Application de la thermodynamique statistique à l'étude des composés non stoechiométriques

Nous présentons dans cet article des méthodes de simulation numérique à l'ordinateur pour l'étude par la thermodynamique statistique d'oxydes non stoechiométriques présentant de grands écarts à la stoechiométrie, donc de fortes teneurs en défauts. Les interactions coulombiennes de longue portée, présentes dans les oxydes partiellement ioniques, sont traitées par la méthode de Monte Carlo et les interactions courte portée par une méthode originale de dénombrement des niveaux d’énergie des défauts dans un petit cristal en équilibre thermodynamique avec un grand réservoir d'oxyde. Des résultats concernant deux types d'oxyde différents, TiO1+x, et CeO2-x sont présentés

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

Experimental approach and atomistic simulations to investigate the radiation tolerance of complex oxides: Application to the amorphization of pyrochlores

Both experimental approach and atomistic simulations are performed in order to investigate the influence of the composition of pyrochlores on their radiation tolerance. Therefore, Gd2Ti2O7 and Gd2Zr2O7 were irradiated with 4 MeV Au and 92 MeV Xe ions in order to study the structural changes induced by low and high-energy irradiations. XRD results show that, for both irradiations, the structural modifications are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Using atomistic simulations with new interatomic potentials derived from the SMTB-Q model, the lattice properties and the defect formation energies were calculated in Gd2Ti2O7 and Gd2Zr2O7. Calculations show that titanates have a more covalent character than zirconates. Moreover, in Gd2Ti2O7 the formation of cation antisite defects leads to strong local distortions around Ti-defects and to a decrease of the Ti coordination number, which are not observed in Gd2Zr2O7. Thus, the radiation resistance is related to the defect stability: the accumulation of structural distortions around Ti-defects could drive the Gd2Ti2O7 amorphization induced by irradiation. 2014 Elsevier B.V. All rights reserve

Phase Transformations in Pyrochlores Irradiated with Swift Heavy Ions: Influence of Composition and Chemical Bonding

International audienceGd2Ti2O7 and Gd2Zr2O7 pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. The XRD results show that the structural modifications induced by irradiation with 93 MeV Xe ions are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd(2)Zr(2)O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Atomistic simulations with the second-moment tight-binding QEq model allow us to calculate the lattice properties of both Gd2Ti2O7 and Gd2Zr2O7, and also to quantify the degree of covalency and ionicity in these compounds. These calculations clearly show that Gd2Ti2O7 is more covalent than Gd2Zr2O7, thus confirming that the amorphization resistance can be related to the covalent character of insulators

Key role of the short-range order on the response of the titanate pyrochlore Y 2 T i 2 O 7 to irradiation

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