Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra
Résumé
Relative Raman scattering intensities are obtained in three samples of vitreous silica of increasing density. The variation of the intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient C-B proportional to omega(2). A comparative intensity and frequency dependence of the Raman spectral lines in the three glasses is performed. Provided the Raman spectra are normalized by C-B, there exists a simple relation between the Si-O-Si bond angle and the frequency of all O-bending motions, including those of fourfold (n = 4) and threefold ( n = 3) rings. For 20% densification we find a reduction of similar to 5.7 degrees of the maximum of the network angle distribution, a value in very close agreement with previous NMR experiments. The threefold and fourfold rings are weakly perturbed by the densification, with a bond angle reduction of similar to 0.5 degrees for the former.