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

The pre-edge features in a system with “even” symmetry, apart from quadrupolar transition contribution, are mainly dipolar in character, associated with the existence of unoccupied states made up of mixed cation np with higher-neighboring cation-(n-1)d orbitals, and reflect the density of states due to the medium-range order of the system, while in “odd” symmetry materials these pre-edge features are the result of a transition from the 1s 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 case, 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 photoabsorber by comparing x-ray absorption near-edge spectra of Fe2SiO4 (fayalite) and Fe2O3 (hematite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinated cations.