A/B-RATIO AND TRANSPORT PROPERTIES OF (La0.85Sr0.15)SCoO3-DELTA PEROVSKITES

The glycine nitrate procedure was used to synthesise small grain size ( < 1 micron) perovskites (La0.85Sr0.15)0.98CoO3 and (La0.85Sr0.15)1.00CoO3 of precise stoichiometry. Electrical conductivity relaxation measurements were used to determine the influence of cation vacancies on transport properties. It was concluded that the oxide ion chemical diffusion coefficient of the two perovskites was the same, with an activation energy of 107 kJ /mol. The Co-rich material exhibited a significantly higher surface exchange coefficient, attributed to the creation of cation vacancies on the surface. Also, electrical conductivity measurements indicated that the stoichiometric sample contained more oxide ion vacancies. Should these vacancies contribute to oxygen incorporation, then an increase at the surface would enhance the surface exchange reaction. It was also possible that a secondary phase catalysed oxygen reduction. The electrical conductivities of the two samples were the same at temperatures above 620 K, whilst the Co-rich material exhibited significantly higher conductivity at lower temperatures. This was attributed to the creation of A-site vacancies in the Co-rich specimen being compensated by the formation of electron-holes at low temperature. At higher temperatures, the A-site vacancies were progressively compensated by oxide ion vacancies, so reducing the electron hole concentration. 13 refs.