We have examined chemical ordering in the glassy ${\mathrm{As}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{-}\mathrm{x}}$ system by determining the effect of composition on the local structure of these chalcogenide glasses using x-ray-absorption spectroscopy. Structural changes associated with composition indicate that with increasing S content, the S-rich glasses on the As site have a similar local structure to crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$ (orpiment), but the As-S-As linkages are replaced by As-S-S linkages at higher S concentration. In As-rich glasses a breakdown of the local ${\mathrm{AsS}}_3$ configuration is evident and the formation of As-As bonds is observed. Further comparison between As-rich alloys and crystalline ${\mathrm{As}}_4$${\mathrm{S}}_4$ (realgar) suggests that a significant fraction of disordered ${\mathrm{As}}_4$${\mathrm{S}}_4$ molecular fragments is contained in the As-rich region.

• Received 21 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.10342