Calculation of Raman spectra and vibrational properties of silicate glasses:: Comparison between Na2Si4O9 and SiO2 glasses

First calculations of the vibrational density of states (VDOS), the character of the vibrational modes, and the polarized Raman spectra of depolymerized alkali silicate glass with 20 mol% Na2O (Na2Si4O9 glass) using Kirkwood-type potential and computer generated models with periodic boundary conditions are presented and compared with amorphous SiO2 (a-SiO2). The effects of depolymerization on the localization, bridging oxygen motion, phase quotient, as well as the newly proposed stretching character and atomic participation ratios of the vibrational modes are quantitatively analyzed. The band edges are shown to correspond to strongly localized modes. All vibrational characteristics have singularities near the band edges. The calculated Raman spectra are, generally, in good agreement with experiment. The parallel (VV) polarized Raman spectra are shown to arise mainly from a bond-stretching scattering mechanism which depends on the derivative of the parallel bond polarizability. On the contrary, the depolarized VH spectra arise from mixed bond-stretching and bond-bending scattering mechanisms. It is shown that the perpendicular polarizability of the silicon-bridging oxygen bonds is an order of magnitude smaller than the derivative of the parallel bond polarizability and that their ratio of about 0.1 is not affected by the presence of short-range disorder and depolymerization. The derivative of the parallel bond polarizability for the silicon-nonbridging oxygen bonds is twice larger than that for the silicon-bridging oxygen bonds. Partial Raman spectra of short-range units with different number of nonbridging oxygens (Q species) in the Na2Si4O9 glass are calculated and compared with empirically established rules for determination of Q species from polarized Raman spectra. [S0163-1829(99)01333-8].