Neutron-scattering andab initiomolecular-dynamics study of vibrations in glassyGeSe2

The dynamics of glassy ${\mathrm{GeSe}}_2$ have been investigated through a combined experimental and theoretical approach. Particular attention was devoted to vibrational modes with energies around 25 meV (200 ${\mathrm{cm}}^{\mathrm{-}1}$), with regard to the interpretation of features (${\mathit{A}}_1$ and ${\mathit{A}}_{1\mathit{c}}$) observed by Raman spectroscopy. Ab initio local-density-functional molecular-dynamics calculations have been carried out for a 63-atom model with periodic boundary conditions. The results are in good agreement with time-of-flight inelastic-neutron-scattering measurements of the vibrational density of states and of the dynamic structure factor of ${\mathrm{GeSe}}_2$. The motions associated with the ${\mathit{A}}_{1\mathit{c}}$ band are shown to be dominated by breathing motions of edge-sharing ${\mathrm{Se}}_4$ tetrahedra around Ge atoms. There is also a substantial contribution from modes localized on network defects.