O K-edge x–ray absorption spectra computed on the theoretical silica glass structures are compared to XRS measurements at small momentum transfer. The structures obtained here will be compared against experimental structure factor at different pressures. This is surprising that very different interpretation of the experimental results can be derived from an apparently identical material! The objective of this contribution is to provide a coherent explanation on the observed results based on a common structure model obtained from First-Principles molecular dynamic calculations. In contrast, a study on the Si– L edge x-ray Raman scattering spectra at high momentum transfer found no significant change in the spectral features and concluded that the 4-fold coordination environment of Si remains up to 74 GPa 10. In one of the studies, the existence of an intermediate 5-fold coordinated structure was proposed 4, 9. Diffraction results obtained from two groups 7, 8, 9 agree that the change in CN should start around 20 GPa and completed at 45–50 GPa. The results further hinted that there may be a denser phase with higher coordination number (CN) above 140 GPa. The acoustic velocity data obtained from Brilluion scattering, however, suggested the onset should started at 30 GPa and the 6-fold coordination is sustained up to 140 GPa 6. From the comparison the O K-edge x-ray Raman scattering spectra (XRS) of silica glass with crystalline quartz and stishovite, it was suggested that a change in the Si environment from 4-fold to 6-fold coordination occurred between 10 to 22 GPa 5. In recent years, conflicting conclusions drawn from different experiments concerning the threshold pressure for the formation of 6-fold coordinated Si have emerged. In spite of these efforts, there is still no consensus on a number of outstanding issues such as the onset for the transformation from 4-fold coordinated quartz-like structure to 6-fold stishovite-like structure, the pressure for the completed transformation, intermediate structures, particularly the possible existence of 5-coordinated Si and mechanisms for densification and the existence of “post-stishovite” polymorphic phase 3, 4. Since then many new results reaching 100 GPa or above have become available. The status of the research on high pressure amorphous silica has been reviewed recently 2. Numerous theoretical and a variety of experimental techniques have been employed to characterize the complex and sometimes anomalous behaviour of silica glass. The study of silica glass under high pressure is particularly important and challenging as it has been used as a zeroth order model of silicate magma in the earth's interior. In the amorphous form, silica glass has become a prototype system for understanding the disordered state. It is the fundamental building block of three-dimensional framework structure found in minerals 1, 2. In the crystalline state, it exists in several polymorphic forms. Silica (SiO 2) is one of the most important and abundant materials.
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