A molecular dynamics study of the vibrational spectra of silica polyamorphs

The vibrational spectra (inelastic neutron scattering, infrared absorption and Raman scattering) of fused silica in the amorphous and in the liquid state are investigated by molecular dynamics simulation in using the TTAM interionic potential (Tsuneyuki et al., Phys. Rev. Lett., 61, 869 (1988)). The simulation predicts in the low frequency region a boson peak, quite similar to the one observed experimentally in different spectroscopies. This mode is attributed to concerted motions between SiO4 tetrahedra and involves the medium-range order in the glass. Upon densification the boson peak shifts to higher frequencies and is less pronounced. Correlatively, the evolution of the density of states with the temperature is found to be strongly dependent on the glass coordination, a finding which suggests an increasing fragility of silica glass with the pressure as recently proposed in the literature. Finally the inability of the TTAM potential to reproduce in details the density of states of silica glass is pointed out.