Oxygen vacancies and valence states of iron in SrFeO$_{3-\delta}$ compounds

The brownmillerite phase has been studied by X-ray diffraction and Mössbauer methods. The available data on single-phase compounds with the perovskite structure in strontium ferrite SrFeO$_{3-\delta}$ are analyzed. All the valence states of iron for any phase state of strontium ferrite are found to be determined by its local oxygen environment. This fact enables us to understand the regularity of transitions of iron from one valence state to another state when adding oxygen vacancies and to explain the structural states of iron in strontium ferrite, such as the single-phase and two-phase compositions. This approach is a more general case for description of all known compounds and phase combinations synthesized in SrFeO$_{3-\delta}$ than the formula for single-phase structures that is considered in the literature and fit well into the scheme proposed in this work.