Far-infrared studies of the amorphous SbxGe28-xSe72 glassy semiconductor

Various parameters in the structural features of the SbxGe28-xSe72 glassy system are studied as a function of average coordination number (r) in the light of recently proposed models for network glasses. The relation between the chemical ordered covalent network model and the constraint theory on the structural features is studied. The overall mean energy (E) of a covalent network for a ternary Sb-Ge-Se chacogenide system has been determined. In amorphous materials, there are non-random structural elements that have the behaviour of a simple chemical ordering. In addition, two topological effects are discussed, the floppy to rigid transition and the structural transition, which resulted in the SbxGe28-xSe72 glassy system and induced some changes in chemical ordering. The infrared transmission spectrum was measured in the wavenumber region 500-200 cm(-1) at room temperature. The results are interpreted in terms of the vibrations of the isolated molecular units in such a way as to preserve fourfold and twofold coordination for germanium and chalcogen atoms, respectively. The infrared features are assigned to Ge-Se bonds in GeSe4 tetrahedral units and Sb-Se bonds in pyramidal molecules. On the other hand, the results explain that a structural model, based on the random covalent network model assuming tetra-coordinated Ge, tri-coordinated Sb and di-coordinated Se, confirms the information obtained from infrared spectroscopy.