Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O

Abstract

Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr 1 − y Ce y CoO 3 − x and mixed oxides of the type (1 − y)SrCeO 3 − ySrCoO 3 − x . Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO 3 − x forms a solid 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-temperature structure of pure SrCoO 3 − x and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr 1 − y Ce y CoO 3 − x increases with temperature and decreases with pO 2 in the ambient atmosphere and with increasing Ce dopant concentration. The pO 2- T- x diagram of the solid solution was determined. The T, pO 2 and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm −1 at 400 °C, E a = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values.