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

Abstract

A combination of neutron and x-ray diffraction has been employedto study the compositional dependence of the atomic structures ofGexAsxS100−2x glasses with S concentration varying between 33.3 and 70.0 at.%. The nearest-neighborcoordination 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 primarilyheteropolar bonding to S atoms in stoichiometric and S-excess glasses withx≤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 remainprimarily bonded to four S atoms, resulting in As-rich regions in the glass structure. Gestarts to participate in metal–metal bonding only in the highly S-deficient glasses with27.5≤x≤33.3. The intermediate-range order and its topological influence on atomic packing in these threecompositional regions, in the order of increasing deficiency in S, are controlled by (a) a mixedGeS2 andAs2S3 network, (b) thecoexistence of a GeS2 network and As clusters, and (c) large Ge–As metal-rich regions. This evolution of theintermediate-range structure with composition is consistent with the correspondingvariation of the position, intensity and width of the first sharp diffraction peak in thestructure factor.