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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.

A long-standing question in condensed matter and material science is understanding how local structural order determines the fate of a liquid on cooling, i.e., whether it crystallizes or experiences dynamic slowing down leading to glass formation. Here we address this issue by focusing on a metallic glass former with concurring crystal and amorphous local order based on icosahedral symmetry. Using atomistic simulations, we study its deep supercooled regime in which we clearly identify spontaneous crystal nucleation close to the glass transition. We find that regions of slow dynamics characterized by a high degree of icosahedral orientational order play a central role in the nucleation process. Our study brings a new perspective into the microscopic mechanism of crystallization from the supercooled phase, which could be of significance for the in silico design of materials with selected glass-forming ability.

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.

The heat of dissolution at infinite dilution of solid Nb in liquid Al was determined indirectly by dissolving small pieces of solid NbAl3 in liquid aluminum in an isothermal calorimeter at 1126 ± 0.5 K. A value of ∞Qdiss(NbAl3,298 K,1126 K) = 75.3 ± 3.0 kJ mol−1 was measured. The derived partial enthalpy of mixing at infinite dilution for Nb in Al is ΔmixH∞ = -243.2 kJ mol−1. A lower limit for the solubility of solid Nb in liquid Al is also obtained from these measurements. The observed limit is 10 times higher than the accepted experimental value in the literature. In addition, heat content measurements for NbAl3 were performed at three different temperatures using drop calorimetry with empty crucibles. The measured heat contents are compared to values obtained from phonon calculations in the quasi-harmonic approximation. The quality of the determined heat of dissolution was checked by measuring the enthalpy of formation of solid NbFe2 and NbNi3. Values of -17.5 ± 2.9 kJ (mol of atoms)−1 and -30.7 ± 3.5 kJ (mol of atoms)−1 were found. These values agree with available experimental and theoretical data from the literature which confirms the validity of our experimental approach. A new, unknown and metastable magnetic configuration was discovered for NbFe2 in ab-initio simulations and will be discussed.

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.

International audience; Ab initio molecular dynamics simulations are performed to investigate dynamic properties of Al-Ni melts. Along the T = 1795 K isotherm, we show that self-diffusion coefficients and the viscosity exhibit a strong non-linear dependence as a function of composition. We further demonstrate that this behavior has an underlying origin in the composition-dependent local structural ordering characterized by a strong interplay between icosahedral short-range order and chemical short-range order in the Ni-rich composition range. Finally we evidence that this structural evolution is at the origin of the breakdown of the Stokes-Einstein relation.

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.

International audience; Different N-substituted phenothiazines have been synthesized and their electrochemical behavior has been investigated in CH3CN in order to design the best polyphenothiazine based cathodic material candidate for lithium batteries. These compounds exhibit two successive reversible one-electron oxidation processes. Ab initio calculations demonstrate that the potential of the first process is a result of both the hybridization effects between the substituent and the phenothiazine unit as well as the change of conformation of the phenothiazine heterocycle during the oxidation process. More specifically, we show that an asymmetric molecular orbital spreading throughout an external cycle of the phenothiazine unit and the alkyl fragment is formed only if the alkyl fragment is long enough (from the methyl moiety onwards) and is at the origin of the bent conformation for N-substituted phenothiazines during oxidation. Electrochemical investigations supported by ab initio calculations allow the selection of a phenothiazinyl unit which is then polymerized by a Suzuki coupling strategy to avoid the common solubilization issue in carbonate-based liquid electrolytes of lithium cells. The first electrochemical measurements performed show that phenothiazine derivatives pave the way for a promising family of redox polymers intended to be used as organic positives for lithium batteries.

