Abstract
The crystal structure of La 1− x Sr x CoO 3− 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 La 0.3Sr 0.7CoO 3− δ , annealed at 650°C in air showed ideal cubic symmetry. HRTEM and SAED studies, however, revealed microdomains with an a c× a c×2 a c 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) AO 4/4 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.
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