STRUCTURE, OXYGEN STOICHIOMETRY AND ELECTRICAL-CONDUCTIVITY IN THE SYSTEM SR-CE-CO-O

Mixed oxides in the system Sr-Ce-Co-O were prepared by solid state rea ction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid s olutions of the type Sr1-yCeyCoO3-x and mixed oxides of the type (1-y) SrCeO3-ySrCoO(3-x). Microstructures and phase compositions were determ ined by electron microscopy and X-ray diffraction. SrCoO3-x forms a so lid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temper ature structure of pure SrCoO3-x and is characterized by high oxygen e xchange and electrical conductivity. The oxygen deficiency x was measu red by solid electrolyte coulometry. The oxygen deficiency of solid so lutions Sr1-yCeyCoO3-x increases with temperature and decreases with p O(2) in the ambient atmosphere and with increasing Ce dopant concentra tion. The pO(2)-T-x diagram of the solid solution was determined. The T, pO(2) and dopant concentration dependencies of electrical conductiv ity were measured by a four-point d.c. technique. By Ce doping stronti um cobaltite becomes a stabilized high-conductive material (maximum co nductivity: 500 S cm(-1) at 400 degrees C, E-a = 0.025 eV, p-type). Ab ove this temperature the T-coefficient of the conductivity changes fro m positive (semiconducting) to negative values.