Effects of higher-coordination shells in garnets detected by x-ray-absorption spectroscopy at the Al K edge

The aluminium 1s x-ray-absorption spectra of a series of garnets, pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), spessartine (Mn3Al2Si3O12), and grossular (Ca3Al2Si3O12), are compared to full multiple scattering calculations using cluster models. An overall good agreement between experiment and calculation, extended also to the edge region, is obtained in the energy range up to 60 eV above the threshold, provided clusters containing at least 40 atoms are used. The analysis of these garnet XAS spectra provides clear evidence on the effect of atoms located far away from the first-coordination shell around the photoabsorber and at the same time, the variation in intensity of the edge is related to the local character of the partial empty density of slates induced by the atoms located in the X sites. As a local probe, XANES spectroscopy at the edge of low Z elements appears to be a perfect tool to investigate the role played by atoms located in higher-coordination shells.