The structure of high-silica alkali-silicate glasses. A Raman spectroscopic investigation

Normal Raman spectra of xM2O · (100 − x)SiO2 glasses (M = Li, Na, K, Rb, Cs and x = 0, 5, 10, 15, 20, 25, 30) and differential Raman spectra of the 5, 10, and 15% M2O glasses are presented. The Raman spectra reflect distinct structural differences in the silicate networks of these glasses caused by the presence of different alkali cation types. The structural origin and localization of vibrational modes producing characteristic spectral bands of these glasses are discussed. Bands in the 900–1200 cm−1 region of the spectra of the alkali-silicated glasses result from highly localized Si-nonbridging oxygen stretching modes and relative intensities of these bands may be used to determine alkali distributions around SiO4 tetrahedra. Comparison of the Raman Spectra of the alkali-silicate glasses has shown that smaller alkali cations exhibit a greater tendency than their larger counterparts to cluster in pairs around SiO4 tetrahedra in these glasses, even at low alkali contents. The high frequency spectral features at 1100 and 1150 cm−1 also indicate the presence of two distinct structural environments in which an SiO4 tetrahedron contains one nonbridging oxygen in all alkali-silicate glasses containing ⩽ 25% M2M except those containing lithium. Intensity of the low frequency spectral band at 440 cm−1 indicates that regional alkali clustering such as that required for phase separation is also more prevalent in glasses containing smaller alkali cations. It is proposed that modeling of the silicate structure in alkali-silicate glasses must account for both long-range and localized alkali cation distribution.