaO/Al2O3 = 1 are investigated in the liquid and undercooled states by neutron diffraction experiments and ab initio molecular dynamics simulations. The results show the presence of AlO5 units and triclusters as well as non-bridging oxygen in the fully charge balanced compositions. Moreover, our findings allow us to identify a possible interplay between these structural units. Finally, we discuss the influence of these defective structural units on the properties of tetrahedral network and more particularly their implication on the evolution of the viscosity and the fragility.

An efficient self-consistent Green's function technique to calculate electronic properties of surfaces of disordered alloys within the local density approximation is presented. The tight-binding version of the linear muffin-tin orbital method is used to describe the electronic states, while the semi-infinite nature of the system is incorporated via the surface Green's function approach. A generalization of the coherent potential approximation method in order to treat inhomogeneous systems is used to study the effect of disorder. The formalism is applied to the study of electronic properties of random CuPd overlayers of the fcc-Cu(001) substrate and of the (001) faces of fcc-CuNi random alloys.

International audience; We report an investigation of the local structure in homovalent-substituted BaTi1−x Zr x O3 relaxors by a combination of experimental and theoretical methods, namely neutron total scattering, X-ray absorption spectroscopy, and supercell ab-initio calculations. It is shown that unlike Zr atoms, Ti atoms are largely displaced in their octahedra, and are thus associated with strong local dipole moments. Besides, we give evidence that the difference in the size of Ti4+ and Zr4+ cations leads to a significant size mismatch of the Ti-O6 and Zr-O6 octahedra. When they link to form the perovskite structure of BaTi1−x Zr x O3, the O6 octahedra undergo slight distortions in order to accommodate their different sizes. It is shown that they are compressed in the direction of Zr neighbors, and expanded in the direction of Ti neighbors. The polar Ti displacements, which are sensitive to the octahedral distortions, then become constrained in their orientation according to the local Zr/Ti distribution. Such constraints impede a perfect alignment of all the Ti displacements as existing in the classic ferroelectric BaTiO3. Our results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi1−x Zr x O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution.

We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a prototype material for relaxors with a homovalent substitution. From a statistical analysis of relaxed structures, we give evidence for four types of Ti-atom polar displacements: along the , , or directions of the cubic unit cell, or almost cancelled. The type of a Ti displacement is entirely determined by the Ti/Zr distribution in the adjacent unit cells. The underlying mechanism involves local strain effects that ensue from the difference in size between the Ti4+ and Zr4+ cations. These results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi(1-x)Zr(x)O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution.

International audience; First-principles electronic structure calculations predict the existence of a crystalline compound in the Au-Ge system. The structure is found by matching the theoretically determined local atomic structure in the liquid state with that for experimentally known crystal structures in other alloys. Subsequently, the best matching crystalline structures were structurally optimized using first-principles methods. Surprisingly, although Au-Ge is known as a non-compound-forming system, a crystal structure was found to be more stable than the terminal phases by about 6 meV/atom at T=0 K. Possibly, this structure can be prepared by a suitably chosen substrate such as Pt5P2.

International audience; We report the results of first-principles molecular-dynamics simulations for liquid and undercooled eutectic Au81Si19 alloys at various temperatures. Through comparisons between Au81Si19 and Au liquids, we show the strong effects of Si alloying on the atomic-scale structure, namely the occurrence of a well-defined chemical short-range order and the slowing of the formation of icosahedral local motifs as a function of temperature. Such a behavior may explain the stability of the liquid phase at the eutectic composition by an enhancement of AuSi interactions. In examining the dynamic properties of both systems, we demonstrate a strong interplay between these structural changes and the evolution of diffusivity at low temperatures. All these results yield a possible scenario for the occurrence of such an unusual deep eutectic point.

International audience; The phenomenon of supercooling in metals-that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point(1)-has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation(2,3). For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid(4,5) and may prevent or lower supercooling(6). This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling(6), no experimental observation of such lateral ordering at interfaces is available(6). Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 x 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and ab initio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid-liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour-liquid-solid growth of semiconductor nanowires

International audience; First principles calculations have been made to theoretically interpret the structural stability of SrTiO3 as a function of pressure or strain. We show that the orthorhombic Pbnm structure is more stable than the ideal perovskite one due to rotations of TiO6 octahedra at high pressures or deformations of TiO6 octahedra at low pressures. Such a study is extended to the structural stability of the Ruddlesden-Popper Srn+1TinO3n+1 phases for n=1, 2 and 3, in which the lattice constraints are induced by the intercalation of SrO layers in the perovskite structure.

