Nonstoichiometric oxides as a continuous homologous series: linear free-energy relationship in oxygen exchange.

A novel methodology for the analysis of oxygen exchange in practically important nonstoichiometric oxides with mixed ionic electronic conductivity (MIEC) is suggested. It is based on the fact that the kinetic and thermodynamic properties of such oxides vary continuously with oxygen stoichiometry. This allows MIEC oxides to be considered as a homologous series, with the difference that traditional series are discrete in their chemical composition whereas MIEC oxides are continuous in oxygen stoichiometry. Analysis of the relations between Gibbs energies of reactions and activation barriers traditionally performed for homologous series can be useful in studies of oxygen exchange in MIEC oxides. To demonstrate the approach, thermodynamic and oxygen-exchange kinetics parameters are measured as functions of oxygen nonstoichiometry δ for two perovskites, SrCo0.8Fe0.2O3-δ and SrFeO3-δ, having metal-like and p-type semiconducting conductivities, respectively. Both oxides are shown to obey linear free energy relationships of the Brønsted-Evans-Polanyi form in spite of their different types of electronic structures. The results open up new possibilities for understanding the mechanism of the rate determining step of oxygen exchange in MIEC oxides.