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

Abstract

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.