Multiple-scattering calculations for SiO2 polymorphs: A comparison to ELNES and XANES spectra

Experimental XANES and ELNES spectra at the silicon K and L(2,3) edges and oxygen K edges in three polymorphs of SiO2 (alpha-quartz, coesite, and stishovite) are compared to calculated multiple-scattering spectra using different cluster sizes and potentials (X(alpha) and complex Hedin-Lundqvist). The comparison demonstrates that relative energies and intensities of the experimental spectra are well reproduced in the energy range up to similar to 50 eV above the edge by cluster sizes of at least 50 atoms and using the X(alpha) potential. For the tetrahedral structures of alpha-quartz and coesite further improvement can be obtained by setting the exchange parameter alpha to 0.828.