Mixed cation effect in Ag 2 S–Tl 2 S–GeS–GeS 2 glasses: Conductivity and tracer diffusion studies M. Bokova a,b, ⁎, I. Alekseev c,d , E. Bychkov a,b a Univ Lille Nord de France, F-59000 Lille, France b ULCO, LPCA, EAC CNRS 4493, F-59140 Dunkerque, France c St. Petersburg University, 199034 St. Petersburg, Russia d V. G. Khlopin Radium Institute, 194021 St. Petersburg, Russia abstract article info Article history: Received 15 June 2014 Received in revised form 30 October 2014 Accepted 31 October 2014 Available online xxxx Keywords: Mixed cation glasses Ionic conductivity 204 Tl tracer diffusion Thallium silver thiogermanate glasses Mixed cation effect has been studied for thallium–silver thiogermanate glasses in the (Ag 2 S) x (Tl 2 S) 50-x (GeS) 25 (GeS 2 ) 25 system, where Ag 2 S fraction r = Ag 2 S / (Ag 2 S + Tl 2 S) = 0, 0.25, 0.5, 0.75, and 1.0. The glass transition temperature changes from 160 °C for r = 0 to 231 °C for r = 1.0 and exhibits a negative deviation from additivity, with a slight minimum T g ≈ 157 °C observed for compositions in the range 0.1 ≤ r ≤ 0.3. The room temperature conductivity σ 298 increases by 5 orders of magnitude with increasing silver concentration, ranging between 10 -8 S cm -1 for the Tl–Ge–S system and 10 -3 S cm -1 for the Ag–Ge–S glass with a minimum at σ 298 ≈ 10 -9 S cm -1 . The activation energy decreases from 0.66 to 0.33 eV, respectively, with the maximum at E σ ≈ 0.69 eV. The minimum of both T g and σ 298 and the E σ maximum are observed in the same composition range, 0.1 ≤ r ≤ 0.3. The 204 Tl tracer diffusion coefficient at 170 °C decreases with silver concentration from 10 -10 cm 2 s -1 for (Tl 2 S) 50 (GeS) 25 (GeS 2 ) 25 glass to 10 -12 cm 2 s -1 for (Ag 2 S) 50 (GeS) 25 (GeS 2 ) 25 glass. The obtained results indicate a classical mixed cation effect. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Chalcogenide glasses containing Ag + , Li + , Na + or Cu + exhibit high ionic conductivity which makes them suitable for many applications, i.e., all-solid-state batteries, chemical sensors, etc. [1–13]. While large effort has been directed toward investigations of ion transport proper- ties of single cation chalcogenide glasses, there are only few studies on the mixed cation effect in chalcogenide systems [14–19]. The mixed cation effect is usually associated with non-linear changes in glass prop- erties (e. g., ionic conductivity and its activation energy, tracer diffusion or glass transition temperature) when one type of mobile cation is replaced by another at fixed total cation content. It has been well documented for oxide glasses (see, for example, [20,21] and references therein) but only a limited number of mixed cation chalcogenide glasses containing alkali, silver and copper ions [14–19] have been studied. To our knowledge, no experiments have been reported for Tl + /Ag + mixed chalcogenide glasses. A significant difference in ion size between silver and thallium cations and therefore a dramatic difference in their ionic mobility makes the Tl + /Ag + mixed glasses an attractive model system to study the mixed cation effect and ion transport in disordered materials. In this work we will present Ag 2 S–Tl 2 S–GeS–GeS 2 glasses, containing 50 mol.% of metal sulphide. The Ag 2 S–GeS–GeS 2 glass is nearly pure Ag + ion conductor [4]. Its Tl-containing counterpart becomes an ionic conductor at ~15 at.% Tl according to our recent electrical and tracer diffusion measurements [22,23]. In order to get additional and complementary information on ion transport in these mixed glasses, we have completed ac impedance measurements and started 204 Tl tracer diffusion experiments. 2. Experimental details 2.1. Glass preparation Thallium thiogermanate glasses were prepared by mixing appropriate quantities of germanium (Aldrich, 99.999%), sulphur (Aldrich, 99.999%), thallium (Fluka, 99.99%) and silver (Neyco, 99.999%). The mixtures were sealed under vacuum (10 -6 mbar) in a cleaned silica tube (with an inner diameter of 8 mm and a wall thickness of 1 mm), heated slowly to 900 °C at 1 K min -1 heating rate and maintained at this temperature for a few days with repeated stirring of the melt. Once homogeneous, the glasses were cooled down to 800 °C and quenched in water at room temperature. To avoid the high vapour pressure of sulphur during initial steps of the synthesis, prolonged heating at 300, 500 and 800 °C was necessary. Typical sample mass was 3 g. The quenched samples were annealed at 20–30 °C below the glass transition temperature, T g , for 24 h. Five compositions of the (Ag 2 S) x (Tl 2 S) 50-x (GeS) 25 (GeS 2 ) 25 Solid State Ionics xxx (2014) xxx–xxx ⁎ Corresponding author at: LPCA, EAC CNRS 4493, 189A Avenue M. Schumann, 59140 Dunkerque, France. Tel.: +33 2 28 65 82 70; fax: +33 3 28 65 82 44. E-mail address: Maria.Bokova@univ-littoral.fr (M. Bokova). SOSI-13517; No of Pages 4 http://dx.doi.org/10.1016/j.ssi.2014.10.032 0167-2738/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Please cite this article as: M. Bokova, et al., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.10.032