Defect formation and transport in SrFe1-xAlxO3-δ

Perovskite-related phases derived from SrFeO3-delta are among known mixed conductors with highest oxygen permeability and are thus of interest as the ceramic membrane materials for oxygen separation and partial oxidation of light hydrocarbons. Dense ceramics of SrFe1-xAlxO3-delta (x = 0.1-0.5) were prepared via the glycine-nitrate process. The cubic solid solution formation was found to occur in the concentration range x = 0 - 0.35. Increasing aluminum content leads to decreasing thermal expansion coefficients (TECs), relative fraction of Fe4+ under oxidizing conditions, and also the total conductivity, predominantly p-type electronic at oxygen pressures close to atmospheric. The TECs vary in the range (13.5-16.4) x 10(-6) K-1 at 373-923 K and increase up to (18.6-31.9) x 10(-6) K-1 at 923-1273 K. The oxygen permeation fluxes decrease moderately with aluminum additions. The Mossbauer spectroscopy data and P(02) dependencies of electrical properties indicate a small-polaron mechanism of electronic transport in SrFe1-xAlxO3-delta. Reducing oxygen partial pressure results in transition from dominant p- to n-type electronic conduction. The low-p(O-2) stability limit of SrFe1-xAlxO3-delta perovskites lies between that of LaFeO3-delta and Fe/Fe1-gammaO boundary.