Effective site-energy model: A thermodynamic approach applied to size-mismatched alloys

International audienceWe present a novel energetic model that takes into account atomistic relaxations to describe the thermodynamic properties of A c B 1−c binary alloys. It requires the calculation of the energies on each site of a random solid solution after relaxation as a function of both the local composition and the nominal concentration. These site energies are obtained by molecular static simulations using N-body interatomic potentials derived from the second-moment approximation (SMA) of the tight-binding scheme. This new model allows us to determine the effective pair interactions (EPIs) that drive the short-range order (SRO) and to analyze the relative role of the EPIs' contribution to the mixing enthalpy, with respect to the contribution due to the lattice mismatch between the constituents. We apply this formalism to Au-Ni and Ag-Cu alloys, both of them tending to phase separate in the bulk and exhibiting a large size mismatch. Rigid-lattice Monte Carlo (MC) simulations lead to phase diagrams that are in good agreement with both those obtained by off-lattice SMA-MC simulations and the experimental ones. While the phase diagrams of Au-Ni and Ag-Cu alloys are very similar, we show that phase separation is mainly driven by the elastic contribution for Au-Ni and by the EPIs' contribution for Ag-Cu. Furthermore, for Au-Ni, the analysis of the SRO shows an inversion between the tendency to order and the tendency to phase separate as a function of the concentration

Superficial segregation in nanoparticles from facets to infinite surfaces

International audienceWe compare the superficial segregations of the Cu-Ag system for a nanoparticle and for surfaces that are structurally equivalent to each of its facet. Based on a lattice-gas model and within a mean-field formalism, we derive segregation isotherms at various temperatures in the canonical ensemble, i.e., for a given overall solute concentration, and in the semi-grand canonical ensemble, i.e., for a given bulk solute concentration. If both processes are very similar for high temperatures, they differ substancially at lower temperatures. Due to the finite-size effect and the indirect coupling between facets and edges, the relative position of the phase transitions of the facets and the corresponding surfaces are inversed when displayed as function of the solute bulk concentration. Moreover, we show that working in the semi-grand canonical ensemble is a much more efficient way to study this phenomenon, although nanoparticles are canonical- objects in essence

Tight-binding variable-charge model for insulating oxides: Application to TiO

We have developed a new variable-charge model aimed at performing large-scale realistic simulations of oxide surfaces and interfaces. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono-covalent character of the metal-oxygen bond by means of a tight-binding analytical approach. We present the first results obtained for TiO2 and ZrO2 polymorphs, which are in very good agreement with the experimental data and recent ab initio results

Use of a variable-charge interatomic potential for atomistic simulations of bulk, oxygen vacancies, and surfaces of rutile TiO 2

International audienc

Hydrogen on screw dislocation in Fe and W: existence of 3D-compound and exotic segregation profile

We present the study of hydrogen atom segregation on screw dislocation in Fe and W metals by means of rigid lattice Monte Carlo simulations using interaction energies obtained from ab initio calculations. These interaction energies are attractive or repulsive depending on direction and distance. This high interaction anisotropy leads to the formation of a 3D compound for both iron and tungsten. A second consequence is that, within a given temperature range, hydrogen enrichment is greater at the outer than at the innermost sites of the dislocation. This unusual behavior is only observed for iron. We propose a mean-field model that accounts for the ordering phenomenon to explain the results and the differences between the two metals

Superficial segregation, wetting, and dynamical equilibrium in bimetallic clusters: A Monte Carlo study

International audienc

Ségrégation interfaciale et rupture : l'apport d'une approche atomistique “mixte”

Dans un alliage, toute modification de l'environnement cristallographique s'accompagne d'une variation des concentrations locales des constituants de l'alliage. La ségrégation peut ainsi être induite par des distorsions de liaisons, comme au coeur des joints de grains, ou par des variations du nombre de coordination, comme en surface. Pour modéliser ces variations de composition interfaciale, les approches vont de la thermodynamique phénoménologique qui ne prend pas en compte la structure atomique des interfaces jusqu'aux calculs ab initio, qui ne permettent guère d'appréhender les forces motrices de la ségrégation interfaciale. Nous montrerons l'apport d'une approche `mixte' couplant modèle de gaz sur réseau et potentiels interatomiques dérivés de la structure électronique. Ceux-ci permettent de calculer les paramètres énergétiques du modèle de gaz sur réseau tout en prenant en compte l'influence des relaxations des distances interatomiques. Grâce à cette approche, nous pourrons comparer les forces motrices responsables des ségrégations superficielle et intergranulaire et discuter du rôle de la ségrégation interfaciale quand un joint de grains se scinde en deux surfaces en tête de fissure

Complete miscibility between different crystallographic structures : Monte Carlo simulations of Cu-Ag deposited on Cu(001)

International audienceMonte Carlo simulations of an Ag$_c$Cu$_{1-c}$ monolayer deposited onto Cu(001) show that a complete solubility of two elements adopting different crystallographic structures can be observed in the surface, while experimental bulk phase diagrams preclude a similar phenomenon in the bulk. While the deposited pure Ag monolayer is pseudo-hexagonal and the pure Cu monolayer is square and pseudomorphic, for intermediate concentrations a disordered state appears in which square and hexagonal environments, respectively due to Cu and Ag coexist. As a result, the surface phase diagram does not present any miscibility gap at 650 K

Ordre intergranulaire et démixtion de volume : une étonnante coexistence

Par simulation Monte-Carlo, nous avons mis en évidence la formation d'un composé intergranulaire bidimensionnel dans le plan du joint de grains de flexion ∑ = 5 (310) pour une solution solide Cu(Ag) modélisée par un potentiel interatomique à N-corps. Ce résultat est d'autant plus surprenant qu'il est obtenu dans un système ayant une très forte tendance à la démixtion. Par une analyse détaillée des forces motrices contrôlant la ségrégation intergranulaire, nous montrons que l'apparition de ce composé est due à la présence de sites géométriquement inéquivalents dans le plan du joint. Ces différences géométriques, par couplage avec l'effet de taille, induisent une ségrégation d'argent spécifique à chaque type de site, conduisant ainsi à l'existence d'un ordre à longue distance dans le plan du joint

Crossover among structural motifs in Pd–Au nanoalloys

International audienc