Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses

The Raman scattering intensity of the 1100 cm−1 polarized band, which appears on the addition of Na2O to SiO2 glass, reaches a maximum at the disilicate composition. The intensity of the polarized band at 950 cm−1 increases sharply as the Na2O concentration increases above 30 mole %. These data were interpreted by normal mode calculations and by IR and Raman intensity calculations for the silicate anion structural units: SiO4 isolated tetrahedra, Si2O7 dimers, Si2O6 chain links, Si2O5 sheet units, and Si2O4 framework units. According to these simplified models, the polarized high frequency band is due to symmetric stretching of Si–O− nonbridging bonds and the frequency increases with degree of polymerization of the tetrahedra. The previous assignments of the 1100 cm−1 band to the symmetric stretch of tetrahedra containing one nonbridging oxygen and of the 950 cm−1 band to the symmetric stretch of tetrahedra containing two nonbridging oxygens were confirmed. The other main feature of the alkali silicate glasses, an intense polarized band in the range of 400–600 cm−1, was shown to be a mixed stretching bending mode of the Si–O–Si bridging bond. The model also accounts for the loss of intensity of the high frequency band with increasing degree of silica polymerization.