Order-disorder transition of Sr2Fe2O5 from brownmillerite to perovskite structure at an elevated temperature

Strontium ferrate (III) of composition Sr₂Fe₂O₅ is well known to have an orthorhombic brownmillerite (Ca₂AlFeO₅)-like structure at room temperature. From the crystallographic viewpoint, this structure is referred to as an “ordered” oxygen-deficient perovskite structure. Heat treatment of strontium ferrate (III) in a stream of nitrogen gas resulted in a violent displacement of the oxygen atoms or the oxygen vacancies throughout its own lattice. When the temperature was elevated up to 700°C, a reversible order-disorder transition of strontium ferrate (III) took place and the compound had a statistically disordered oxygen-deficient perovskite structure with cubic symmetry.