Oxygen nonstoichiometry, conductivity, and Seebeck coefficient of La0.3Sr0.7Fe1-xGaxO2.65+δ perovskites

The total electrical conductivity and the Seebeck coefficient of perovskite phases La0.3Sr0.7Fe1-xGaxO2.65+delta (x = 0-0.4) were determined as functions of oxygen nonstoichiometry in the temperature range 650-950degreesC 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 Ga3+. The nonstoichiometry dependencies of the partial molar enthalpy and entropy of oxygen in La0.3Sr0.7(Fe,Ga)O2.65+delta 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 longrange 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 La0.3Sr0.7 (Fe,Ga)O2.65+delta, which increases with temperature, suggest a small-polaron conduction mechanism. (C) 2002 Elsevier Science (USA).