Abstract
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.
Highlights
The mixed valence oxide LuFe2O4 is among the most studied multiferroic candidates since a new type of ferroelectricity based on charge ordering (CO) was postulated in this compound below TCO ≈ 320 K [1]
If there is coexistence of Fe+2 and Fe+3 valence states as in heterogeneous mixed valence compounds, the x-ray absorption near edge structure (XANES) of LuFe2O4 should agree with the 1:1 addition of the individual XANES spectra of appropriate reference compounds with the respective formal valences
At the Fe K-edge we have taken as reference XANES of Fe2+ the spectrum of LuFeCoO4 shifted -4 eV according to the chemical shift (∆E) of 4 eV found empirically between Fe2+ and Fe3+ [7]
Summary
The mixed valence oxide LuFe2O4 is among the most studied multiferroic candidates since a new type of ferroelectricity based on charge ordering (CO) was postulated in this compound below TCO ≈ 320 K [1]. At the Fe L2,3-edges (L2 ≈ 720 eV and L3 ≈ 707 eV) and O K-edge (543 eV) non-polarized and polarized XANES spectra were measured in total electron yield on sintered pieces and oriented single crystals, respectively.
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