A combined neutron and x-ray diffraction study of short- and intermediate-range structural characteristics of Ge–As sulfide glasses

A combination of neutron and x-ray diffraction has been employed to study the compositional dependence of the atomic structures of GexAsxS100-2x glasses with S concentration varying between 33.3 and 70.0 at.%. The nearest-neighbor coordination numbers of Ge and As atoms are always found to be 4 and 3, respectively, irrespective of the glass composition. Ge and As atoms have primarily heteropolar bonding to S atoms in stoichiometric and S-excess glasses with x < =18.2. Low and intermediate levels of deficiency of S (20 < =x < =25) are accommodated via the formation of homopolar As-As bonds while Ge atoms remain primarily bonded to four S atoms, resulting in As-rich regions in the glass structure. Ge starts to participate in metal-metal bonding only in the highly S-deficient glasses with 27.5 < =x < =33.3. The intermediate-range order and its topological influence on atomic packing in these three compositional regions, in the order of increasing deficiency in S, are controlled by (a) a mixed GeS2 and As2S3 network, (b) the coexistence of a GeS2 network and As clusters, and (c) large Ge-As metal-rich regions. This evolution of the intermediate-range structure with composition is consistent with the corresponding variation of the position, intensity and width of the first sharp diffraction peak in the structure factor.