Phase formation and iron oxidation states in SrFe(Al)O3−δ perovskites

Mössbauer spectroscopy, total conductivity measurements and X-ray diffraction (XRD) have been used to evaluate phase formation and Fe oxidation and coordination states in the Sr(Fe1−xAlx)O3−δ system. A solid-solution range of cubic perovskite (ABO3) with random B-cation distribution forms in the approximate limits, 0≤x≤0.35. The Fe3+/Fe ratio increases with increasing Al concentration fomenting a B-O network with isolated Fe4+ species, resulting in p-type electronic conductivity via a hopping mechanism. The activation energy for hole transport, 28-30 kJ/mol at 300-700 K in air, exhibits little variation with x. Thermal expansion coefficients, (13.5-16.4)×10−6 K−1 at 373-923 K and (18.6-31.9)×10−6 K−1 at 923-1273 K, decrease with increasing Al content and oxygen nonstoichiometry.