X-ray Raman scattering for structural investigation of silica/silicate minerals
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We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: α-quartz, α-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.
KeywordsX-ray Raman scattering SiO2 polymorphs Short-range structure Intermediate structure Electronic structure
Hirofumi Ishii and Ignace Jarrige are appreciated for their help on the experiments. We thank Isao Tanaka and Tomoyuki Yamamoto for their advice on calculation of spectra. Alfred Q.R. Baron and Osamu Ohtaka kindly read this manuscript. A series of experiments were performed under proposals to National Synchrotron Radiation Research Center. This work was supported by Grants-in-Aid for Young Scientists of Japan Society for the Promotion of Science and by the 21st Century COE Program of Institute of Study for the Earth’s Interior of Okayama University.
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