Oxygen K-edge XANES of germanates investigated using first-principles calculations

O K-edge x-ray absorption near-edge structure (XANES) spectra of alpha-quartz-type and rutile-type GeO2 polymorphs and of K2Ge8O17 have been analyzed using first-principles plane-wave pseudopotential calculations. XANES spectra have been calculated using supercell including core-hole effects and good agreement with experiment has been obtained. In the the case of GeO2 polymorphs, local density of empty states has been performed and peaks in the experimental spectra can be assigned to transitions involving hybridization of the O p orbitals with the Ge s, Ge p, Ge sp, and Ge d orbitals. Furthermore, peak positions in the theoretical spectra appear to be correlated with changes in the Ge-O-Ge angle as well as indirectly with the Ge coordination geometry. Analysis of O K-edge XANES spectra for individual O sites in K2Ge8O17 shows that oxygens shared between two fivefold Ge atoms or one fourfold and one fivefold Ge atom exhibit subtle shifts to lower energy of the peaks, which have been previously observed in alkali germanate glasses at and above the germanate anomaly.