Mössbauer and magnetization studies of Fe2BO4

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Abstract
Polycrystalline Fe2 BO4 prepared by solid-state reaction was investigated over a wide temperature range using Mössbauer spectroscopy and magnetization and resistivity measurements. The Mössbauer data below 270 K reveal four iron sites (two Fe2+ and two Fe3+ ) in a 1:1:1:1 ratio. Above this temperature, electron delocalization sets in between the divalent-trivalent iron ions and up to 400 K both delocalized and localized iron states are observed. The magnetization and Mössbauer data indicate that this oxoborate mixed-valence system is an L-type ferrimagnet below Tc = 155 K, with the iron atoms in each sublattice having different point symmetries and different magnetic interactions. The resistivity data show the first-order nature of the charge-ordering transition (Tco = 316 K).