Mixed conductivity and Mössbauer spectra of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (x=0–0.05, y=0–0.30)

Aluminum incorporation in the rhombohedrally distorted perovskite lattice of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (image, image) decreases the unit cell volume and partial ionic and p-type electronic conductivities, while the oxygen nonstoichiometry and thermal expansion at 900–1200 K increase on doping. The creation of A-site cation vacancies has an opposite effect on the transport properties of Al-substituted ceramics. The maximum A-site deficiency tolerated by the (La,Sr)(Fe,Al)O3−δ structure is however limited, close to 3–4%. The Mössbauer spectroscopy revealed progressive localization of electron holes and a mixed charge-compensation mechanism, which results in higher average oxidation state of iron when Al3+ concentration increases. The average thermal expansion coefficients of (La0.5Sr0.5)1−xFe1−yAlyO3−δ are (12.2–13.0)×10−6 K−1 at 300–900 K and (20.1–30.0)×10−6 K−1 at 900–1200 K in air. The steady-state oxygen permeability (OP) of dense Al-containing membranes is determined mainly by the bulk ionic conductivity. The ion transference numbers at 973–1223 K in air, calculated from the oxygen permeation and faradaic efficiency (FE) data, vary in the range 1×10−4–3×10−3, increasing with temperature.