Porous hollow PtM/C electrocatalysts (where M is a 3d transition metal element) have proven to enhance remarkably the oxygen reduction reaction (ORR) kinetics in proton exchange membrane fuel cells. However, their mechanism of formation, the nature of adequate alloying elements and carbon supports as well as the rationale for their interesting electrocatalytic properties remain open questions. Herein, we describe the mechanism of formation and growth of porous hollow PtNi/C NPs using operando wide angle X-ray scattering, scanning transmission electron microscopy and X-ray energy dispersive spectroscopy 1. The results show that Ni-rich NPs covered by a NixByOz shell form first before acting as a sacrificial template for the deposition of Pt2+ ions via galvanic replacement (Figure 1). Finally, the solid Ni-rich core@Pt-rich shell NPs are transformed into alloy hollow PtNi/C NPs via the nanoscale Kirkendall effect (a vacancy mediated interdiffusion mechanism). Density Functional Theory (DFT) calculations were then used to gain insights on the impact of atomic vacancies on the surface structure and the reactivity of the porous hollow PtNi/C nanoparticles 2. The DFT and experimental results show that porous hollow PtM/C nanoparticles feature unique catalytic advantages (i) open percolated structure exposing two catalytic surfaces (inner and outer), and (ii) structural defects that distort the metal lattice and produce catalytic sites that are highly active for both electrooxidation (COadsmonolayer electrooxidation, methanol and ethanol electrooxidation) and electroreduction (ORR) reactions. 1. R. Chattot, T. Asset, J. Drnec, P. Bordet, J. Nelayah, L. Dubau, F. Maillard, Nano Lett. (2017). DOI: 10.1021/acs.nanolett.7b00119 2. O. Le Bacq, A. Pasturel, R. Chattot, B. Previdello, J. Nelayah, T. Asset, L. Dubau, F. Maillard, ChemCatChem. (2017). DOI: 10.1002/cctc.201601672 Figure 1.Conventional and scanning transmission electron microscopy images and elemental maps of the different intermediate nanostructures forming during the synthesis of hollow PtNi/C nanoparticles. Figure 1

Molecular dynamics investigations of the structure and dynamics of Cu64.5Zr35.5 metallic glass-forming alloy have been carried out using five different semi-empirical, many-body interaction potentials based on the Finnis-Sinclair model [M. I. Mendelev et al., J. Appl. Phys. 102, 043501 (2007) (MSK); M. I. Mendelev et al., Philos. Mag. 89, 967 (2009) (MKOSYP); L. Ward et al., e-print arXiv:1209.0619 (2012) (WAFW)] and the embedded-atom model [Y. Q. Cheng et al., Phys. Rev. Lett. 102, 245501 (2009) (CMS) and N. Jakse et al., Phys. Rev. B 85, 174201 (2012) (JNP)]. Although the total static structure factor of the alloy for all the five interaction potentials is, in general, found to be in good agreement with the experimental results, the investigation of a local structure in terms of icosahedral short-range order reveals that the effect of the interaction potential (especially the cohesive part) on the structure of the alloy is not as trivial as it seems. For MSK and JNP potentials, the self-intermediate scattering function Fs(q, t), q-dependence of the structural relaxation time τα in the low-q region, and the self-diffusion coefficient, Ds, for Cu-atoms in the alloy are in excellent agreement with the experimental results. The results for MKOSYP, CMS, and WAFW potentials deviate significantly from the experiment and suggest the dynamics of the alloy to be faster. The difference in the description of the dynamics of the alloy by different potentials is found to be due to the difference in the relevant energy scales corresponding to the temperature scales. τα and Ds exhibit Arrhenius temperature dependence in the high temperature regime above the melting temperature. We also suggest that the attractive forces influence the dynamics of the liquid alloy significantly, which is against the mere perturbative role assigned to the attractive forces in the van der Waals picture of liquids that has been challenged in the recent years. As the five interaction potentials are frequently employed to study thermodynamic, mechanical, and transport properties of Cu-Zr alloys, our study also provides a suitability check for these potentials.

1st French Conference on Catalysis (FCCat), Frejus, FRANCE, MAY 23-27, 2016; International audience; We present a joint experimental and density functional theory (DFT) study on the effect of atomic vacancies on the restructuring of platinum-transition metal alloy nanocatalysts and the associated changes in electrocatalytic activity. Atomic vacancies were introduced into slabs composed of pure Pt monolayers, and the structures were relaxed using the Vienna ab initio simulation package code. Effects of i) the concentration and ii) the spatial distribution of atomic vacancies in the slabs on surface and bulk restructuring were investigated. Highly disordered nanostructures featuring large variations of the in-plane and out-of-plane nearest-neighbour distances around the mean were observed upon relaxation. These findings were confirmed experimentally by using hollow PtNi/C nanoparticles synthesized by a combination of galvanic replacement and the nanoscale Kirkendall effect (a vacancy-mediated interdiffusion mechanism). The experimental results also show that hollow PtNi/C nanoparticles feature a combination of oxophilic and oxophobic catalytic sites on their surface and are thus highly active both for electrochemical oxidation and reduction reactions.

