Structural studies of glassy „Li 2 S… 0.5 „SiS 2 … 0.5 by isotopic-substitution neutron diffraction J. H. Lee Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom A. Pradel, G. Taillades, and M. Ribes Laboratoire de Physicochimie de la Matie´re Condense´e, UMR 5617 CNRS, Universite´ de Montpellier II, 34095 Montpellier Cedex 05, France S. R. Elliott Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom ~Received 24 April 1997! The structure of the superionic glass ~Li 2 S! 0.5 ~SiS 2 ! 0.5 has been studied using Li-isotopic-substitution neutron diffraction. A marked difference has been observed in the intensity of the first sharp diffraction peak ~FSDP! in the structure factor of two isotopically substituted glasses containing the Li isotopes nat Li and 6 Li, having opposite signs of the scattering length. It was found that this behavior can be explained by a void model, in which the FSDP can be regarded as originating from atomic-density fluctuations arising from the chemical ordering of interstitial voids. Peak identifications at short-length scales in the total correlation functions were made and an understanding of the environment of the mobile ion Li 1 has been attempted by means of a first-difference analysis. It was found that probably three S 2 anions surround each Li 1 cation. @S0163-1829~97!03341-9# I. INTRODUCTION One of the most extensively studied ionic conductors is the alkali-silicate system which comprises the network former silica and an alkali-oxide modifier. The ionic conduc- tivity of modified oxide glasses is quite low at room temperature. 1 However, if the network former is replaced with its sulphide analogue, SiS 2 in this study, the ionic con- ductivity increases considerably since the sulphur ion is more polarizable than the oxygen ion. 2,3 An example of this kind of glass system is (Li 2 S) x (SiS 2 ) 1 2x . This system is attractive as the mobile cation, lithium, is a good anode material for battery applications. These glasses exhibit fairly high ionic dc conductivities at room temperature, e.g., 1 310 24 V 21 cm 21 for the composition of ~Li 2 S! 0.5 ~SiS 2 ! 0.5 . 4 As for many other ionic conductors, the conductivity increases with modifier content and also with incorporation of doping salts, e.g., LiI. 5,6 This system is interesting also from a theoretical point of view since an increase of the modifier content results not only in a higher ionic conductivity but also produces signifi- cant changes in the medium-range order ~MRO! of the thio- silicate network. 7,8 Such MRO changes are expected conse- quently to change the ionic conduction pathways. Unlike alkali silicates, however, there has not been much work done on the structure of thiosilicate systems despite their interest. This is mainly due to technical problems re- lated to the hygroscopic characteristic of these compounds. Two crystalline compositions in the Li 2 S-SiS 2 system 9 are known to exist, namely Li 2 SiS 3 and Li 4 SiS 4 , but neither structure has yet been fully determined by any crystallo- graphic methods. However, an attempt has been made to investigate the crystalline structure of these two modifica- tions by ‘‘simulating’’ x-ray powder diffraction patterns. 9 This was performed by using atomic coordinates of the sili- cate analogue, e.g., Li 2 SiO 3 , followed by rescaling the inter- atomic distances according to the unit-cell parameters of the thiosalt. A structural understanding of Li 2 SiS 3 and Li 4 SiS 4 was made possible by packing atoms into special positions in the unit cell. These authors have suggested 9 that the fully edge-sharing SiS 4 tetrahedra in the SiS 2 crystal ~there are no corner-sharing SiS 4 units nor bridges between the chains in this structure 10 ! transform into corner-sharing chains on in- corporation of Li 2 S. When the concentration of Li 2 S reaches 50%, it is suggested that the structure becomes a one- dimensional chain structure comprised of corner-sharing ~SiS 4 ! 22 tetrahedra each with two nonbridging sulphurs, but further incorporation of Li 2 S breaks the corner-sharing chains and forms fully nonbridging tetrahedra. 9 It has also been reported that there are two crystalline phases ~stable and metastable! in the Li 2 SiS 3 composition, 11 both being based on the chain structure built up with corner-sharing tet- rahedra. In comparison, a 29 Si MASNMR study on these materials has shown that the structure of the stable phase of Li 2 SiS 3 ~low-temperature form! contains dimers of ~SiS 4 ! 22 built up with two edge-sharing tetrahedra, and thus differs from the corner-sharing based structure of the metastable modification. 8 Earlier 29 Si MASNMR studies of glasses in the (Li 2 S) x (SiS 2 ) 1 2x system indicate that an increasing content of Li 2 S decreases the amount of edge sharing dramatically by converting the structure to one comprising corner sharing. 7 This change can be represented as E (2) →E (1) →E (0) , where E refers to edge-sharing units sharing the number of edges given in the superscript ~Fig. 1!. This characteristic distin- guishes the thiosilicate system from the analogous oxygen silicates which are known to comprise corner-sharing units only. However, no E (2) units remain for compositions PHYSICAL REVIEW B 1 NOVEMBER 1997-I VOLUME 56, NUMBER 17 56 0163-1829/97/56~17!/10934~8!/$10.00 10 934 © 1997 The American Physical Society