Reinterpretation of the magnetic structures of the perovskites SrFeO2.710 and Sr2LaFe3O8.417

Abstract

The mixed-valence oxygen-deficient perovskite SrFeO_2.710 has been prepared essentially free of other phases by lengthy annealing of rapidly quenched material. The Mössbauer spectra confirm the essential features of two distinct types of iron which can be assigned as Fe³⁺ and Fe⁴⁺ high-spin cations. The unusual characteristics of the spectra from the antiferromagnetic phase, and their similarity to those from Sr₂LaFe₃O_8.417, have been interpreted in terms of a structural model with layers of Fe³⁺ in octahedral coordination separated by layers of Fe⁴⁺ in square-pyramidal coordination. Ferromagnetic and antiferromagnetic superexchange interactions within the Fe⁴⁺ layers can lead to significant frustration and can explain the absence of magnetic hyperfine splitting from these cations below the ordering temperature where the Fe³⁺ cations show long-range coupling. The magnetism is therefore approximately two-dimensional in character.