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

The perovskite type oxides of the system Sr-Ce-Fe-O were prepared by m ixing and thermal treatment in air. Microstructure and phase compositi on were determined by scanning electron microscopy and X-ray diffracti on. Besides the pure SrCeO3 and SrFeO3-x a solid solution range of Sr1 -yCeyFeO3-x was observed between y = 0.01 divided by 0.2. This solid s olution is the active phase responsible for high oxygen exchange and e lectrical conductivity as required for cathode materials of high-tempe rature electrochemical cells. The oxygen exchange and oxygen deficienc y x were measured by solid electrolyte coulometry, From pO(2)-T-x diag rams the partial molar enthalpies and entropies of oxygen were calcula ted for the air-oxidized states of the solid solution and of the two-p hase region: Delta H-O of the two-phase material 0.8 SrCeO3-0.2 SrFeO3 -x is -23 to -29 kJ mol(-1) and Delta S-O is -3 to -8 J mol(-1) K-1. F or the solid solution Sr,0.9Ce0.1FeO3-x the values depend on x: Delta H-O increases from -33.5 to -28.4 kJ mol(-1) and Delta S-O increases f rom -10 to +20 J mol(-1) K-1 in the range 0.24 < x < 0.36. The de elec trical conductivity of the monophase material Sr1-yCeyFeO3-x show a ma ximum near 500 degrees C at y(Ce) = 0.05 mol (80 S cm(-1), E-a = 0.088 eV). The conductivity is of p-type in the region of higher oxygen par tial pressures and n-type at reducing conditions.