Perovskite-like system (Sr,La)(Fe,Ga)O3-δ:: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La1-xSrxFe1-yGayO3-delta (x=0.40-0.80; y=0-0.60) was found to vary in the approximate range y = 0.25-0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O3-delta in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7-16.0) x 10(-6) K-1 at 300-800 K and (19.3-26.7) x 10(-6) K-1 at 800-1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La1-xSrxFe1-yGayO3-delta (x=0.7-0.8; y=0.2-0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La1-xSrxFe1-yGayO3-delta increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport. (C) 2002 Elsevier Science B.V All rights reserved.