Symmetry role on the preedge X-ray absorption fine structure at the metal K edge

The preedge features in a system with "even" symmetry, apart from quadrupol ar transition contribution, are mainly dipolar in character, associated wit h the existence of unoccupied states made up of mixed cation-4p with higher -neighboring cation-3d orbitals, and reflect the density of states due to t he medium-range order of the system. In "odd" symmetry materials these pree dge 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 f unctions with the central cation 3d orbitals, similar to atetrahedral confi guration. These results are validated for Fe as a photoabsorber by comparin g XAS spectra of Fe2SiO4 (fayalite) to ab initio full multiple scattering c alculations at the Fe K edge, but pertain to all systems containing sixfold -coordinated cations.