CONDUCTIVITY AND SEEBECK MEASUREMENTS ON STRONTIUM FERRATES

The Seebeck coefficient of nine strontium ferrate ceramics with the nominal compositions: SrFeyO3-x (where y = 0.909, 0.952, 1, 1.05, 1.10), and NazSr1-zFeO3-x and LazSr1-zFeO3-x, (where z=0.05, 0.1) were measured in air from 300-1000 degrees C. The deficiency in oxygen stoichiometry, x, adjusts itself in air from approximately 0.15 to 0.35 in this temperature range. Most of the materials have reasonably high electronic conductivities, and some samples of lanthanum-doped ferrate reached conductivities of 50-90 S/cm at 25 degrees C. The total spread in conductivity between the 9 samples was 4 decades at 25 degrees C. Negative temperature coefficients for the total conductivity were observed above 5-600 degrees C. The thermopowers (-5 to +120 mu V/K) show similar (strong) temperature dependencies for all materials and increase with increasing temperature. The temperature dependency of conductivity and thermopower can be explained by the strong variation of oxygen content (hole concentration decreases with increasing temperature). A simple defect model for SrFeO3-x at high P-O2 was consistent with the observed oxygen partial pressure dependence of the conductivity at 800 degrees C.