Structural aspects of the ionic conductivity of La1−xSrxCoO3−δ

The crystal structure of La1−xSrxCoO3−δ in the composition range 0≤x≤0.8 was investigated by means of powder XRD. A more detailed study was performed on the x=0.7 composition by neutron diffraction, HRTEM, SAED, image analysis and PEELS. The XRD results indicate a long-range cubic perovskite symmetry (spacegroup Pm3̄m) for x>0.5 and a rhombohedrally distorted perovskite structure (spacegroup R3̄c) for x≤0.5. Neutron diffraction of La0.3Sr0.7CoO3−δ, annealed at 650°C in air showed ideal cubic symmetry. HRTEM and SAED studies, however, revealed microdomains with an ac×ac×2ac superstructure in addition to regions without such a superstructure. The superstructure could be explained by assuming the removal of all the oxygen ions from every other (001) AO4/4 plane. PEELS measurements indicated a lower oxidation state for Co in the superstructure regions than in the non-superstructure regions. The importance of oxygen-vacancy ordering on the magnitude of ionic conductivity is discussed.