Neutron studies of an inorganic plastic glass

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 Ge3As52S45 is shown to have a structure based on As4S3 molecules, whereas that of its selenide analogue, vitreous Ge3As52Se45, involves far fewer As4Se3 molecules. Two As2X3 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 Ge3As52S45 is evident from the enhanced first diffraction peak relative to the As2S3 reference glass, and has been further investigated by comparing its interference function, Qi(Q), with that expected for an isolated As4S3 molecule. On the other hand, the second diffraction peak for vitreous Ge3As52S45 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 As4S3 crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge3As52S45, obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As4S3 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 As4S3 molecules; i.e. that, at ambient temperature, vitreous Ge3As52S45 behaves as a plastic glass. A possible structural model for vitreous Ge3As52S45 comprises a two-dimensional tangled GeS2 net with As4S3 molecules trapped in its folds whereas, for vitreous Ge3As52Se45, a clathrate model appears more appropriate.