Structural Properties of SrCo0.2Fe0.8O3–delta Oxide: Oxygen Nonstoichiometry, Vacancy Ordering and Broken Local Symmetry

Results of X-ray, Mössbauer and thermogravimetric studies per formed for SrCo0.2Fe0.8O3-delta oxide are presented. The initial “as prepared”, vacancy ordered, orthorhombic SrCo0.2Fe0.8O2.75 upon reduction at high temperatures in low hydrogen content, H2/Ar gas mixture, transforms into brownmillerite type SrCo0.2Fe0.8O2.5. While heating in air, at around 400 graduate C, SrCo0.2Fe0.8O2.5 transforms into distorted cubic structure. This trans formation is accompanied by a massive oxygen up take (similar to 0.28 mol/mol). Further heating of the sample leads to a decrease of the oxygen content. Additionally, Rietveld refinement of the XRD data indicates practically no distortion of the cubic structure at 800 graduate C. Upon cooling, the initial orthorhombic SrCo0.2Fe0.8O2.75 structure is rebuilt. Local hyperfine inter actions within the B sublattice of the perovskite structured SrCo0.2Fe0.8O2.75, which were studied by means of transmission 57Fe Mössbauer spectroscopy, revealed presence of an unexpected, high electric field gradient. This can be ascribed to a significantly lower local symmetry brought about by the presence of or ered oxygen vacancies.