Extremely Low Lattice Thermal Conductivity and Point Defect Scattering of Phonons in Ag-doped (SnSe)_1–x(SnS)_x Compounds

Abstract

Single crystalline SnSe has been reported to exhibit the high thermoelectric zT value of 2.6 at 923 K along the b-axis direction, due to its low thermal conductivity [Zhao, L. D.; et al. Nature 2014, 508, 373]. However, the strongly anisotropic properties of the orthorhombic structure degrade the thermoelectric performance of polycrystalline SnSe, resulting in a low zT of 0.6 and 0.8 for Ag- and Na-doped SnSe, respectively. Here, we prepared Ag0.01Sn0.99Se1–x S x (x = 0, 0.10, 0.15, 0.20, and 0.35) compounds by melting and hot press sintering. The compounds showed extremely low thermal conductivity (0.11 W m–1 K–1 at 825 K for x = 0.15). Using transmission electron microscopy images, we found that SnS alloying induced numerous nanoscale point defects. A Debye–Callaway model analysis supported the conclusion that the extremely low lattice thermal conductivity could be attributed to the point defect scattering of phonons. This resulted in a high zT of 1.67 at 823 K for the x = 0.15 sample, which is the state-of-the-art zT value for polycrystalline SnSe. Because the compounds are based on the environmentally friendly and cheap materials Sn, Se, and S, they make promising candidates for thermoelectric applications.

Lattice dynamics of Pnma Sn(S1–xSex) solid solutions: energetics, phonon spectra and thermal transport

“…However, the calculations predict the JDoS of all three mixed compositions to be on a similar scale to the endpoints, which suggests that the broader range of phonon frequencies in the mixed compositions does not necessarily lead to as large an enhancement in the number of scattering channels as might be expected. This is, however, consistent with the relatively small absolute variation in k latt observed among different alloy compositions in experiments [12,14].…”

“…n are the phonon occupation numbers at the calculation temperature (equation (15)) and the functions d and Δ enforce conservation of energy and crystal momentum, respectively (see equations (14)/ (16)). Finally, we further define a function¯( ) w l N 2 and component parts¯(…”

“…There has therefore been significant effort devoted to alternative systems including the more earth-abundant SnS [2] and metal oxides [7][8][9][10].Alloying is commonly used as a means to enhance thermoelectric performance, as a suitable choice of components can maintain or improve a favourable electronic structure while reducing k latt by introducing variation in atomic masses and chemical bond strength to promote stronger phonon scattering [11]. Pb(S, Se, Te), Sn(S, Se) and (Bi, Sb) 2 (Se, Te) 3 alloys have all been studied as thermoelectrics and the alloying shown to improve ZT [12][13][14][15][16][17]. Due to the record-breaking high-temperature ZT of SnS, the Te-free Sn(S, Se) alloys are of particular interest, and experimental studies have demonstrated a reduction in the thermal conductivity of mixed compositions [12,14].Thermoelectricity is unique in that all four of the terms in the figure of merit ZT are amenable to calculation using first-principles electronic-structure methods such as density-functional theory (DFT) [18].…”

“…Pb(S, Se, Te), Sn(S, Se) and (Bi, Sb) 2 (Se, Te) 3 alloys have all been studied as thermoelectrics and the alloying shown to improve ZT [12][13][14][15][16][17]. Due to the record-breaking high-temperature ZT of SnS, the Te-free Sn(S, Se) alloys are of particular interest, and experimental studies have demonstrated a reduction in the thermal conductivity of mixed compositions [12,14].Thermoelectricity is unique in that all four of the terms in the figure of merit ZT are amenable to calculation using first-principles electronic-structure methods such as density-functional theory (DFT) [18]. There have been a number of theoretical studies on existing and potential new single-component bulk thermoelectrics [6,[19][20][21][22][23], and there have recently been efforts to use high-throughput modelling to screen large numbers of compounds in a bid to identify new candidate thermoelectrics [24,25].Studying multicomponent alloy systems with first-principles methods is challenging, however, as building a realistic model requires calculations on a large number of configurations that for complex systems with lowsymmetry unit cells can easily approach the cost of a screening study [26].…”

“…Alloying is commonly used as a means to enhance thermoelectric performance, as a suitable choice of components can maintain or improve a favourable electronic structure while reducing k latt by introducing variation in atomic masses and chemical bond strength to promote stronger phonon scattering [11]. Pb(S, Se, Te), Sn(S, Se) and (Bi, Sb) 2 (Se, Te) 3 alloys have all been studied as thermoelectrics and the alloying shown to improve ZT [12][13][14][15][16][17]. Due to the record-breaking high-temperature ZT of SnS, the Te-free Sn(S, Se) alloys are of particular interest, and experimental studies have demonstrated a reduction in the thermal conductivity of mixed compositions [12,14].…”