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A neutron diffraction investigation has been performed of the structure of four chalcogenide glasses, using the D4c diffractometer on the high-flux reactor at the Institut Laue-Langevin (ILL). Vitreous Ge 3 As 52 S 45 is shown to have a structure based on As 4 S 3 molecules, whereas that of its selenide analogue, vitreous Ge 3 As 52 Se 45 , involves far fewer As 4 Se 3 molecules. Two As 2 X 3 reference glasses (X = S or Se) have purely network structures. As–X, and As–As bond lengths have been extracted, together with their associated co-ordination numbers, from peak fits to the real space correlation functions, T ( r ), and suggest that all four glasses are chemically ordered; i.e. that they contain the maximum possible number of As–X (and Ge–X) bonds. The molecular nature of vitreous Ge 3 As 52 S 45 is evident from the enhanced first diffraction peak relative to the As 2 S 3 reference glass, and has been further investigated by comparing its interference function, Qi ( Q ), with that expected for an isolated As 4 S 3 molecule. On the other hand, the second diffraction peak for vitreous Ge 3 As 52 S 45 is not reproduced by a random orientation molecular model, indicating that there is orientational correlation between adjacent molecules, and this is discussed with respect to the structure of the corresponding As 4 S 3 crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge 3 As 52 S 45 , obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As 4 S 3 molecules, as revealed by a peak at ~ 34 meV arising from the triangle of As atoms that form the base of the molecule. MARI data recorded at ambient temperature also reveal a strong quasi-elastic component in the scattered intensity, which has been further investigated using the IN5 quasi-elastic scattering spectrometer (ILL) and indicates the presence of rotational diffusion of the As 4 S 3 molecules; i.e. that, at ambient temperature, vitreous Ge 3 As 52 S 45 behaves as a plastic glass. A possible structural model for vitreous Ge 3 As 52 S 45 comprises a two-dimensional tangled GeS 2 net with As 4 S 3 molecules trapped in its folds whereas, for vitreous Ge 3 As 52 Se 45 , a clathrate model appears more appropriate. |
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