Symmetry dependence of x-ray absorption near-edge structure at the metal Kedge of 3d transition metal compounds

The pre-edge features in a system with "even" symmetry, apart from quadrupo lar transition contribution, are mainly dipolar in character, associated wi th the existence of unoccupied states made up of mixed cation np with highe r-neighboring cation- (n-1) d orbitals, and reflect the density of states d ue to the medium-range order of the system, while in "odd" symmetry materia ls these pre-edge features are the result of a transition from the Is to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation d orbitals. In the latter ca se, they contain not only the p but also the d base of orbitals, similar to a tetrahedral configuration. These results are validated for Fe as a photo absorber by comparing x-ray absorption near-edge spectra of Fe2SiO4 (fayali te) and Fe2O3 (hematite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinate d cations. (C) 2001 American Institute of Physics.