Infrared spectroscopy has been used to analyze the structural changes in samples prepared by the sol-gel method. Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. The IR spectra of these gels, in the Si–O bond stretching region, shows that their structure strongly depends on the H2O/TEOS ratio. The relative change in intensity of the Si–O stretching modes, in samples prepared using different H2O/TEOS ratios, are interpreted in terms of different degrees of structural disorder. According to our infrared absorption data, a decrease in the H2O/TEOS ratio from about 7, the SiO2 structure evolves from a three-dimensional network toward a chainlike structure.

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The crystallization of bulk amorphous SiO 2 samples, prepared by the sol-gel method, was obtained by heat treatments in air at temperatures as low as 500°C. This occurs when silver is added to the precursor solutions in an amount such that it forms aggregates embedded in the glass. Another requirement to observe the low-temperature glass crystallization is that the bulk samples must be prepared from precursor solutions with specific compositions. These compositions, have a high H 2 O/TEOS ratio, which produces an amorphous SiO 2 structure with some structural similarities to cristobalite, the phase in which the SiO 2 glass crystallizes.