A MONTE-CARLO INVESTIGATION OF THE ROLE OF COULOMB INTERACTIONS ON THERMODYNAMIC PROPERTIES AND DEFECT STRUCTURE OF M1-DELTA-O CUBIC OXIDES

We have performed a Monte Carlo investigation of the thermodynamic pro perties and defect structure of nonstoichiometric rocksalt-structured transition metal oxides M1-deltaO (M = Ni, Co, Mn). Our aim is to go b eyond the approximate models usually used, based on the ideal mass act ion law formalism or on the Debye-Huckel theory. We have considered a defect model as simple as possible: metal vacancies and positive holes localized on the metal sublattice. These defects are subject to pure Coulomb interactions through a unique dielectric constant which is con sidered to be an adjustable parameter. We show that this model allows us to satisfactorily account for the variations of the partial molar f unctions DELTAG(O2) = RT In PO2 and DELTAH(O2) according to the compos ition for this family of oxides. The defect structure is described in term of radial distribution functions and clustering. In the first par t of the paper, the Monte Carlo methods aimed at the study of very sma ll departures from stoichiometry are detailed. Especially, we have ada pted some methods developed in the simulations of fluids: the ''partic le insertion method'' and ''umbrella sampling''.