Atomic structure and chemical order in Ge–As sulfide glasses: a combined Ge and As K-edge EXAFS study

Abstract

The coordination environments of Ge and As atoms in Ge x As x S 100−2 x glasses with 13.3⩽ x⩽32.5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold- and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS 4 tetrahedra and AsS 3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the first coordination shell. As–As homopolar bonds are found to appear at low and intermediate levels of S-deficiency. Ge–Ge bonds are formed in extremely S-deficient glasses only after all As atoms participate in homopolar As–As bonds, implying clustering of like metal atoms and violation of chemical order in S-deficient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses.