Synthesis and magnetic characterisation of Fe1−xMgxSb2O4 (x = 0.25, 0.50, 0.75) and their oxygen-excess derivatives, Fe1−xMgxSb2O4+y

Abstract

Three new materials of composition Fe_1−xMg_xSb₂O₄ (x = 0.25, 0.50, 0.75) with the tetragonal schafarzikite structure have been synthesised. Magnetic susceptibility measurements suggest that Fe_1−xMg_xSb₂O₄ (x = 0.25, 0.50) are canted antiferromagnets whilst Fe_0.25Mg_0.75Sb₂O₄ is paramagnetic. The magnetic ordering temperatures decrease as the Mg²⁺ concentration increases. The materials form oxygen-excess phases when heated in oxygen-rich atmospheres at temperatures of ∼350 °C. ⁵⁷Fe Mössbauer spectroscopy shows that the oxidation process involves the oxidation of Fe²⁺ to Fe³⁺. Powder neutron diffraction confirms the location of the excess oxygen within the structural channels and reveals a change in magnetic order at low temperatures from A-type (magnetic moments along 〈100〉) for Fe_1−xMg_xSb₂O₄ to C-type (magnetic moments along [001]) for the oxidised materials. The change is attributed to a weakening of the antiferromagnetic exchange interactions between edge-linked FeO₆ octahedra for the Fe³⁺-containing materials.