Mossbauer and magnetization studies of Fe2BO4

Polycrystalline Fe2BO4 prepared by solid-state reaction was investigated over a wide temperature range using Mossbauer spectroscopy and magnetization and resistivity measurements. The Mossbauer 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 Mossbauer data indicate that this oxoborate mixed-valence system is an L-type ferrimagnet below T-c = 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 (T-co = 316 K).