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

Abstract

The perovskite type oxides of the system Sr-Ce-Fe-O were prepared by mixing and thermal treatment in air. Microstructure and phase composition were determined by scanning electron microscopy and X-ray diffraction. Besides the pure SrCeO 3 and SrFeO 3 − x a solid solution range of Sr 1 − y Ce y FeO 3 − x was observed between y = 0.01 ÷ 0.2. This solid solution is the active phase responsible for high oxygen exchange and electrical conductivity as required for cathode materials of high-temperature electrochemical cells. The oxygen exchange and oxygen deficiency x were measured by solid electrolyte coulometry. From pO 2- T- x diagrams the partial molar enthalpies and entropies of oxygen were calculated for the air-oxidized states of the solid solution and of the two-phase region: ΔH o of the two-phase material 0.8 SrCeO 3 − 0.2 SrFeO 3 − x is − 23 to − 29 kJ mol −1 and ΔS o is − 3 to − 8 J mol −1 K −1. For the solid solution Sr 0.9Ce 0.1FeO 3 − x the values depend on x: ΔH o increases from − 33.5 to − 28.4 kJ mol −1 and ΔS o increases from − 10 to + 20 J mol −1 K −1 in the range 0.24 < x < 0.36. The dc electrical conductivity of the monophase material Sr 1 − y Ce y FeO 3 − x show a maximum near 500 °C at y c e = 0.05 mol (80 S cm −1, E a = 0.088 eV). The conductivity is of p-type in the region of higher oxygen partial pressures and n-type at reducing conditions.