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-coordina ted alpha quartz, alpha cristobalite, coesite, amorphous silica (a-SiO 2), and opal. The SiK and SiL near-edge features are interpreted on th e basis of a qualitative MO model of SiO44- and SCF-Xalpha calculation of model molecules. Some edge features are attributed to the multiple scattering effect of the more distant shell atoms in the crystal stru cture. The K- and L-edge features reflect the maximum densities of uno ccupied Si 3s, 3p, and 3d states in the conduction band and are qualit atively in agreement with calculated densities of states. Comparison o f 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 sili ca, the transition of Si 2p electrons to the t2 State Of high Si 3p ch aracter becomes dipole allowed. For stishovite and coesite, states dom inated by Si 3s apparently have a large amount of Si 3p orbital charac ter, probably because of pressure-induced mixing of Si 3s and 3p orbit als. The SiK- and SiL-edge shifts are systematically related to the co ordination 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 polymor phs. The SiK- and SiL-edge XANES indicate that the local structure of two opals investigated is a mixture of a-SiO2 and alpha cristobalite s tructural units, and the relative proportions of the two structural co mponents are semiquantitatively determined. EXAFS structure parameters (bond distances, coordination number, and Debye-Waller factor) of qua rtz 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 characteriz ing the structure and bonding of the mantle silicates and silicate gla sses.