Infrared-reflectance spectra of heat-treated sol-gel-derived silica

The infrared-reflectance spectra of silica glasses were measured and analyzed with a Kramers-Kronig transformation for a better understanding of the response of silica, especially in the high-frequency region (1000--1300 cm^-1). The silica samples were prepared by the sol-gel technique and subsequently subjected to heat treatments at selected temperatures to induce the formation of structures with a variable connectivity of the silicate network. The infrared reflectivity was found to increase with heat-treatment temperature. This was attributed to glass densification, resulting from condensation of Si-OH groups into Si-O-Si network bridging units. This densification mechanism was also confirmed by Raman measurements. The profiles of the high-frequency parts of the transverse-optic (TO) and longitudinal-optic (LO) spectra were found consistent with the presence of two coupled modes (AS₁ and AS₂) for the asymmetric stretching of Si-O-Si bridges, as proposed in previous works. Along these lines, the shoulder at {~}1200 cm^-1 in the infrared-absorption spectrum of silica, which is particularly enhanced in the spectra of silica gels, can be understood as arising from the TO response of the AS₂ mode. The LO-TO splitting of the various infrared-active modes was found to depend on heat-treatment temperature. This effect was attributed to variations of silicate network connectivity with temperature, which is presumably affecting the range of the Coulombic forces inducing the LO-TO splitting.