Thermoelectric properties of the Pnma and R3m phases of GeS and GeSe Min Zhang, Joseph M. Flitcroft, and Jonathan M. Skelton University of Manchester, UK As the impact of climate change becomes ever more apparent, coupled with dwindling fossil fuel reserves and an ever-growing demand for more energy, there is an intense focus on finding technologies that can provide clean and sustainable energy. As large amounts of current energy usage is wasted as heat (60 %), using thermoelectric (TE) generators to capture waste heat and recover it as useful electrical energy could significantly enhance efficiency and sustainability. Furthermore, thermoelectric generators are easily scalable to a wide range of applications including in the automotive, manufacturing and power industries. 1 Tin monoselenide SnSe is one of the most promising TE materials and was reported to have a very high ZT of 2.6 at 923 K. 2 The germanium analogues GeS and GeSe have also been predicted to have good TE performance, 3-4 and, recently, the rhombohedral ( R3m ) phase of GeSe, was reported to have a ZT value > 1. 5 However, the TE properties of R3m GeS, a structural analogue of R3m GeSe, have not yet been investigated. We have used first-principles modelling to perform a comprehensive evaluation of the structural and electronic properties of the orthorhombic ( Pnma ) and R3m phases of GeS and GeSe. Our results show that n-type Pnma GeSe can support a large ZT of 2.23 at 940 K, due to a combination of its low lattice thermal conductivity and comparatively high electrical conductivity. This suggests that, if n-type doping of Pnma GeSe is possible, it could be a good match for p-type Pnma SnSe 6 in a thermoelectric couple device. References 1. Freer R, Powell AV. Realising the potential of thermoelectric technology: A Roadmap. J.Mater.Chem.C . 2020;8(2):441-63. 2. Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014, 508 (7496), 373-377. 3. Hao, S.; Shi, F.; Dravid, V. P.; Kanatzidis, M. G.; Wolverton, C. Computational prediction of high thermoelectric performance in hole doped layered GeSe. Chem.Mater. 2016, 28 (9), 3218-3226. 4. Ding, G.; Gao, G.; Yao, K. High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds. Sci. Rep. 2015, 5 (1), 9567. 5. Sarkar, D.; Ghosh, T.; Roychowdhury, S.; Arora, R.; Sajan, S.; Sheet, G.; Waghmare, U. V.; Biswas, K. Ferroelectric instability induced ultralow thermal conductivity and high thermoelectric performance in rhombohedral p-type GeSe crystal. J.Am.Chem.Soc. 2020, 142 (28), 12237-12244. 6. Flitcroft, J. M.; Pallikara, I.; Skelton, J. M. Thermoelectric Properties of Pnma and Rocksalt SnS and SnSe. Solids 2022, 3 (1), 155-176.
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