C. Berne, M. Sluiter, Y. Kawazoe, T. Hansen, and A. Pasturel Commissariat a l’Energie Atomique, Direction de la Recherche Technologique, Departement des Techniques des Energies Nouvelles, 17 Rue des Martyrs, 38054 Grenoble-Cedex 09, France Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Institut Laue Langevin, BP 156, 38042 Grenoble-Cedex 09, France Laboratoire de Physique et Modelisation des Milieux Condenses, Maison des Magisteres, BP 166 CNRS, 38042 Grenoble-Cedex 09, France ~Received 23 April 2001; published 19 September 2001!

Thèse préparée dans le cadre d’une cotutelle entre la Communauté Université Grenoble Alpes et Nanyang Technological University, Singapour; The development of portable electronics, electric vehicles and renewable energies has attracted great interest in the search for new solutions for energy storage. The autonomy of current Li-ion batteries is not yet sufficient to meet the needs of these different applications. Indeed, graphite, the most used anode material, has a relatively low specific capacity of 372 mAh.g-1 which limits the battery life. It is therefore necessary to substitute graphite with a more efficient material. The ternary oxides of the AB2O4 type (A, B: transition metals) are particularly interesting candidates because of their relatively high specific capacity (from 750 to 1000 mAh.g-1). In particular, ZnFe2O4 is a promising material with a theoretical capacity of 1000 mAh.g-1 and a low working voltage (1.5V vs Li / Li +). It is also a cheap, abundant and non-toxic material. In addition, the nanostructuring of this material would ensure good mechanical stability and improve the kinetics of lithium during charge / discharge cycles. The objective of this thesis is the synthesis of ZnFe2O4 nanoparticles by a laser pyrolysis process and the evaluation of their electrochemical performances against metallic lithium. This thesis work is divided into two parts: firstly, the influence of various synthesis parameters (nature and flow rates of the gases, precursors used, etc.) on the crystalline phases and the morphologies obtained is studied. In a second step, several of the nanopowders obtained by laser pyrolysis are tested in half-cell cycling to evaluate the electrochemical performance and understand the mechanisms governing the storage of lithium in these materials. This thesis work was carried out jointly by the Grenoble-Alpes University and the Nanyang Technological University in Singapore, and in collaboration with the AIME laboratory of the University of Montpellier; Le développement de l’électronique portable, des véhicules électriques et des énergies renouvelables a suscité un grand intérêt dans la recherche de nouvelles solutions pour le stockage de l’énergie. L’autonomie des batteries Li-ion actuelles n’est pas encore suffisante pour répondre aux besoins de ces différentes applications. En effet, le graphite, matériau d’anode le plus utilisé, possède une capacité spécifique relativement faible de 372 mAh.g-1 qui limite l’autonomie des batteries. Il est donc nécessaire de substituer le graphite par un matériau plus performant. Les oxydes ternaires de type AB2O4 (A, B : métaux de transition) sont des candidats particulièrement intéressants de par leur capacité spécifique relativement élevée (de 750 à 1000 mAh.g-1). En particulier, ZnFe2O4 est un matériau prometteur avec une capacité théorique de 1000 mAh.g-1 et une faible tension de travail (1,5V vs Li/Li+). C’est aussi un matériau peu cher, abondant et non toxique. De plus, la nanostructuration de ce matériau permettrait d’assurer une bonne stabilité mécanique et d’améliorer la cinétique du lithium lors des cycles charge/décharge.L’objectif de cette thèse est la synthèse de nanoparticules de ZnFe2O4 par un procédé de pyrolyse laser et l’évaluation de leurs performances électrochimiques face à du lithium métallique. Ce travail de thèse se divise en deux parties : tout d’abord, l’influence de différents paramètres de synthèse (nature et débits des gaz, précurseurs utilisés, …) sur les phases cristallines et les morphologies obtenues est étudiée. Dans un second temps, plusieurs des nanopoudres obtenues par pyrolyse laser sont testées en cyclage en demi-cellule pour en évaluer les performances électrochimiques et comprendre les mécanismes régissant le stockage du lithium dans ces matériaux.Ce travail de thèse a été réalisé en cotutelle entre l’université Grenoble-Alpes et l’université technologique de Nanyang à Singapour, et en collaboration avec le laboratoire AIME de l’université de Montpellier.

