Mixed conductivity and stability of A-site-deficient Sr(Fe,Ti)O_3–δ perovskites

Abstract.

Deficiency in the A sublattice of perovskite-type Sr_{1– y} Fe_0.8Ti_0.2O_3–δ (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 Sr_{1– y} Fe_0.8Ti_0.2O_3–δ 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 Sr_0.97Fe_0.8Ti_0.2O_3–δ ceramics. In the system Sr_0.97Fe_{1– x} Ti _x O_3–δ (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)O_3–δ 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)O_3–δ 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 Sr_{1– y} Ti_{1– x} Fe _x O_3–δ (x=0.10–0.60, y=0–0.06) in air are in the range (11.7–16.5)×10^–6 K^–1 at 350–750 K and (16.6–31.1)×10^–6 K^–1 at 750–1050 K.