New assignments of Raman spectra of tetrahedra and the effects of electronegativity. II. Silicate glasses

Our density functional theory calculations (by GAUSSIAN) on the Si4O104- moiety and Na4Si4O10 molecule reproduce the experimental Raman frequencies and relative intensities for the three A(1) modes in the experimental Raman spectra of Na2Si2O5 and BaSi2O5 crystals and glasses. Taking the center of mass into consideration, accompanied by visualization using GaussView, these calculations provide a firm assignment of these three peaks: the 1060 cm(-1) peak to the Si-NBO stretch (NBO = Si-O or Si-O-Na), the similar to 600 cm(-1) peak to the Si-BO stretch (BO = Si-O-Si), and the similar to 500 cm(-1) peak to the Si-BO-Si bend. Our GAUSSIAN calculations on modified "non-equilibrium" SiO44- tetrahedra, constrained to have the same Si-O bond lengths and O-Si-O bond angles as in the Si4O104- moiety, confirm the above-mentioned assignment and show reasonable agreement with the experimental Raman frequencies and relative intensities. We then examine the regular trends in the Si-NBO and Si-BO stretch frequencies for all Q(n) (n = 0-4) species in silicate glasses and crystals. The large systematic change in the differences between these frequencies (Delta(1)) is explained by electronic changes within the SiO4 tetrahedron. For example, the larger Delta(1) for the Q(3) tetrahedra than the Q(2) tetrahedra is due to the changes in Si, BO, and NBO electron densities on the tetrahedral Si and O atoms, as measured by Si 2p and O 1s x-ray photoelectron spectra. This trend strongly suggests that the Si-BO stretch frequency for the Q(4) species in v-SiO2 should be similar to 500 cm(-1), close to the A(1) breathing mode frequency of 495 cm(-1) for four membered rings previously assigned.