Lattice distortion and anisotropic thermoelectric properties in hot- deformed CuI-doped Bi 2 Te 2$7 Se 0.3 Jin Hee Kim a, 1 , Hyunyong Cho a, 1 , Song Yi Back a , Jae Hyun Yun a , Ho Seong Lee b , Jong-Soo Rhyee a, * a Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yong-in, Gyeong-gi,17104, South Korea b School of Materials Science and Engineering, Kyungpook National University, Daegu, 41566, South Korea article info Article history: Received 29 July 2019 Received in revised form 9 October 2019 Accepted 10 October 2019 Available online 11 October 2019 Keywords: Thermoelectric High ZT Lattice distortion Hot deformation abstract We investigated anisotropic thermoelectric properties of (CuI) x Bi 2 Te 2$7 Se 0.3 (x ¼ 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of poly- crystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x ¼ 0.3 mol.% shows the maximum power factor (3.8 mW m 1 K 2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric perfor- mance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x ¼ 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high res- olution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x ¼ 0.3% compound. Therefore, the unconventional anisotropic lattice thermal con- ductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Thermoelectric devices can be used for waste heat recovery and solid-state refrigeration, so that much attention has been increased steadily, due to demands for renewable energy and eco-friendly system. The thermoelectric generator directly converts heat into electric energy by a temperature gradient between the ends of a device. Also, a thermoelectric refrigerator can transport heat from the end of the device to the opposite end by electric bias. The thermoelectric device has the advantage of solid-state operation, no mechanical moving parts with no vibration, no release of greenhouse gases, and extended operating lifetime [1]. High thermoelectric performance is needed for efficient waste- heat recovery and to widen the application fields near room tem- perature. The performance of the thermoelectric devices mainly depends on the thermoelectric figure of merit (ZT) defined by ZT ¼ S 2 sT/k, where S, s, T , and k are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal con- ductivity, respectively. The high ZT materials which have maximum performance at room temperature can be used for a wide range of applications, including not only the conventional thermoelectric generator or refrigerator but also wearable and flexible thermo- electric devices [2,3]. Furthermore, the thermoelectric materials that have high performance near room temperature can be applied as low-grade waste heat recovery (below 150  C) [4], which is abundant in solar-thermal, body heat, and many mechanical systems. The bismuth telluride based compounds are well known high ZT materials operating near room temperature [5]. The p-type poly- crystalline bismuth tellurides were reported as high ZT values by Hot-deformation (ZT ¼ 1.3 at 380 K) [6], hot-press sintering with nanoparticles (ZT ¼ 1.4 at 373 K) [7] and melt-spinning and spark plasma sintering (SPS) (ZT ¼ 1.56 at 300 K [8], ZT ¼ 1.86 at 320 K) [9]. On the other hand, n-type bismuth telluride based compounds also reported high ZT values in the compounds such as hot- deformed Bi 2 Te 2$3 Se 0.7 (ZT ¼ 1.2 at 445 K and ZT ¼ 1.04 at 398 K) [6, 10], I-doped polycrystalline Bi 2 Te 2$7 Se 0.3 (ZT ¼ 1.13 at 423 K) [11], Cu-doped polycrystalline Bi 2 Te 2$7 Se 0.3 (ZT ¼ 1.10 at 373 K) [12] and Cu-doped single-crystalline Bi 2 Te 3 (ZT ¼ 1.15 near 300 K) [13]. * Corresponding author. E-mail address: jsrhyee@khu.ac.kr (J.-S. Rhyee). 1 Two authors (J.H.K. and H.C.) are equally contributed on this work. 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.2019.152649 0925-8388/© 2019 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 815 (2020) 152649