The development of portable electronics, electric vehicles and renewable energies has attracted great interest in the search for new solutions for energy storage. The autonomy of current Li-ion batteries is not yet sufficient to meet the needs of these different applications. Indeed, graphite, the most used anode material, has a relatively low specific capacity of 372 mAh.g-1 which limits the battery life. It is therefore necessary to substitute graphite with a more efficient material. The ternary oxides of the AB2O4 type (A, B: transition metals) are particularly interesting candidates because of their relatively high specific capacity (from 750 to 1000 mAh.g-1). In particular, ZnFe2O4 is a promising material with a theoretical capacity of 1000 mAh.g-1 and a low working voltage (1.5V vs Li / Li +). It is also a cheap, abundant and non-toxic material. In addition, the nanostructuring of this material would ensure good mechanical stability and improve the kinetics of lithium during charge / discharge cycles. The objective of this thesis is the synthesis of ZnFe2O4 nanoparticles by a laser pyrolysis process and the evaluation of their electrochemical performances against metallic lithium. This thesis work is divided into two parts: firstly, the influence of various synthesis parameters (nature and flow rates of the gases, precursors used, etc.) on the crystalline phases and the morphologies obtained is studied. In a second step, several of the nanopowders obtained by laser pyrolysis are tested in half-cell cycling to evaluate the electrochemical performance and understand the mechanisms governing the storage of lithium in these materials. This thesis work was carried out jointly by the Grenoble-Alpes University and the Nanyang Technological University in Singapore, and in collaboration with the AIME laboratory of the University of Montpellier, Le développement de l’électronique portable, des véhicules électriques et des énergies renouvelables a suscité un grand intérêt dans la recherche de nouvelles solutions pour le stockage de l’énergie. L’autonomie des batteries Li-ion actuelles n’est pas encore suffisante pour répondre aux besoins de ces différentes applications. En effet, le graphite, matériau d’anode le plus utilisé, possède une capacité spécifique relativement faible de 372 mAh.g-1 qui limite l’autonomie des batteries. Il est donc nécessaire de substituer le graphite par un matériau plus performant. Les oxydes ternaires de type AB2O4 (A, B : métaux de transition) sont des candidats particulièrement intéressants de par leur capacité spécifique relativement élevée (de 750 à 1000 mAh.g-1). En particulier, ZnFe2O4 est un matériau prometteur avec une capacité théorique de 1000 mAh.g-1 et une faible tension de travail (1,5V vs Li/Li+). C’est aussi un matériau peu cher, abondant et non toxique. De plus, la nanostructuration de ce matériau permettrait d’assurer une bonne stabilité mécanique et d’améliorer la cinétique du lithium lors des cycles charge/décharge.L’objectif de cette thèse est la synthèse de nanoparticules de ZnFe2O4 par un procédé de pyrolyse laser et l’évaluation de leurs performances électrochimiques face à du lithium métallique. Ce travail de thèse se divise en deux parties : tout d’abord, l’influence de différents paramètres de synthèse (nature et débits des gaz, précurseurs utilisés, …) sur les phases cristallines et les morphologies obtenues est étudiée. Dans un second temps, plusieurs des nanopoudres obtenues par pyrolyse laser sont testées en cyclage en demi-cellule pour en évaluer les performances électrochimiques et comprendre les mécanismes régissant le stockage du lithium dans ces matériaux.Ce travail de thèse a été réalisé en cotutelle entre l’université Grenoble-Alpes et l’université technologique de Nanyang à Singapour, et en collaboration avec le laboratoire AIME de l’université de Montpellier.

