Mixed conductivity and stability of A-site-deficient Sr(Fe,Ti)O3-δ perovskites

Deficiency in the A sublattice of perovskite-type Sr1-yFe0.8Ti0.2O3-delta(y = 0-0.06) leads to suppression of oxygen-vacancy ordering and to increasing oxygen ionic conductivity, unit cell volume, thermal expansion, and stability in CO2-containing atmospheres. The total electrical conductivity, predominantly p-type electronic in air, decreases with increasing A-site deficiency at 300-700 K and is essentially independent of the cation vacancy concentration at higher temperatures. Oxygen ion transference numbers for Sr1-yFe0.8Ti0.2O3-delta in air, estimated from the faradaic efficiency and oxygen permeation data, vary in the range from 0.002 to 0.015 at 1073-1223 K, increasing with temperature. The maximum ionic conductivity was observed for Sr0.97Fe0.8Ti0.2O3-delta ceramics. In the system Sr0.97Fe1-xTixO3-delta (x=0.1-0.6), thermal expansion and electron-hole conductivity both decrease with x. Moderate additions of titanium (up to 20%) in Sr-0.97(Fe,Ti)O3-delta result in higher ionic conductivity and lower activation energy for ionic transport, owing to disordering in the oxygen sublattice; further doping decreases the ionic conduction. It was shown that time degradation of the oxygen permeability, characteristic of Sr(Fe,Ti)O3-delta membranes and resulting from partial ordering processes., can be reduced by cycling of the oxygen pressure at the membrane permeate side. Thermal expansion coefficients of Sr1-yTi1-xFexO3-delta (x=0.10-0.60, y=0-0.06) in air are in the range (11.7-16.5)x10(-6) K-1 at 350-750 K and(16.6-31.1)x10(-6) K-1 at 750-1050 K.