Atomic structure and chemical order in Ge-As sulfide glasses: A combined Ge and AsK-edge EXAFS study

The coordination environments of Ge and As atoms in GexAsxS100 (2x) glasses with 13.3 less than or equal to x less than or equal to 32.5 have been stu died with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold - and threefold-coordinated, respectively, in all glasses. The atomic struc tures of the stoichiometric and S-excess glasses are found to consist or Ge S4 tetrahedra and AsS3 trigonal pyramids implying the preservation of chemi cal order at least over the length scale of the first coordination shell. A s-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 clusterin g of like metal atoms and violation of chemical order in S-deficient glasse s. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physi cal properties in these glasses. (C) 2001 Elsevier Science B.V. All rights reserved.