JOURNAL OF Journal of Non-Crystalline Solids 156-158 (1993)720-724 I,1,I North-Holland Structure of fast-ion conducting chalcogenide glasses: the Ag-As-Se system Chris J. Benmore and Philip S. Salmon School of Physics, University of East Anglia, Norwich NR4 7TJ, UK The coordination environment of Ag in bulk-quenched mg4As37.6Se5s.3 and mgzAs3Se 4 glasses is investigated by using the method of isotopic substitution in neutron diffraction. It is found that there is little change in the short-range order about Ag between the two compositions: Ag is covalently bonded to an average of = 3 Se atoms at a distance of = 2.68 ~, in both samples. The absence of a pronounced change in the structure about the metal atom in the Ag-As-Se system, between glass compositions where the electrical conductivity is electronic (4 at.% Ag) and where it is predominantly ionic (22.2 at.% Ag), is by direct contrast with the case observed for glasses in the Ag-As-S system. 1. Introduction The object of this paper is to present new results on the coordination environment of Ag, in melt-quenched glasses of the Ag-As-Se system, as measured by using the method of isotopic substitution in neutron diffraction. Motivation for the work has been provided in part by recent neutron diffraction results on the coordination environment of Ag in the two different homoge- neous glass-forming regions of the Ag-As-S sys- tem. These regions are distinct and are centred on the (Ag2S)x(AS2S3)l_x ( 0 _~<x _~< 1) pseudo-bi- nary tie-line, one occurring at small and the other at high (x = 0.5) Ag concentration. The neutron results showed that there is a reduction from = 4 to = 3 in the number of S atoms covalently bonded to Ag as x is increased from 0.096 to 0.500 [1]. Hence the occurrence of two homoge- neous glass-forming regions and the change in the nature of the electrical conductivity as x is increased (the ionic component is negligible at x < 0.1 but it is dominant at x = 0.5 where Ag + Correspondence to: Dr P.S. Salmon, School of Physics, Univer- sity of East Anglia, Norwich NR4 7TJ, UK. Tel: +44-603 592 580. Telefax: +44-603 259 515. E-mail: HP170 @ UK.AC.UEA.CPC865. has been identified as the mobile species and the glasses are fast-ion conductors) are associated with a change in the coordination environment of the metal atom. It is therefore of interest to investigate the generality of this observation by studying glasses in the Ag-As-Se system since, while there is a change in the nature of the chalcogen species, the electronegativities of S and Se are very similar (respectively 2.58 and 2.55 on the Pauling scale [2]). Glasses having the compo- sitions mg4As37.6Se58.4 (corresponding to x= 0.096 on the (Ag2Se)x(As2Se3)l_ x tie-line) and Ag2As3Se 4 (or Ag22.2As33.3Se44.4 corresponding to x = 0.25 on the (Ag2Se)x(AsSe)l_x tie-line) were chosen for investigation by using the neu- tron method and will be denoted by, respectively, Ag-4 and Ag-22.2. As for the case of the Ag-As-S system, the electrical conductivity in glassy Ag-As-Se re- mains essentially electronic when small quantities (~ 4 at.%) of Ag are incorporated into As-Se [3-5]. Likewise, with increasing Ag content, the conductivity changes from electronic (or = 2.5 x 10-10 f~-i cm-~ at 20°C for Ag4.lAS37.6Se58.3 [3]) to predominantly ionic (~r---5 X 10 -6 1) -1 cm -1 at 20°C for Ag/AsaSe 4 [5,6]) and the silver trans- port number approaches unity [5-7]. However, by contrast with the Ag-As-S system, glass forma- 0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved