Oxygen Vacancies in the Perovskite-type Ferrites. II. Mössbauer Effect in the SrFeO2.5 -LaFeO3 Solid-solution System

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Abstract
The Mössbauer effect was studied for the solid-solution Sr1−xLaFeO2.5+x⁄2 system, prepared by firing in a vacuum. The intensity of the characteristic six absorption lines of the tetrahedral-iron site in the brownmilleritetype phase (0.0≤x≤0.02) was weakened with an increase in the La content because of an increasing incorporation of oxygen, while the Mössbauer parameters remained constant. SrFeO2.5 with a brownmillerite-type structure, completely changed into a cubic perovskite-type structure at the composition with x=0.30; however, the original absorption peaks for the tetrahedral-iron site still exist even if the intensity is considerably decreased. Moreover, new peaks came out, with Mössbauer parameters different from those for the octahedral- or the tetrahedraliron sites. On the basis of the values of their Mössbauer parameters, the origin of the new peak can be assigned to the presence of a five-coordinated iron site surrounded by a trigonal bipyramidal polyhedron of oxygen rather than a square pyramidal one. The three kinds of iron site lead to a single octahedral one when most of Sr is replaced with La (x≥0.50). The oxygen-deficient, perovskite-type phase with a cubic symmetry may be stabilized by the presence of the mixed-coordination scheme of Fe3+, i.e., the octahedral, trigonal bipyramidal, and tetrahedral sites.