Redox energetics of perovskite-related oxides

The present paper focuses on the redox properties of perovskite-related oxides and presents a solution model that connects integral thermodynamic properties that are measured calorimetrically with partial thermodynamic quantities that are measured by equilibration methods. The model allows us to extract significant features of the redox energetics of non-stoichiometric oxides. It is shown that the redox behavior, e. g. the composition-partial-pressure isotherms, is independent of the stability of the non-stoichiometric oxides and as a first approximation is directly given by the relative stability of the oxidation states involved. The stability of a given oxidation state is related to the structure of the oxide, and the large difference in redox behavior between hexagonal and cubic SrMnO3-delta is rationalized. Whereas both the enthalpy and entropy of oxidation in general depend on temperature, a number of systems can be adequately described using an ideal solution approach. This implies that composition-independent enthalpic and entropic terms can be used as first approximations to describe the redox energetics of non-stoichiometric oxides. In order to illustrate the approach an overview of the redox energetics of selected La(1-x)Ae(x)MO(3-delta) phases (Ae alkaline earth, M transition metal) of interest in connection with solid oxide fuel cell and gas separation membrane applications is given.