Oxygen nonstoichiometry and mixed conductivity of SrFe 1− x[formula omitted]O 3− δ ( M=Al, Ga): Effects of B-site doping

Abstract

The oxygen nonstoichiometry, electrical conductivity and Seebeck coefficient of perovskite-related SrFe 1− x Ga x O 3− δ ( x = 0 – 0.2 ) and SrFe 1− x Al x O 3− δ ( x = 0.1 – 0.3 ) were studied in the temperature range 923–1223 K at oxygen partial pressures varying from 10 −20 to 0.3 atm. The substitution of iron with gallium was found to promote long-range ordering of oxygen vacancies, resulting in the perovskite → brownmillerite transition at moderate oxygen pressures, and to decrease the mobility of the p-type electronic charge carriers. Opposite tendencies are observed for aluminum doping, which causes a higher oxygen nonstoichiometry in oxidizing atmospheres. As suggested by atomistic simulations of highly oxygen-deficient ferrite lattices, this behavior is associated with oxygen-vacancy trapping near Al 3+ having energetically favorable tetrahedral coordination, whereas Ga 3+ cations with preferential octahedral coordination in the perovskite lattice tend to repulse vacancies towards oxygen sites surrounded by iron and thus destabilize the structure. The relatively low hole mobility, with activation energies of 0.18–0.33 eV, indicates a small-polaron conduction mechanism. Under reducing conditions the oxygen content and ionic transport in brownmillerite-type polymorphs of SrFe 1− x Ga x O 3− δ , exhibiting a relatively wide range of oxygen stoichiometry variation, are both lower compared to the Al-containing materials characterized by the co-existence of perovskite- and brownmillerite-like domains.