Point defects in oxide spinel solid solutions of the type (Co,Fe,Mn)3 − δO4 at 1200 °C

The composition dependence of the concentrations of the majority point defects (cation vacancies and cation interstitials) in cobalt-iron-manganese oxide spinels at 1200 °C and constant oxygen partial pressure was studied. The energies of point defect formation in oxide spinel solid solutions depend on temperature and chemical composition, and can be related to so-called defect constants. Values for such defect constants were determined by fitting experimental nonstoichiometry data to equations derived from point defect thermodynamics. The applicability of two different approaches to modeling the composition dependence of the defect constants were investigated. The first approach is a model, originally proposed by Aragón and McCallister for titanomagnetite, which assumes that the equilibrium constants of defect formation reactions in Fe3O4 are the same in iron-based spinel solid solutions. This model works relatively well for cobalt-iron oxide spinels, especially at low Co concentrations, but not for iron-manganese spinel solid solutions. The other approach is more general, and relates the defect constants to the standard free energies for the oxidation of rock-salt structure oxide phases to spinels. This model works essentially for all spinel compositions investigated.