We investigate the connection between local structure and dynamics in the liquid and undercooled $\mathrm{A}{\mathrm{l}}_{93}\mathrm{C}{\mathrm{r}}_{7}$ alloy using ab initio molecular dynamics simulations. In the liquid phase, we show the striking effects of Cr alloying on the atomic-scale structure, characterized by a heterogeneous local ordering around each component, due to a strong interplay between chemical short-range order and icosahedral short-range order. In the undercooled phase, this interplay leads to the formation of an icosahedral-based medium-range order (IMRO) referring to Cr atoms. In examining the dynamic properties, we observe that this chemically induced structural heterogeneity gives rise to a substantial decoupling of component diffusion in the liquid phase, far above the liquidus temperature. In the undercooled regime, we find the breakdown of the Stokes-Einstein relation and a pronounced increase of the non-Gaussian parameter, indicating the onset of dynamic heterogeneities. Using the isoconfigurational ensemble method, we evidence that the structural origin of dynamics heterogeneities is clearly related to the formation of IMRO. Finally, we discuss the role played by IMRO in nucleation on the preexisting icosahedral cluster in the liquid phase.

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.

International audience; It is becoming common practice to consider that the Stokes-Einstein relation D/T similar to eta(-1) usually works for liquids above their melting temperatures although there is also experimental evidence for its failure. Here we investigate numerically this commonly-invoked assumption for simple liquid metals as well as for their liquid alloys. Using ab initio molecular dynamics simulations we show how entropy scaling relationships developed by Rosenfeld can be used to predict the conditions for the validity of the Stokes-Einstein relation in the liquid phase. Specifically, we demonstrate the Stokes-Einstein relation may break down in the liquid phase of some liquid alloys mainly due to the presence of local structural ordering as evidenced in their partial two-body excess entropies. Our findings shed new light on the understanding of transport properties of liquid materials and will trigger more experimental and theoretical studies since excess entropy and its two-body approximation are readily obtainable from standard experiments and simulations.

We perform ab initio molecular dynamics simulations to study structural and transport properties in liquid $\mathrm{A}{\mathrm{l}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}$ alloys, with copper composition $x\ensuremath{\le}0.4$, in relation to the applicability of the Stokes-Einstein (SE) equation in these melts. To begin, we find that self-diffusion coefficients and viscosity are composition dependent, while their temperature dependence follows an Arrhenius-type behavior, except for $x=0.4$ at low temperature. Then, we find that the applicability of the SE equation is also composition dependent, and its breakdown in the liquid regime above the liquidus temperature can be related to different local ordering around each species. In this case, we emphasize the difficulty of extracting effective atomic radii from interatomic distances found in liquid phases, but we see a clear correlation between transport properties and local ordering described through the structural entropy approximated by the two-body contribution. We use these findings to reformulate the SE equation within the framework of Rosenfeld's scaling law in terms of partial structural entropies, and we demonstrate that the breakdown of the SE relation can be related to their temperature dependence. Finally, we also use this framework to derive a simple relation between the ratio of the self-diffusivities of the components and the ratio of their partial structural entropies.

International audience; We use ab initio molecular dynamics simulations to study the transport properties and the validity of the Stokes-Einstein relation in Al-rich liquid alloys with Ni, Cu, and Zn as alloying elements. First, we show that the composition and temperature dependence of their transport properties present different behaviors, which can be related to their local structural ordering. Then, we evidence that the competition between the local icosahedral ordering and the local chemical ordering may cause the breakdown of the Stokes-Einstein relation even in the liquid phase. We demonstrate that this breakdown can be captured by entropy-scaling relationships developed by Rosenfeld and using the two-body excess entropy. Our findings provide a unique framework to study the relation between structure, thermodynamics, and dynamics in metallic melts and pave the way towards the explanation of various complex transport properties in metallic melts. Published by AIP Publishing.

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.

International audience; Zirconium alloys are used as fuel cladding in nuclear reactors, where they are subject to corrosion. In this article, based on first-principles calculations, we examine the effect of hydrogenation on the thermodynamic stability of various defects in ZrO(2) oxide in dry and aqueous environments. We found a defect complex consisting of an oxygen vacancy and a hydrogen atom, V(O-H), to be particularly stable for a wide range of Fermi levels for all the environmental conditions tested. We suggest that this V(O-H) complex may be responsible for the increase in the kinetics of oxidation of zirconium alloys frequently observed in aqueous environments. (C) 2011 Elsevier B.V. All rights reserved.

