A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5-25 K), Cp, and low-frequency Raman scattering (<150 cm-1) has been performed in amorphous silica samples synthesized by sol-gel method (xerogels) and thermally densified in a range of densities, from ? = 1250 kg m-3 to ? = 2100 kg m-3, close to the density of the melt quenched vitreous silica (v-SiO2). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO2, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870 °C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm-1 and of a peak in Cp(T)/T3, very close to those observed in v-SiO2; (ii) in silica xerogels treated at temperatures less than about 800 °C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.