Crystal Structure, Oxygen Deficiency, and Oxygen Diffusion Path of Perovskite-Type Lanthanum Cobaltites La0.4Ba0.6CoO3-δ and La0.6Sr0.4CoO3-δ

Crystal structures of perovskite-type lanthanum cobaltites, La0.4Ba0.6CoO3-delta and La0.6Sr0.4CoO3-delta, have been studied by in situ neutron powder diffractometry from 27 to 1258 C. La0.4Ba0.6CoO3-delta has a cubic Pm (3) over barm structure in the whole temperature range, while La0.6Sr0.4CoO3-delta exhibits a phase transition from hexagonal (R (3) over barc) to cubic (Pm (3) over barm) symmetry between 400 and 600 degrees C. Geometric information on oxygen diffusion is essential to understand the facile electrode reaction of perovskite-type cobaltites, but previous approaches have been limited to computational predictions. Here we provide long-awaited experimental evidence for a curved three-dimensional oxygen diffusion path in an ABO(3) mixed ionic-electronic conductor, La0.4Ba0.6CoO3-delta. The oxide ions diffuse in the (100) direction near the stable position, while the path is along (110) around the middle point of the path. The connected path as not visualized in La0.6Sr0.4CoO3-delta but in La0.4Ba0.6CoO3-delta, which indicates the higher oxygen diffusivity in La0.4Ba0.6CoO3-delta. The oxygen defect concentration, bottleneck size for oxygen diffusion, and oxygen atomic displacement parameter of La0.4Ba0.6CoO3-delta are higher than those of La0.6Sr0.4CoO3-delta, which are responsible for the higher oxygen diffusivity in La0.4Ba0.6CoO3-delta.