International audience; Fission product stability in nuclear fuels is investigated using density functional theory (DFT). In particular, incorporation and solution energies of He, Kr, Xe, I, Te, Ru, Sr and Ce in pre-existing trap sites of UO(2) (vacancies, interstitials, U-O divacancy, and Schottky trio defects) are calculated using the projector-augmented-wave method as implemented in the Vienna ab initio simulation package. Correlation effects are taken into account within the DFT+U approach. The stability of many binary and ternary compounds in comparison to soluted atoms is also explored. Finally the involvement of FP in the formation of metallic and oxide precipitates in oxide fuels is discussed in the light of experimental results. (C) 2011 Elsevier B.V. All rights reserved

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd2(MoO4)3] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure-transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure-transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO4 stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short-range and long-range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure-transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A1 symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm−1, soften continuously through the first three phases when increasing pressure. The strong A2 mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm−1 at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the MoO stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.

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).

International audience; The behaviour of caesium in nuclear fuels is investigated using density functional theory (DFT). In a first step, the incorporation and solution energies of Cs in pre-existing trap sites of UO2 (vacancies, interstitials, U-O di-vacancy and Schottky trio defects) are calculated using the projector-augmented-wave (PAW) derived pseudo potentia Is as implemented in the Vienna ab initio simulation package (VASP). Correlation effects are taken into account within the DFT + U approach. The solubility of caesium is found to be very low, in agreement with experimental data. The migration of Cs is found to be highly anisotropic, it is controlled by uranium diffusion with an Arrhenius activation energy of 4.8 eV in hyperstoichiometric UO2+x, in good agreement with experimental values.

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.

An overview of a recent series of ab initio molecular dynamics (MD) simulations for pure liquid transition metals as well as for transition metals (TM) based liquid alloys is presented. The aim is to investigate the local structure of these systems and their evolution upon undercooling, and our results are analysed through a three-dimensional image of the short-ranger order (SRO) by means of the common-neighbour analysis. Recent diffraction experiments indicate that the structure of both pure metals and alloys in undercooled states is dominated by an icosahedral SRO. Such a SRO is predicted to influence the energy of the interface between the liquid and a solid nucleus, depending on the structure of the solid phase. This, in turn, decisively impacts the nucleation behaviour of solid phases from the undercooled melts. We find that the five-fold symmetry is already present in the liquid state of all the studied systems. However, our findings show that the five-fold symmetry in the liquid state as well as it...

International audience; Using ab initio molecular dynamics simulations, we study structural and dynamic properties of liquid Al-Ni alloys at T = 1795 K as a function of concentration. From partial pair-correlation functions and Bhatia-Thornton structure factors, we show a non-linear composition dependence of chemical short-range order. Additional analysis based on three-dimensional pair-analysis techniques evidences a strong interplay between icosahedral short-range order and chemical short-range order in the Ni-rich range. In examining transport properties like self-diffusion and inter-diffusion coefficients, we demonstrate a close relationship between the structural changes and the non-linear evolution of dynamic properties as a function of composition.

International audience; We use ab initio molecular dynamics simulations to study the correlation between the local ordering and the dynamic properties of liquid Al80Ni20 alloy upon cooling. Our results evidence a huge increase of local icosahedral ordering (ISRO) in the undercooled regime which is more developed around Ni than Al atoms. We show that ISRO has a strong impact on self-diffusion coefficients of both species and is at the origin of their crossover from Arrhenius to non-Arrhenius behavior around a crossover temperature T-X = 1000 K, located in the undercooled region. We also clearly identify that this temperature corresponds to the development of dynamic heterogeneities and to the breakdown of the Stokes-Einstein relation. At temperatures below this crossover, we find that the behavior of the diffusion and relaxation dynamics is mostly incompatible with predictions of the mode-coupling theory. Finally, an analysis of the van Hove function indicates that the crossover temperature TX marks the onset of a change in the diffusion mechanism from a normal flow to an activated process with hopping. From these results, the glass-forming ability of the alloy is discussed.

