Hard and soft x-rays XAS characterization of charge ordered LuFe2O4

This work presents a thorough characterization of the mixed valence compound LuFe2O4 by means of x-ray absorption spectroscopy (XAS). Polarized XAS measurements at the Fe K-edge, Fe L2,3-edges and O K-edge have been carried out to validate the bimodal Fe+2/Fe+3 charge ordering (CO) proposed to give rise to ferroelectricity at TCO ≈ 320 K and also to study the anisotropy and temperature dependence of the Fe local structure. Our results discard a bimodal CO below TCO and agree with the presence of a Fe+2.5±δ distribution with δ ≤ 0.25. Unexpectedly, the strong anisotropy of the hexagonal crystallographic structure is not reflected in either the Fe (4p, 3d) or the O (2p) density of unoccupied states. Finally, the so- called CO transition is originated by the ordering of local distortions as it is revealed by the temperature evolution of the XAS spectra.