A Raman spectroscopic study of shock-wave densification of vitreous silica

The densification processes in SiO2 glass induced by shock-wave compression up to 33.4 GPa are investigated by Raman spectroscopy. At first, densifica tion increases with increasing shock pressure. A maximum densification of 1 1% is obtained for a shock pressure of 26.3 GPa. This densification is attr ibuted to the reduction of the average Si-O-Si angle, which occurs first by the collapse of the largest ring cavities, then by further reduction of th e average rin size. For higher shock pressures, a different structural modi fication is observed, resulting in decreasing densification with increasing shock pressure. Indeed, the recovered densification becomes very small, wi th values of 1.8 and 0.5% at 32 and 43.4 GPa, respectively. This is attribu ted to partial annealing of the samples due to high after shock residual te mperatures. The study of the annealing process of the most densified glass by in situ high temperature Raman spectroscopy confirms that relaxation of the Si-O-Si angle starts at a lower temperature (about 800 K) than that of the siloxane rings (about 1000 K), thus explaining the high intensity of th e siloxane defect bands in the samples schocked at compressions of 32 and 4 3.4 GPa. The large intensity of the siloxane bands in the nearly undensifie d samples shocked by compressions above 30 GPa may be explained by the rela xation during decompression of five- and six-fold coordinated silicon speci es formed at high pressure and high temperature during the shock event.