Neutron diffraction studies of the structure of Ge-based multicomponent sulphide glasses

We have used high real-space resolution pulsed neutron diffraction to inves tigate the structure of fibre-forming Ge-based multicomponent sulphide glas ses. Diffraction patterns were measured up to a maximum momentum transfer o f 40 Angstrom(-1) for a series of seven samples from the stoichiometric sys tem Ge-25(As,Ga)(10)S-65, with Ga contents from 0 to 10 at.%. The correlati on functions all have three principal peaks at 2.23, 2.97 and 3.49 Angstrom . The first principal peak in the correlation functions is predominantly du e to Ge-S bonds, together with smaller contributions from As-S and Ga-S bon ds. The second principal peak is due to cation-cation distances between edg e-sharing structural units. The third principal peak is due mostly to S-S d istances within structural units. There is also fine structure around the f irst peak of the correlation functions, consisting of a small 'pre-peak' at similar to 2.0 Angstrom and a larger and broader shoulder at similar to 2. 5 Angstrom. There are no detectable changes with composition in the pre-pea k, whilst the shoulder grows as gallium is added to the glass. The position of the pre-peak, 2.0 Angstrom, is consistent with either S-2(2-) disulphid e groups within a structural unit, or with S-S bonds in the network, due to chemical disorder. According to the chemical disorder model, the peak at 2 .5 Angstrom is due to Ge-Ge bonds in the network. If gallium is tetrahedral ly coordinated then the number of these bonds must increase as gallium repl aces arsenic. Such an increase is observed in the experimental data and hen ce the chemical disorder model is preferred over the disulphide model. (C) 1999 Elsevier Science B.V. All rights reserved.