Raman and XANES spectroscopy of permanently densified vitreous silica

Vitreous SiO2 was permanently densified at pressures ranging up to 8 GPa at 700 °C, where its density reached a value of 2.58 g/cm3. The series of glasses formed were analyzed by Raman spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy at both the oxygen and silicon K-edges. Changes in the Raman spectra are most evident in the Si–O–Si bending and rocking region near 500 cm-1, as the broad band becomes narrower and shifts to higher frequency, indicating a narrower distribution of Si–O–Si bond angles in the compacted glasses. A pronounced shift to lower frequency of the weak high-frequency bands associated with Si–O stretching motions is also observed, indicating a lengthening of Si–O bonds with increasing densification. More subtle changes are observed in both the Si and O K-edge spectra, with the O K-edge appearing more sensitive to short-range structural modifications such as narrowing of the intertetrahedral angle. In contrast to the structure of permanently densified v-SiO2 formed at room temperature, in which medium-range order is mostly affected, densification at higher temperatures is accomplished at shorter range, although at the pressures of this study, the glass remains principally a fully polymerized tetrahedral framework glass.