Magnetic exchange interactions in perovskite solid solutions. Part 5. The unusual defect structure of SrFeO3 –y

Abstract

A major investigation of the oxygen-deficient perovskite phase SrFeO_3–y(0.15 < y < 0.25) has been carried out using ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction, and magnetic susceptibility measurements. The Mössbauer spectra have been recorded for a variety of samples at many temperatures in the range 4.2–900 K, and are extremely complex. Evidence has been found for three chemically distinct types of iron below 550 K, but above this temperature a thermally activated electron-transfer process takes place which results in a single narrow resonance. It is shown that ordering of oxygen vacancies can take place to a degree which depends upon both the thermal history and the oxygen content of a given sample. The magnetic properties are very unusual, and it is believed that aggregation of oxygen vacancies produces layers of tetrahedrally co-ordinated Fe³⁺ cations which show two-dimensional long-range order below 220 K. It is tentatively proposed that in the idealised structure of SrFeO_2.75 electron delocalisation can occur in the intervening layers which contain two distinct types of iron atom in octahedral co-ordination. The model leads to a plausible explanation for the observed disproportionation which occurs for y > 0.28.