X-ray absorption spectroscopy of silicon dioxide (SiO₂) polymorphs: The structural characterization of opal

SiK- and SiL-edge X-ray absorption spectra obtained using synchrotron radiation are reported for 6:3-coordinated stishovite and 4:2-coordinated α quartz, α cristobalite, coesite, amorphous silica (a-SiO₂), and opal. The SiK and SiL near-edge features are interpreted on the basis of a qualitative MO model of $SiO44−$ and SCF-Xα calculation of model molecules. Some edge features are attributed to the multiple scattering effect of the more distant shell atoms in the crystal structure. The K- and L-edge features reflect the maximum densities of unoccupied Si 3s, 3p, and 3d states in the conduction band and are qualitatively in agreement with calculated densities of states. Comparison of SiK- and SiL- edge XANES demonstrates the bond mixing of Si 3p and 3s orbitals and of Si 3p and 3d orbitals. Also, for 4:2-coordinated silica, the transition of Si 2p electrons to the t₂ state of high Si 3p character becomes dipole allowed. For stishovite and coesite, states dominated by Si 3s apparently have a large amount of Si 3p orbital character, probably because of pressure-induced mixing of Si 3s and 3p orbitals. The SiK- and SiL-edge shifts are systematically related to the coordination number of Si atoms, Si-O bond length, Si-Si distance, Si-O-Si angle, Si-O bond valence, and Si NMR chemical shift of SiO₂ polymorphs. The SiK- and SiL-edge XANES indicate that the local structure of two opals investigated is a mixture of a-SiO₂ and α cristobalite structural units, and the relative proportions of the two structural components are semiquantitatively determined. EXAFS structure parameters (bond distances, coordination number, and Debye-Waller factor) of quartz and stishovite are obtained and shown to be in good agreement with the X-ray structures. Si in sixfold and fourfold coordination can be distinguished unambiguously from SiK- and SiL- edge XANES features and SiK-edge EXAFS analysis. These results are very useful for characterizing the structure and bonding of the mantle silicates and silicate glasses.