POINT-DEFECTS IN OXIDE SPINEL SOLID-SOLUTIONS OF THE TYPE (CO,FE,MN)(3-DELTA)O-4 AT 1200-DEGREES-C

The composition dependence of the concentrations of the majority point defects (cation vacancies and cation interstitials) in cobalt-iron-ma nganese oxide spinels at 1200 degrees C and constant oxygen partial pr essure was studied. The energies of point defect formation in oxide sp inel solid solutions depend on temperature and chemical composition, a nd can be related to so-called defect constants. Values for such defec t constants were determined by fitting experimental nonstoichiometry d ata to equations derived from point defect thermodynamics. The applica bility of two different approaches to modeling the composition depende nce of the defect constants were investigated. The first approach is a model, originally proposed by Aragon and McCallister for titanomagnet ite, 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, especi ally 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-sal t structure oxide phases to spinels. This model works essentially for all spinel compositions investigated.