Phonon spectra in SiO2 glasses

Phonon spectra in SiO2 sol-gel made glasses annealed under different condit ions are investigated using infrared absorption and Raman scattering. These data are compared with those obtained in commercial optical-quality quartz . All the materials exhibit the same phonon bands, the exact position and t he intensity depend on the measuring technique and on the sample preparatio n method. The phonon spectra in this material are interpreted on the basis of a simple quasi-linear description of elastic waves in an O-Si-O chain. I t is shown that the main features observed in the range 400-1400 cm(-1) can be predicted using a quasi-linear chain model in which the band at 1070 cm (-1) is assigned to the longitudinal optical waves in the O-Si-O chain with the smallest possible wavelength at the Brillouin zone boundary, the band located around 450 cm(-1) is assigned to the transversal optical waves and the band at 800 cm(-1) to the longitudinal acoustical waves with the same w avelength. The degree of structural disorder can be also deduced within the framework of the proposed model.