Mössbauer and magnetization studies of Fe₂BO₄

Polycrystalline Fe 2 BO 4 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 Fe 2+ and two Fe 3+ ) 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 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).