The aim of this study is to compute structural, dynamic and thermodynamic properties in the liquid and undercooled states of Au-Si alloys using molecular dynamics simulations. The interactions are described via a modified embedded-atom model (MEAM) refined to take into account the liquid properties. In a first step, for the eutectic composition, the local structure is characterized by a strong Au-Si affinity, namely a well-pronounced chemical short-range order which leads to the slowing down of the formation of icosahedral local motifs in the undercooled regime. Moreover we have shown that this short range order strongly influences dynamic and thermodynamic properties of this liquid alloy. A more general study including compositions around the eutectic composition confirms the peculiar behavior of the eutectic alloy. In a second step, we study the behavior of the eutectic alloy in interaction with different substrates of silicon. We show that the liquid mimics the orientation of the substrate, using a one-atomic layer and a chemical composition that may differ from the eutectic one. This peculiar behavior is related to the undercooling properties experimentally observed in these systems., Ce travail a pour but l'étude des propriétés structurales, dynamiques et thermodynamiques de l'alliage Au-Si dans l'état liquide et surfondu. Nous avons utilisé des simulations de dynamique moléculaire pour déterminer ces propriétés. Les interactions interatomiques nécessaires à ces simulations ont été construites dans un modèle de type MEAM. Dans une première partie de ce travail, nous avons montré que pour la composition eutectique, la structure locale de l'alliage liquide est caractérisée par une forte affinité entre l'or et le silicium, conduisant à un ordre chimique local très important qui ralentit la formation des motifs icosaédriques, caractéristique de l'ordre structural des systèmes métalliques surfondus. Nous avons également montré que cet ordre local influence fortement les propriétés thermodynamiques et dynamiques de cet alliage liquide. Une étude plus générale autour de la composition eutectique confirme les propriétés particulières du liquide à la composition eutectique. Dans une seconde partie, nous avons étudié les propriétés de l'alliage eutectique Au-Si en interaction avec des substrats de silicium. Nous avons mis en évidence une forte structuration du liquide à l'interface, le liquide ayant la propriété de reproduire sur une couche atomique la topologie de la surface du substrat en modifiant parfois sa composition chimique. Ce comportement très particulier est relié aux propriétés de surfusion observées expérimentalement dans ces systèmes.

Semiconductor (SC) nanowires (NMs) have been identified as important components for futureelectronic and sensor nanodevices. They are produced using a metal catalyst (Au) that forms a loweutectic point with the SC phase (Si) and enables their nucleation and their growth at lowtemperatures. The aim of this study is to investigate the formation, the structure as well as thebehaviour of such liquid eutectic droplets in interaction with the substrates on which the futur NMsproperties will later depend. This work has been performed in-situ, using synchrotron radiation whichis the perfect tool to characterize this mechanism at the atomistic scale. The eutectic droplets havebeen obtained through the dewetting of thin gold films. This process has been found to modify theepitaxial relationships between the gold and the silicon substrate. The obtained droplets areaccompanied by a wetting layer (WL) whose conditions of formation as well as atomic structure,which turned to be a surface reconstruction, have been determined. During the cooling process, thesupercooling effects in such AuSi eutectic droplets have been found to be enhanced in the presenceof this reconstruction. It has been explained by its specific atomic structure which is likely to stabilizethe liquid phase. Similar experiments on other systems (Au-Ge or Al-Si) were performed and asynthesis has been made in order to present the current knowledge on this topic in comparison withthe results of this work., Les nanofils (NFs) de semi-conducteur (SC) sont des objets possédant des propriétés trèsintéressantes pour la fabrication de futurs composants électroniques à l’échelle nanométrique. Ilssont élaborés grâce à l’utilisation d’un catalyseur métallique (Au) formant un point eutectiqueprofond en s’alliant à la phase SC (Si) permettant la germination et la croissance du NF à bassestempératures. Ce travail a pour but l’étude de la formation, de la structure et du comportement deces gouttes d’alliage eutectique en interaction avec le substrat dont les propriétés futures du NF vontdépendre. L’étude a été menée in-situ, par utilisation du rayonnement synchrotron qui est un outilparfaitement dédié à la caractérisation de structures à l’échelle atomistique. Les gouttes d’eutectiqueont été obtenues par démouillage du film d’or. L’étude de ce procédé a révélé des changements dansles relations d’épitaxies entre l’or et le silicium. Les gouttes ainsi formées sont accompagnées d’unecouche de mouillage, se révélant être une reconstruction de surface, dont les conditions de formationainsi que la structure atomique ont été déterminées. Lors du refroidissement, une augmentation deseffets de surfusion a été observée en présence de cette reconstruction et a été expliquée par lastructure même de cette dernière qui semble stabiliser l’état liquide. Des expériences similaires ontété faites sur d’autres systèmes (Au-Ge, Al-Si) et une synthèse a été réalisée pour comparer lesconnaissances actuelles sur le sujet avec les résultats de ce travail.