Defect structure and defect-induced expansion of doped perovskite La0.7Sr0.3Co0.9Fe0.1O3−δ

Abstract

The results of chemical expansion measured as a function of oxygen partial pressure, pO2, and temperature by means of dilatometric technique are presented for the perovskite-type doped cobaltite La0.7Sr0.3Co0.9Fe0.1O3−δ. The modeling of the defect structure of this perovskite was carried out. Within the framework of the model all iron sites are assumed to be occupied by localized holes whereas both electrons and holes can be localized on cobalt sites in La0.7Sr0.3Co0.9Fe0.1O3−δ. The defect structure model proposed was shown to fit perfectly well available experimental data on oxygen nonstoichiometry of the perovskite studied. Equilibrium constants of the appropriate defects reactions were, therefore, determined and concentrations of all defect species defined within the framework of the model proposed were calculated as functions of temperature and oxygen nonstoichiometry. These concentrations were employed in the chemical expansion model derived by us earlier in order to compute the chemical expansion of the La0.7Sr0.3Co0.9Fe0.1O3−δ lattice as a function of its oxygen nonstoichiometry. Cobalt ions transition from low spin (LS) state to high spin (HS) one induced by temperature increase was taken into account as well. The model proposed was shown to coincide completely with experimental data on chemical expansion for the La0.7Sr0.3Co0.9Fe0.1O3−δ at all temperatures investigated. As a result, the spin state distribution of cobalt was calculated depending on temperature for the oxide studied.