Oxygen nonstoichiometry and defect structure of undoped and doped lanthanum cobaltites

The results of oxygen nonstoichiometry are presented for the perovskite-type undoped cobaltite LaCoO3-δ, doped with strontium La0.7Sr0.3CoO3-δ, and doped with copper LaCo0.7Cu0.3O3-δ as a function of oxygen partial pressure \({p_{\rm {O_2}}}\) and temperature. The modeling of the defect structure of these oxygen deficient perovskites is carried out. Two different defect structure models are evolved. Within the framework of the first model electrons are treated as quasi-free in LaCoO3-δ, partly trapped in La0.7Sr0.3CoO3-δ and LaCo0.7Cu0.3O3-δ, whereas holes are assumed to be itinerant in all oxides studied. The intrinsic electronic disordering process is taken into account as well. According to the second model electrons and holes are assumed to be localized. Thermal excited charge disproportionation of cobalt is considered as well. The corresponding equations for the law of mass action, electroneutrality condition, and mass balance give a set of nonlinear equations in the both cases. The analytical solutions of these sets yield general expressions which are used for the verification of the defect models proposed by nonlinear curve fitting. The latter showed that both models of the defect structure of all lanthanum cobaltites studied fit the experimental data about equally good. It is, therefore, difficult to rule out either of defect structure models on the basis of equilibrium data on oxygen nonstoichiometry solely.