ON THE RELIABILITY OF THE IDEAL MASS-ACTION LAW FORMALISM FOR THE ANALYSIS OF NONSTOICHIOMETRY AND RELATED PROPERTIES IN IONIC COMPOUNDS

We have investigated the reliability and the limits of the ideal mass action law formalism to describe thermodynamic and electrical properti es of nonstoichiometric compounds in the case where Coulomb interactio ns between defects are predominant. To do this, we have compared the p redictions yielded by this procedure with accurate Monte Carlo calcula tions. We have first shown that the mass action constants must be corr ectly evaluated by fitting both enthalpic and entropic contributions. We have also shown that the ideal mass action formalism has to be used carefully and that its range of application is limited to the case wh ere the interactions are not too large (for instance, the dielectric c onstant must be greater-than-or-equal-to 17 in the case of M1-deltaO c ubic oxides) and for moderate departures from stoichiometry i.e. neith er too small nor too large.