NEUTRON-SCATTERING AND AB-INITIO MOLECULAR-DYNAMICS STUDY OF VIBRATIONS IN GLASSY GESE2

The dynamics of glassy GeSe2 have been investigated through a combined experimental and theoretical approach. Particular attention was devoted to vibrational modes with energies around 25 meV (200 cm(-1)), with regard to the interpretation of features (A(1) and A(1c)) 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 GeSe2. The motions associated with the A(1c) band are shown to be dominated by breathing motions of edge-sharing Se-4 tetrahedra around Ge atoms. There is also a substantial contribution from modes localized on network defects.