A NEUTRON INELASTIC-SCATTERING STUDY OF INTERMEDIATE RANGE ORDER AND LOW-ENERGY EXCITATIONS INVITREOUS GEXSE1-X ALLOYS

Medium range order in three GexSe1-x glasses has been studied by the inelastic scattering of low energy neutrons. The main Ge-Se-Ge bond angle stretching mode of GeSe2 is observed as a major feature in the generalized density of states but the Raman 'companion line', which has been associated with either edge-sharing tetrahedra or a vibrational mode of the Se-Se bonds on the edges of outriggers rafts, is not received as a separate peak. Limits are set for the relative strength of this vibration. Low frequency modes are observed for all three alloys as a shoulder in the density of states which increases in magnitude and energy as x is decreased. The Q dependence of these modes in GeSe2 indicates that they arise from the rotational motion of coupled tetrahedra.