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

The crystal structure of La1-xSrxCoO3-delta in the composition range 0 less than or equal to x less than or equal to 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 Pm (3) over bar m) for x > 0.5 and a rhombohedrally distorted perovskite structure (spacegroup R(3) over bar c) for x less than or equal to 0.5. Neutron diffraction of La0.3Sr0.7CoO3-delta, annealed at 650 degrees C in air showed ideal cubic symmetry. HRTEM and SAED studies, however, revealed microdomains with an a(c) X a(c) X 2a(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. (C) 2000 Published by Elsevier Science BN. All rights reserved.