Ge and As x-ray absorption fine structure spectroscopic study of homopolar bonding, chemical order, and topology in Ge-As-S chalcogenide glasses S. Sen, C. W. Ponader, and B. G. Aitken Glass Research Division, Corning Incorporated, Corning, New York 14831 Received 14 March 2001; published 23 August 2001 The coordination environments of Ge and As atoms in Ge x As y S 1-x -y glasses with x : y =1:2, 1:1, and 2.5:1 and with wide-ranging S contents have been studied with Ge and As K-edge x-ray absorption fine structure spectroscopy. The coordination numbers of Ge and As atoms are found to be 4 and 3, respectively, in all glasses. The first coordination shells of Ge and As atoms in the stoichiometric and S-excess glasses consist of S atoms only, 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, whereas Ge-Ge bonds are formed only in strongly S-deficient glasses indicating clustering of metal atoms and violation of chemical order in S-deficient glasses. The composition-dependent variation in chemical order in chalcogen- ide glasses has been hypothesized to result in topological changes in the intermediate-range structural units. The role of such topological transitions in controlling the structure-property relationships in chalcogenide glasses is discussed. DOI: 10.1103/PhysRevB.64.104202 PACS numbers: 61.43.Fs, 61.43.Dq, 61.25.Mv, 61.10.Ht I. INTRODUCTION The physicochemical properties of chalcogenide glasses in the system Ge-As-S,Se have been studied intensively in the last 20 years because of their technological importance in the field of optical devices. 1,2 The chalcogenide glasses have also been used as model systems for studying the influence of topological constraints on the compositional variation of various physical properties in covalently bonded amorphous materials. 3,4 It has been argued on the basis of the theory of constraint counting that the physical properties of these glasses would be predominantly controlled by the average coordination number r, irrespective of their actual chemical compositions. 3,4 The Ge, As, and SSe atoms in a Ge x As y (S,Se) 1 -x -y glass are 4, 3, and 2 coordinated, re- spectively. Therefore the average coordination number r for such a glass can be written as  r  =4 x +3 y +2(1 -x - y ). A simple counting of the bond length and angle constraints on the total number of degrees of freedom available to a mole of three-dimensionally connected atoms shows that the system of atoms would undergo a rigidity percolation type of tran- sition at  r  =2.4. 3,4 Detailed studies of the compositional dependence of elastic, thermodynamic, transport, and elec- tronic properties in chalcogenide glasses indicate that some properties indeed show such a transition at  r  2.4, al- though another important transition or extremum is almost always present at  r  2.67. 5,6 The latter transition has re- cently been speculated to be a manifestation of a topological phase transition in the chalcogenide glass structure. Accord- ing to this model, for  r  2.7, the topological dimensional- ity D of the glass structure is 2 whereas for  r  2.7 the glasses are characterized by a three-dimensionally connected structure, i.e., D =3. 6 However, these topological arguments do not take into account the differences between the characters of various homopolar and heteropolar bonds and the degree of chemical order in the chalcogenide glass structure. Recent spectro- scopic studies have clearly indicated the presence of ho- mopolar bonds in the atomic structure of nonstoichiometric chalcogenide glasses. 7–9 Homopolar bonding may result in a violation of the chemical order in the structure with cluster- ing of the constituent atoms. 9 Such clustering of constituent atoms would have important influences on the nature and dimensionality of the intermediate-range structural units in the glass. A systematic study of the bonding environment and chemical order in chalcogenide glasses over a wide compo- sition range and r values is therefore needed in order to test these hypotheses. We report here the results of a detailed Ge and As K-edge x-ray absorption fine structure EXAFS spectroscopic study of the local coordination environments of Ge and As atoms in Ge-As-S glasses. Three series of Ge x As y S 1 -x -y glass compositions with x : y =1:1, 2.5:1, and 1:2 and with r values ranging between 2.351 and 2.975 have been studied. We also report the Ge EXAFS results for a number of S-deficient binary Ge-sulfide glasses in order to make a structural comparison between these and the S-deficient ternary glasses. II. EXPERIMENT A. Sample preparation The Ge-As-sulfide glasses were prepared by melting mix- tures of the constituent elements 99.9995% purity, metals basis. These mixtures were contained in evacuated (10 -6 Torr) flame-sealed fused-silica ampoules and were melted in a rocking furnace at 1200 K for at least 24 h. Cylindrical glass rods were obtained by quenching the am- poules in water followed by annealing for 1 h at the respec- tive glass-transition temperatures. The nominal compositions of the GeAsS glasses studied here are shown in Fig. 1 and are also listed in Tables I, II, and III. The compositions of these glasses have also been denoted in this paper in terms of ‘‘% S in excess of stoichiometry’’ which corresponds to the PHYSICAL REVIEW B, VOLUME 64, 104202 0163-1829/2001/6410/10420210/$20.00 ©2001 The American Physical Society 64 104202-1