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, La 0.4 Ba 0.6 CoO 3−δ and La 0.6 Sr 0.4 CoO 3−δ, have been studied by in situ neutron powder diffractometry from 27 to 1258 °C. La 0.4 Ba 0.6 CoO 3−δ has a cubic Pm 3̅ m structure in the whole temperature range, while La 0.6 Sr 0.4 CoO 3−δ exhibits a phase transition from hexagonal ( R 3̅ c ) to cubic ( Pm 3̅ m ) 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 AB O 3 mixed ionic-electronic conductor, La 0.4 Ba 0.6 CoO 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 0.6 Sr 0.4 CoO 3−δ but in La 0.4 Ba 0.6 CoO 3−δ, which indicates the higher oxygen diffusivity in La 0.4 Ba 0.6 CoO 3−δ . The oxygen defect concentration, bottleneck size for oxygen diffusion, and oxygen atomic displacement parameter of La 0.4 Ba 0.6 CoO 3−δ are higher than those of La 0.6 Sr 0.4 CoO 3−δ, which are responsible for the higher oxygen diffusivity in La 0.4 Ba 0.6 CoO 3−δ .