Structural Properties of SrCo_(0.2)Fe_(0.8)O_(3-δ) Oxide:Oxygen Nonstoichiometry,Vacancy Ordering and Broken Local Symmetry

摘要

Results of X-ray,Mossbauer and thermogravimetric studies performed for SrCo_(0.2)Fe_(0.8)O_(3-δ) oxide are presented. The initial "as prepared",vacancy ordered,orthorhombic SrCo_(0.2)Fe_(0.8)O_(2.75) upon reduction at high temperatures in low hydrogen content,H2/Ar gas mixture,transforms into brownmillerite type SrCo_(0.2)Fe_(0.8)O_(2.5). While heating in air,at around 400°C,SrCo_(0.2)Fe_(0.8)O_(2.5) transforms into distorted cubic structure. This transformation is accompanied by a massive oxygen uptake (-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°C. Upon cooling,the initial orthorhombic SrCo_(0.2)Fe_(0.8)O_(2.75) structure is rebuilt. Local hyperfine interactions within the B sublattice of the perovskite structured SrCo_(0.2)Fe_(0.8)O_(2.75),which were studied by means of transmission Fe Mossbauer 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 ordered oxygen vacancies.