DYNAMICS OF VITREOUS AG-GE-SE

The dynamic scattering function S (Q, E) for vitreous g-Ag4Ge3Se9 at T = 15 K has been measured by time-of-flight inelastic neutron scattering using an incident energy of approximately 60 meV. The neutron measurements have been complemented by room-temperature Raman-scattering measurements. The data are presented and discussed in terms of the elastic structure factor S(el)(Q), the total structure factor S(Q), the neutron-weighted generalized one-phonon vibrational density of states (GVDOS), and the Q dependence of the dynamic structure factor S(Q,E) at selected energy excitations. Comparisons are made with experimental results for pure GeSe2 glass, and with calculations based on the method of normal-mode analysis to interpret the data. The introduction of Ag into the Ge-Se glassy network has a profound effect on the vibrational spectrum and the intermediate-range order. The GeSe4/2 tetrahedron, however, generally accepted as the local structural element of pure GeSe2 glass, remains a valid structural component also in the heavily Ag-doped glass. We find that the so-called companion line in the Raman spectrum for g-GeSe2 disappears upon doping with Ag, indicating a change of the intermediate-range order. Moreover, there is a large increase in the population of vibrational modes below approximately 200 cm-1 in both the GVDOS and the Raman spectrum. These modes can be associated with vibrational modes of Ag atoms interacting with three Se in a nearly planar pyramidal arrangement. We derive 30 kdyn/cm for the Ag-Se stretching constant.