Influence of alkaline earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite type oxides La 0.6 A 0.4 FeO 3 delta A Ca, Sr and Ba

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties ofperovskite type oxides La0.6A0.4FeO3 d A Ca, Sr, and Ba were investigated. La0.6Ca0.4FeO3 d LCF64 and La0.6Sr0.4FeO3 d LSF64 show a phase transformation in air at elevated temperature,i.e., fromorthorhombic Pnma to rhombohedral R 3c and from rhombohedral to cubic Pm 3m , respectively,while La0.6Ba0.4FeO3 d LBF64 remains cubic over the entire temperature range from room temperatureto 10001C. The different phase behaviour of the solids is interpreted to reflect the decreased tendencyfor octahedral tilting with increasing alkaline earth metal dopant ion radius. The electrical conductivityof LSF64 is 191 S cm 1in air at 8001C, decreasing to a value of 114 S cm 1at apO2of 0.01 atm, andfound over thispO2range roughly twice as high as those of LCF64 and LBF64. Failure to describe thedata of electrical conductivity using Holstein s small polaron theory is briefly discussed. Chemicaldiffusion coefficients and surface exchange coefficients of the materials in the range 650 9001C wereextracted from data of electrical conductivity relaxation. Data of oxygen nonstoichiometry was used tocalculate the vacancy diffusion coefficients from the measured chemical diffusion coefficients. Thecalculated migration enthalpies are found to decrease in the order LCF64 1.08 0.04 eV 4LSF64 0.95 0.01 eV 4LBF64 0.81 0.01 eV . The estimated ionic conductivities of the materials, at9001C, are within a factor of 1.