Synthesis and magnetic characterisation of Fe1xMgxSb2O4 (x = 0.25, 0.50, 0.75) and their oxygen-excess derivatives, Fe1xMgxSb2O4+y

Three new materials of composition Fe1 xMgxSb2O4 (x = 0.25, 0.50, 0.75) with the tetragonal schafarzikite structure have been synthesised. Magnetic susceptibility measurements suggest that Fe1 xMgxSb2O4 (x = 0.25, 0.50) are canted antiferromagnets whilst Fe0.25Mg0.75Sb2O4 is paramagnetic. The magnetic ordering temperatures decrease as the Mg concentration increases. The materials form oxygen-excess phases when heated in oxygen-rich atmospheres at temperatures of B350 1C. Fe Mössbauer spectroscopy shows that the oxidation process involves the oxidation of Fe to Fe. 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 h100i) for Fe1 xMgxSb2O4 to C-type (magnetic moments along [001]) for the oxidised materials. The change is attributed to a weakening of the antiferromagnetic exchange interactions between edgelinked FeO6 octahedra for the Fe -containing materials.