Structural studies of inorganic glasses
The present work has been concerned with a study of the structure of germanium and silicon dichalcogenide glasses. Initially, the aim of this research was to obtain direct structural evidence of reversible photostructural changes in bulk germanium chalcogenide glasses. However, having confirmed the existence of such changes using EXAFS and neutron scattering techniques, it became obvious that a more comprehensive understanding of the as-quenched, annealed glass structure is required before a valid interpretation of the data can be made; in particular, the extent of medium-range order in these systems is of interest. Although limited to few examples at the moment, the potenital of both structural modelling and NMR studies for solving such problems has been investigated in a variety of inorganic glass systems, and these approaches can now be extended to those glasses exhibiting photostructural changes. Modelling studies of the structure of a-SiSe2 have shown that the total correlation function T(r) is sensitive to different structural features in the generated glass structure. The present studies heavily favour a structure based on chains of edge-sharing (SiSe4) tetrahedra. A degree of spatial correlation is required between the chains, such as can only be obtained by short lengths of parallel chains 'pinned' by corner-sharing tetrahedra. 29Si NMR studies of vitreous silica have shown that water, incorporated into the SiO2 network as OH, acts as a major source of spin-lattice relaxation in this system. Numerical methods of self-calibration ('phasing') and removal of data truncation and lineshape apodization effects have been investigated, thereby enabling quantitative information concerning the Si-O-Si bond-angle distribution in silica to be obtained. Although these algorithms are presented with reference to NMR spectroscopy, their extension to other branches of spectroscopy is obvious. NMR has also been used to probe defect states in a-Se. 77Se spin-lattice relaxation time data provide evidence of a defect state (perhaps a VAP) in equilibrium with C10 centres in the glass.