Raman and infrared spectra on silica gel evolving toward glass

Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40-800°C temperature range, are reported and discussed with reference to the spectra of fused quartz. The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C. Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes. In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm-1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature. © 1982.