Phase Stability and Oxygen Nonstoichiometry of Highly Oxygen-Deficient Perovskite-Type Oxides: A Case Study of (Ba,Sr)(Co,Fe)O3−δ

Highly oxygen-deficient perovskite-type oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF5582), are excellent mixed ionic and electronic conductors, with high concentrations of mobile oxygen vacancies and electronic charge carriers. Literature data for the oxygen nonstoichiometry δ of BSCF5582 at a given temperature and oxygen partial pressure show considerable scatter. In addition, the decomposition of cubic BSCF5582 (into hexagonal and cubic phases of differing compositions) at temperatures below 1073 K was not taken into account. In this study we investigated the phase stability and the oxygen nonstoichiometry of BSCF5582 using thermogravimetry (TG) and coulometric titration (CT). TG experiments, based on rapid temperature jumps, permitted the relative oxygen nonstoichiometry Δδ of cubic BSCF5582 to be extracted, free from the effects of the aforementioned slow decomposition. The CT results suggest the existence, at low oxygen partial pressures, of a stoichiometric phase with composition Ba0.5Sr0.5Co0.8Fe0.2O2 in which all the cations are in the lowest oxidation states possible in a solid compound. It is proposed to use this point as a reference to calibrate both CT and TG data, yielding absolute values for δ for the cubic BSCF5582 without any uncertainties posed by the decomposition. The TG experiments also yielded the decomposition temperature for cubic BSCF as a function of oxygen partial pressure.