Defect chemistry and transport properties of perovskite-type oxides La1-xCax FeO3- δ

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties of perovskite-type oxides La1-xCaxFeO3-δ (x = 0.05, 0.10, 0.15, 0.20, 0.30 and 0.40) are investigated. All investigated compositions exhibit, under ambient air, a phase transition from room-temperature orthorhombic (space group Pbnm) to rhombohedral (space group R3c) at elevated temperature. The transition temperature is found to decrease gradually from 900 °C for x = 0.05 to 625 °C for x = 0.40. Analysis of the data of oxygen nonstoichiometry obtained by thermogravimetry shows that under the given experimental conditions the Ca dopant is predominantly compensated by formation of electron holes rather than by oxygen vacancies. Maximum electrical conductivity under air is found for the composition with x = 0.30 (123 S cm-1 at 650 °C). Analysis of the temperature dependence of the mobility of the electron holes in terms of Emin-Holstein's theory indicates that small polaron theory fails for the compositions with high Ca contents x = 0.30 and x = 0.40. This is tentatively explained by the increased delocalization of charge carriers with increasing Ca dopant concentration. The oxygen transport properties of La1-xCaxFeO3-δ in the range 650-900 °C are evaluated using the electrical conductivity relaxation (ECR) technique. Combined with data of oxygen non-stoichiometry, the obtained results enable calculation of the oxygen vacancy diffusion coefficient and associated ionic conductivity. Both parameters increase with increasing Ca content in La1-xCaxFeO3-δ, while it is found that the effective migration barrier for oxygen diffusion decreases with decreasing oxygen vacancy formation enthalpy