Oxygen Nonstoichiometry, Defect Structure, Thermal and Chemical Expansion of Pseudo-Cubic La0.8Sr0.2Co0.9Ni0.1O3−δ and Double Perovskite GdBaCo2O6−δ

Abstract

The results of thermal and chemical expansion measured as a function of oxygen partial pressure, pO2, and temperature by means of in situ XRD and dilatometric techniques are presented for the pseudo-cubic cobaltite La0.8Sr0.2Co0.9Ni0.1O3−δ and the double perovskite GdBaCo2O6−δ. The modeling of the defect structure of La0.8Sr0.2Co0.9Ni0.1O3−δ was carried out. Within the framework of the model all nickel sites are assumed to be occupied by localized electrons whereas both electrons and holes can be localized on cobalt sites. The defect structure model proposed was shown to fit pretty well available experimental data on oxygen nonstoichiometry of the perovskite studied. Concentrations of all defect species were calculated and then employed to compute the chemical expansion of the La0.8Sr0.2Co0.9Ni0.1O3−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 La0.8Sr0.2Co0.9Ni0.1O3−δ at all temperatures investigated. As a result, the spin state distribution of cobalt was calculated depending on temperature for the oxide studied. Double perovskite lattice chemical expansion along a axis and contraction along c axis with decreasing oxygen content, 6−δ, were found to compensate each other completely and, therefore, volume chemical expansion becomes negligible. As a result the cell volume linearly increases with temperature contrary to the simple pseudo-cubic perovskite.