Oxygen Nonstoichiometry, Conductivity, and Seebeck Coefficient of La 0.3Sr 0.7Fe 1− xGa xO 2.65+ δ Perovskites

Abstract

The total electrical conductivity and the Seebeck coefficient of perovskite phases La 0.3Sr 0.7Fe 1− x Ga x O 2.65+ δ ( x=0–0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650–950°C at oxygen partial pressures varying from 10 −4 to 0.5 atm. Doping with gallium was found to decrease oxygen content, p-type electronic conduction and mobility of electron holes. The results on the oxygen nonstoichiometry and electrical properties clearly show that the role of gallium cations in the lattice is not passive, as it could be expected from the constant oxidation state of Ga 3+. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La 0.3Sr 0.7(Fe,Ga)O 2.65+ δ are indicative of local inhomogeneities, such as local lattice distortions or defect clusters, induced by gallium incorporation. Due to B-site cation disorder, this effect may be responsible for suppressing long-range ordering of oxygen vacancies and for enhanced stability of the perovskite phases at low oxygen pressures, confirmed by high-temperature X-ray diffraction and Seebeck coefficient data. The values of the electron–hole mobility in La 0.3Sr 0.7(Fe,Ga)O 2.65+ δ , which increases with temperature, suggest a small-polaron conduction mechanism.