International audience; We performed ab initio molecular dynamics simulations to study the relationship between dynamic and structural properties of liquid Al1-xNix alloys along the T = 1795 K isotherm. Our findings show a strong non-linear dependence of self-diffusion coefficients and the viscosity as a function of composition, in fair agreement with available experimental data. We demonstrate that this non-linear dependence can be related to the detailed description of local structural ordering and more particularly to a pronounced increase of icosahedral short range order (ISRO) in the Ni-rich composition range. We evidence that the composition dependence of local structural ordering is not captured by the Stokes-Einstein relation but can be understood in terms of the partial excess entropy approximated by the two-body contribution.

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.

An overview of a recent series of ab initio molecular dynamics (MD) simulations for pure liquid transition metals as well as for transition metals (TM) based liquid alloys is presented. The aim is to investigate the local structure of these systems and their evolution upon undercooling, and our results are analyzed through a three-dimensional picture of the short-ranger order (SRO) by means of the common-neighbor analysis. Recent diffraction experiments indicate that the structure of both pure metals and alloys in undercooled states is dominated by an icosahedral SRO. We find that the five-fold symmetry is already present in the liquid state of all the studied systems. However our findings show that the five-fold symmetry in the liquid state as well as its evolution upon undercooling depends on the system under consideration. For Ni , Zr , and Ta , local configurations are more complex than that given by the simple icosahedron. For Al 80 Ni 20 and Al 80 Mn 20 alloys, local configurations are the result of a strong competition between chemical and topological effects; the key role played by the occurrence of localized magnetic moments of Mn atoms to interpret their short-range arrangements is emphasized, and the time evolution of the configurations is examined in terms of the mean square displacements.

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.

Ordering phenomena have been investigated in liquid Al–Zn alloys performing molecular dynamics (MD) simulations using “empirical oscillating pair potentials.” The local structural order is studied by computing two microscopic functions, namely, the concentration fluctuation function and the Warren-Cowley short-range order parameter. We also study the influence of ordering phenomena on transport properties like diffusivity and viscosity. The MD results are confronted to those determined from measurements and in the framework of the quasi-lattice theory.

The defect population is important for understanding the microstructure of δ-plutonium. Using spin-polarized density-functional theory, we calculate the formation energies of monovacancies as well as divacancies. We show that the unrelaxed values are quite independent of the magnetic configuration while atomic relaxations lead to smaller values, relaxation effects being larger for the disordered magnetic structure.

We present results on the structural stability of Pu1− x Ga x alloys based on the local spin-density approximation and including non-local corrections to the exchange–correlation functional (generalized gradient approximation). First, we used both the linearized augmented plane-wave and the projector augmented wave (PAW) methods to calculate the energies of formation of five intermediate phases which occur in the Pu–Ga phase diagram. Then, PAW calculations were performed for various superstructures based on the fcc lattice to get the interaction parameters to describe the energetics of the δ-Pu–Ga solid solution. To discuss the effects of the chemical short-range order on the structural stability of the δ-Pu–Ga solid solution as a function of temperature, the cluster variation method configurational entropy has been introduced using the regular tetrahedron approximation. The solid part of the Pu-rich side of the phase diagram involving phase equilibria between the Pu phases, the δ-Pu–Ga solid solution and...

The formation energies of Pu–X (X = Al, Ga, In) compounds are calculated in the framework of density functional theory within the local spin-density approximation and non-local corrections to the exchange-correlation functional. The factors that govern the stability of δ-(Pu–X) solid solutions are analysed using fcc superstructures for which total energy calculations are performed. The cluster variation method is used to determine the influence of chemical short range order on the structural stability of δ-(Pu–X) solid solutions as a function of temperature and composition. The differences between the three systems are discussed. The calculated Pu–X (X = Al, Ga, In) phase diagrams in the Pu-rich side are compared with the experimental ones.

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; Ab initio molecular dynamics simulations are used to describe the diffusion of hydrogen in liquid aluminum at different temperatures. Quasi-instantaneous jumps separating periods of localized vibrations around a mean position are found to characterize the hydrogen motion at the microscopic scale. The hydrogen motion is furthermore analyzed using the van Hove function. We highlight a non-Fickian behavior for the hydrogen diffusion due to a large spatial distribution of hydrogen jumps. We show that a generalized continuous time random walk (CTRW) model describes the experimental diffusion coefficients in a satisfactory manner. Finally, the impact of impurities and alloying elements on hydrogen diffusion in aluminum is discussed.