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 A1 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 ∼600 cm−1 peak to the Si-BO stretch (BO = Si–O–Si), and the ∼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 Qn (n = 0–4) species in silicate glasses and crystals. The large systematic change in the differences between these frequencies (Δ1) is explained by electronic changes within the SiO4 tetrahedron. For example, the larger Δ1 for the Q3 tetrahedra than the Q2 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 Q4 species in v-SiO2 should be ∼500 cm−1, close to the A1 breathing mode frequency of 495 cm−1 for four membered rings previously assigned.

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