R. Tétot

Molecular dynamics simulations for the Ag/Cu (111) system: from segregated to constitutive interfacial vacancies

The occurrence of an Ag layer on top of a Cu (111) substrate can result from either deposition (Ag/Cu) or strong segregation at the surface of a dilute Cu(Ag) alloy. Molecular dynamics simulations within many-body tight-binding potentials show that two structures compete: one corresponding to a rather uniform Ag (111) layer on top of a Cu (111) substrate leading to a Moire structure and the other involving constitutive vacancies forming triangular loops of partial dislocations in the first Cu substrate layer. A useful guide to predict the most efficient relaxation mechanism is to characterize the regions which undergo the largest stress. Thus, the local pressure map in the first Cu underlayer allows to predict a complementarity between the segregation of vacancies in compressive sites and the Ag enrichment, due to the larger atomic radius of the Ag atoms, in the tensile ones. Segregation energy maps, both for vacancies and Ag atoms, confirm this prediction and lead to several perspectives for the kinetics of dissolution of an Ag deposit over a Cu (111) substrate.

Order and disorder in the highly defective oxides TiOx, VOx and NbOx

Abstract Order and disorder of vacancies in highly defective monoxides TiO x , VO x and NbO x are investigated in the framework of the lattice gas Ising model with effective pair interactions. It is shown that the interactions are insignificant in VO x which is completely disordered. In the case of NbO x and TiO x , the interaction parameters are selected (i) to ensure the stability of the ordered structures present in these systems and (ii) to account for experimental oxygen partial molar free energy and enthalpy as a function of the composition in TiO x , by means of Monte Carlo simulations.

Defect modelling in non-stoichiometric oxides

The mass action law formalism has been extensively used as an efficient way to rationalize a large spectrum of properties of non-stoichiometric oxides. This approach is still extremely useful in spite of some shortcomings. In fact most of the improvements proposed in the literature have lost the clarity of the initial model and convey an extremely heavy formalism. We show that an obvious improvement is to take into account the electrostatic interactions, which allows to better understand not only thermodynamic properties but also transport properties.

Evaluation of Defect-Defect Pair Interactions in Nonstoichiometric Oxides by Cvm and Monte Carlo Calculations

A distinctive feature of oxides, in comparison with other nonstoichiometric compounds (including metallic alloys), is that ?G(O2) and ?H(O2), the relative partial molar free energy and enthalpy of oxygen, are measurable as a function of both temperature, T, and departure from stoichiometry, x, for a great number of systems1. ?G(O2) and ?H(O2) are respectively the variations of the free energy and the enthalpy during the dissolving of one oxygen mole in an infinite amount of oxide at constant T. As a consequence of the interactions between nonstoichiometry defects, these functions are generally heavily composition dependent. By means of statistical methods, the defect-defect pair interactions (DDPIs) may be related to ?G(O2) and ?H(O2). This allows us either to evaluate the DDPIs by an inverse approach, or to check defect models proposed in the litterature. In view of the shape of ??(O2)(x) for small x, e.g. for small concentrations of defects, two limiting cases may be distinguished: - metallic oxides: ?H(O2) is constant in a certain range of x, as in solid solutions of oxygen in transition metals (see refs in ref 1). This behaviour implies that the DDPIs are relatively short ranged. In this case, both Monte Carlo simulations and the CVM (cluster variation method) appear to be appropriate to deal with defect statistic.