Ultra-low thermal conductivity and improved thermoelectric performance in disordered nanostructured copper tin sulphide (Cu 2 SnS 3 , CTS) K. Lohani a , E. Isotta a , N. Ataollahi a , C. Fanciulli b , A. Chiappini c , P. Scardi a, * a Department of Civil, Environmental and Mechanical Engineering, University of Trento, Via Mesiano 77, 38123, Trento, Italy b National Research Council of Italy-Institute of Condensed Matter Chemistry and Technologies for Energy (CNR-ICMATE), Lecco Unit, Via Previati 1/E, 23900, Lecco, Italy c Institute of Photonics and Nanotechnologies IFN, National Research Council CNR CSMFO Lab. & Fondazione Bruno Kessler FBK, Centro Materiali e Microsistemi CMM, Via alla Cascata 56/C, 38123, Trento, Italy article info Article history: Received 30 October 2019 Received in revised form 28 January 2020 Accepted 29 February 2020 Available online 4 March 2020 Keywords: Copper tin sulphide Cu 2 SnS 3 Reactive ball-milling Ordered and disordered structure Ultra-low thermal conductivity Phonon-glass-electron-crystal Cubic CTS abstract Copper tin sulphide (Cu 2 SnS 3, CTS) is a promising p-type thermoelectric material. In the present work, we have investigated the cation disorder in CTS disks made by sintering powders produced via high energy reactive ball-milling. The crystalline structures, electronic and thermal properties were system- atically investigated. As-milled CTS shows a disordered cubic structure (c-CTS), preserved with thermal treatment up to 500  C. By increasing the thermal treatment temperature, CTS gradually evolves towards the ordered monoclinic structure (m-CTS), reaching complete order at 650  C. The disordered CTS has several times higher zT than the ordered CTS. In fact, ordered CTS has high thermopower, up to 700 mV/K, and high electrical resistivity leading to zT < 0.05 above 700K; whereas, disordered (c-CTS) has comparatively high zT~0.30 above 700K. This has been related to the lower electrical resistivity, and the ultra-low thermal conductivity (k~0.26 W/m-K), resulting from the disordered structure, promoting Phonon-Glass-Electron-Crystal (PGEC) characteristics. In our best knowledge, zT~0.30 above 700K is the highest in CTS samples without acting on the chemistry of the system. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Thermoelectric materials are used to exploit temperature gra- dients, turning thermal energy into electrical energy based on the Seebeck effect [1]. Besides direct energy production in remote conditions, like deep space probes, they find increasingly more applications in devices for waste heat recovery [2]. The energy conversion efficiency of a thermoelectric material can be expressed by the dimensionless figure of merit, zT, defined as zT ¼ S 2 sT/k [3], where S is the Seebeck coefficient, s is the electrical conductivity, T is absolute temperature, and k is thermal conductivity, made of a phononic (k lat ) and of an electronic (k ele ) component [1 ,4,5]. Pa- rameters S, s and k in zT are interrelated through the carrier con- centration (n) so that it is difficult to optimize them independently [6]. Copper tin sulphide (Cu2SnS3, CTS) is an earth-abundant, eco- friendly, non-toxic p-type ceramic material [4]. CTS has a 3-D conductive network, showing Phonon-Glass-Electron-Crystal (PGEC) characteristic, with enhanced carrier transport and reduced thermal conductivity [7]. This makes CTS a promising thermoelectric material [8]. CTS is reported in polymorphic crystal structures, cubic (c-CTS; SG: F-43 m), monoclinic (m-CTS; SG: Cc) and, tetragonal (t-CTS; SG: I-42 m). These polymorphs are identified as disordered, ordered and pseudo-ordered structures, respectively [9e12]. Different CTS polymorphs have different band gap energies [13]. Recent work on similar CZTS (Cu 2 SnZnS 4 ) chalcogenides has put forward that cation disordered structures could improve thermo- electric properties of this class of semiconducting materials [14, 15]. These results reveal that a disordered structure could enhance the density of state effective mass (m* DOS ) with an increase in n, while k lat is largely suppressed [2, 14]. Transitions to structurally disor- dered polymorphs, obtained via acceptor doping, have been stud- ied in CTS systems by many researchers [7 ,9, 16]. The ordered m-CTS * Corresponding author. E-mail address: Paolo.Scardi@unitn.it (P. Scardi). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2020.154604 0925-8388/© 2020 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 830 (2020) 154604