ORIGIN OF THE RAMAN BANDS, D1 AND D2, IN HIGH SURFACE-AREA AND VITREOUS SILICAS

The Raman bands of silicas at 491 and 607 cm-1, observed for high surface area and vitreous samples and usually ascribed, respectively, to defects D1 and D2, are discussed by comparison with infrared spectra. On the basis of rehydration experiments of silica powders, the D1 band is assigned to a vibrational mode of [SiO4] tetrahedra with a non-bridging oxygen atom. The structure giving band D2 is related to the surface condensation, between 200 and 550-degrees-C, of two weakly interacting silanols which absorb in the 3600-3680 cm-1 range. The condensation, described in a model based on the true beta-cristobalite structure, generates a (SiO)3 ring D2 and a free silanol which appears in the infrared spectra at about 3740 cm-1. Mechanisms of formation of D1 and D2 in vitreous silica are suggested in agreement with previous experiments.