Crystal structure, oxygen deficiency, and oxygen diffusion path of perovskite-type lanthanum cobaltites La _0.4Ba _0.6CoO _3-δ and La _0.6Sr _0.4CoO _3-δ

Crystal structures of perovskite-type lanthanum cobaltites, La _0.4Ba _0.6CoO _3-δ and La _0.6Sr _0.4CoO _3-δ, have been studied by in situ neutron powder diffractometry from 27 to 1258 °C. La _0.4Ba _0.6CoO _3-δ has a cubic Pm3̄m structure in the whole temperature range, while La _0.6Sr _0.4CoO _3-δ exhibits a phase transition from hexagonal (R3̄c) to cubic (Pm3̄m) symmetry between 400 and 600 °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, La _0.4Ba _0.6CoO _3-δ. 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 was not visualized in La _0.6Sr _0.4CoO _3-δ but in La _0.4Ba _0.6CoO _3-δ, which indicates the higher oxygen diffusivity in La _0.4Ba _0.6CoO _3-δ. The oxygen defect concentration, bottleneck size for oxygen diffusion, and oxygen atomic displacement parameter of La _0.4Ba _0.6CoO _3-δ are higher than those of La _0.6Sr _0.4CoO _3-δ, which are responsible for the higher oxygen diffusivity in La _0.4Ba _0.6CoO _3-δ.