Oxygen-Vacancy-Related Structural Phase Transition of Ba_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ

Ba 0.8 Sr 0.2 Co 0.8 Fe 0.2 O 3-δ exists as a single cubic phase at 1000 °C in air. Upon slow cooling, it transforms, between 900 and 700 °C, to a mixture of cubic, hexagonal, and rhombohedral phases, as evidenced by X-ray diffraction and transmission electron microscopy. The reversible phase change is accompanied by a large change of the oxygen content (Δδ = 0.14) of Ba 0.8 Sr 0.2 Co 0.8 Fe 0.2 O 3-δ and a volume change of 1.2 vol% at 700–900 °C. The multiphase material shows lamellar shaped structures in the HRTEM micrographs due to the partial transformation of the material from the cubic to the hexagonal and rhombohedral symmetries. The electrical conductivity changes from p -type semiconductivity between room temperature and 700 °C to metal-like conductivity at higher temperatures up to 1000 °C. The electrical conductivity is irreversibly changed upon thermal cycling due to the formation of microcracks.