Calculation of optical and K pre-edge absorption spectra for ferrous iron of distorted sites in oxide crystals
Résumé
Advanced semi-empirical calculations have been performed to compute simultaneously optical absorption and K pre-edge X-ray Absorption spectra of Fe 2+ in four distinct site symmetries found in minerals. The four symmetries, i.e. a distorted octahedron, a distorted tetrahedron, a square planar site, and a trigonal bipyramidal site, are representative of the Fe 2+ sites found in crystals and glasses. A particular attention has been paid to the definition of the p-d hybridization Hamiltonian which occurs for non-centrosymmetric symmetries in order to account for electric dipole transitions. For the different sites under study, an excellent agreement between calculations and experiments was found for both optical and X-ray absorption spectra, in particular in terms of relative intensities and energy positions of electronic transitions. To our knowledge, these are the first calculations of optical absorption spectra on Fe 2+ placed in such diverse site symmetries, including centrosymmetric sites. The proposed theoretical model should help to interpret the features of both the optical absorption and the K pre-edge absorption spectra of 3d transition metal ions and to go beyond the usual fingerprint interpretation.
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