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 (α-quartz, coesite, and stishovite) are compared to calculated multiple-scattering spectra using different cluster sizes and potentials (X_α 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 ∼ 50 eV above the edge by cluster sizes of at least 50 atoms and using the X_α potential. For the tetrahedral structures of α-quartz and coesite further improvement can be obtained by setting the exchange parameter α to 0.828.