Atomic structure and chemical order in Ge-As selenide and sulfoselenide glasses: An x-ray absorption fine structure spectroscopic study

The nearest-neighbor coordination environments of Ge and As atoms in GexAsySe1-x-y glasses with x:y=1:2 and 1:1 and in Ge0.154As0.308 (S,Se)0.538 sulfoselenide glasses with wide-ranging Se contents have been studied with Ge and As K-edge extended x-ray absorption time spectra spectroscopy. The average coordination numbers of Ge and As atoms are found to be 4 and 3, respectively, in all glasses. The compositional makeup of the first coordination shells of Ge and As atoms indicate that chemical order is largely preserved in stoichiometric and Se-excess GexAsySe1-x-y glasses. On the other hand, chemical order is found to be strongly violated in the case of Se-deficient GexAsySe1-x-y glasses where the chalcogen deficiency is entirely taken up by the formation of As-As homopolar bonds at low and intermediate levels of Se deficiency. The Ge atoms take part in homopolar bonding only in strongly Se-deficient glasses indicating clustering of As atoms. In the case of sulfoselenide glasses the distribution of S and Se atoms in the first coordination shells of Ge and As atoms is found to be random, signifying a chemically ordered chalcogen 'sublattice'. It is shown that the intermediate-range structural and topological aspects of such compositional variation of chemical order may play a central role in controlling properties such as the molar volume of chalcogenide glasses