High Thermoelectric Performance in Sulfide‐Type Argyrodites Compound Ag₈Sn(S_1−xSe_x)₆ Enabled by Ultralow Lattice Thermal Conductivity and Extended Cubic Phase Regime

Abstract

Argyrodites with a general chemical formula of A 8 BC 6 are known for complex phase transitions, ultralow lattice thermal conductivity, and mixed electronic and ionic conduction. The coexistence of ionic conduction and promising thermoelectric performance have recently been reported in selenide and telluride argyrodites, but scarcely in sulfide argyrodites. Here, the thermoelectric properties of Ag 8 Sn(S 1−x Se x ) 6 are reported. Specifically, Ag 8 SnS 6 exhibits intrinsically ultralow lattice thermal conductivities of 0.61-0.31 W m −1 K −1 over the whole temperature range from 32 to 773 K due to distorted local crystal structure, relatively weak chemical bonding, rattler-like Ag atoms, low-lying optical modes, and dynamic disorder of Ag ions at high temperatures. Se doping shifts the orthorhombic-cubic phase transition from 457 K at x = 0 to 430 K at x = 0.10, thereby expanding the temperature range of the thermoelectrically favored cubic phase. A figure of merit zT value ≈ 0.80 is achieved at 773 K in Ag 8 Sn(S 1−x Se x ) 6 (x = 0.03), the highest zT value reported in sulfide argyrodites. These results fill a knowledge gap of the thermoelectric study of argyrodites and contribute to a comprehensive understanding of the chemical bonding, lattice dynamics, and thermal transport of argyrodites.a potential for practical applications. Yet, it is still a challenge to decouple the adversely correlated {σ, S, κ e } toward high zT values via electronic and phonon band engineering. [3][4][5][6][7][8] Many efforts have been devoted to discover novel TE materials such as cage-like filled skutterudites (e.g., CeFe 4 Sb 12 , CoSb 3 , Sr 8 Ga 16 Ge 30 , etc.), [9][10][11][12] Zintl-phase compounds (e.g., CaZn 2 Sb 2 , Mg 3 Sb 2 , YbMg 2 Bi 2 , etc.), [13][14][15][16] and defective half-Heusler compounds (e.g., Nb 0.8 CoSb, Ti 0.9 NiSb, V 0.9 CoSb, etc.) [17] that possess a long-range order favoring charge carrier mobility while disordered in short range to inhibit heat-carrying phonons in the context of "phonon-glass-electron-crystal". [18] Recently, a class of "liquid-like" TE materials with complex phase transitions, ultralow lattice thermal conductivity, and liquidlike ionic conduction have been reported, Cu 2−x (S, Se, Te), [19][20][21] Ag 2 (S, Se, Te), [22,23] argyrodite, [24][25][26][27][28][29][30][31][32][33][34][35][36] AgCrSe 2 , [37,38] CuAgSe, [39] and AgCuTe [40] to name a few. Argyrodites are good examples of liquid-like TE materials. Argyrodites include 716, 816, and 916 phases in chemistry, with a general chemical formula A 12−n

N-type thermoelectric Ag8SnSe6 with extremely low lattice thermal conductivity by replacing Ag with Cu

“…As a result, a peak figure of merit ZT ( ZT ∼ 1.5) is attained at 850 K. 34 Another similar compound Cu 8 GeSe 6 has a low κ L value of 0.2 W m −1 K −1 and ZT value as high as 1.0. 24 For the Ag 8 SnS 6 compound, it is reported that lattice part κ L is 0.31–0.61 W m −1 K −1 , 37 much higher than those observed in Ag 9 GaSe 6 and Cu 8 GeSe 6 and that the recorded ZT value is only 0.8. Therefore, it is highly likely to improve the TE performance of Ag 8 SnS 6 if its κ L value can be further reduced.…”

“…The stronger the bonding is, the higher the sound velocity. 23,66 In this regard, there is a dual effect that regulates k L : one is point defects formed by cation substitution, which reduces the lattice part k L , 37 and the other is the introduction of stronger Cu-Se bond than Ag-Se, which increases the k L value. That is why we observed the minimum k L $ 0.12 W K À1 m À1 at x ¼ 0.05 and 645 K.…”

Ductile Ag20S7Te3 with Excellent Shape‐Conformability and High Thermoelectric Performance

“…Similar feature has been also observed in other superionic compounds such as Cu 2‐ δ (S, Se, Te) [ 28–31 ] and argyrodites. [ 32 ]…”

Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In‐doped GeSb2Te4 Single Crystals

“…In addition, it is also quite crucial to seek potential materials with intrinsically low κ L , such as liquid‐like materials [ 18 ] and low‐sound‐velocity materials. [ 19 ]…”

“…In addition, it is also quite crucial to seek potential materials with intrinsically low κ L , such as liquid-like materials [18] and low-sound-velocity materials. [19] In the past decade, materials with layered structures, such as SnSe, [20][21][22][23][24] In 4 Se 3 , [25] and SnS, [26,27] have sparked significant research owing to their excellent TE performance. Their layered structure and strong lattice anharmonicity naturally lead to ultralow κ L .…”