Low Thermal Conductivity and High Thermoelectric Performance in (GeTe)_1–2x(GeSe)_x(GeS)_x: Competition between Solid Solution and Phase Separation

Abstract

GeTe and its derivatives constituting Pb-free elements have been well known as potential thermoelectric materials for the last five decades, which offer paramount technological importance. The main constraint in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (κ). Herein, we demonstrate low κ (∼0.7 W/m·K) and a significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in the Sb-doped pseudoternary (GeTe)(GeSe)(GeS) system by two-step strategies. The (GeTe)(GeSe)(GeS) system provides an excellent podium to investigate competition between an entropy-driven solid solution and enthalpy-driven phase separation. In the first step, small concentrations of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the κ by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly, the system deviates from a solid solution, and phase separation of the GeSSe (5-20 μm) precipitates in the GeTeSe matrix occurs, which does not participate in phonon scattering. In the second stage, κ of the optimized sample is further reduced to 0.7 W/m·K by Sb alloying and spark plasma sintering (SPS), which introduce additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low κ in Sb-doped (GeTe)(GeSe)(GeS) is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence, the SPS-processed GeSbTeSeS sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb-doped (GeTe)(GeSe)(GeS) exhibits significantly higher Vickers microhardness (mechanical stability) compared to that of pristine GeTe.

Achieving zT > 2 in p‐Type AgSbTe2−xSex Alloys via Exploring the Extra Light Valence Band and Introducing Dense Stacking Faults

“…Note that pristine AgSbTe 2 has multivalence bands, which are responsible for the electronic transport coefficients. [24] b) The comparison of our highest zT of AgSbTe 1.85 Se 0.15 with the reported state-of-the-arts lead-free thermoelectric materials of Bi 0.5 Sb 1.5 Te 3 , [25] Bi 2 Te 2.79 Te 0.21 , [26] Yb 0.25 Co 4 Sb 12 , [23] Na-doped SnSe, [22] Mg 2 Sn 0.75 Ge 0.25 , [27] Ge 0.9 Sb 0.1 Te 0.9 Se 0.05 S 0.05 , [28] and Sn 0.91 Mn 0.14 Te(Cu 2 Te) 0.05 . a) Temperature-dependent zT for our AgSbTe 2−x Se x and the reported reference data for AgSbTe 2−x Se x with x = 0 and 0.02.…”

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n‐Type BiTe

“…Due to the intertwined relationship among S , σ and κ el , independent control over κ lat is imperative to realize high TE performance. Solid‐solution alloying and nanostructuring are proven to be effective strategies in reducing κ lat of a TE system . However, these extrinsic strategies also lead to charge carrier scattering, resulting in reduced charge carrier mobility ( μ ) .…”

“…[1][2][3][4][5] Due to the intertwined relationship among S, s and k el ,i ndependent control over k lat is imperative to realize high TE performance.S olid-solution alloying and nanostructuring are proven to be effective strategies in reducing k lat of aT Es ystem. [6][7][8][9] However,t hese extrinsic strategies also lead to charge carrier scattering, resulting in reduced charge carrier mobility (m). [10] Hence, identification of crystalline solids with intrinsically low k lat [10][11][12][13][14][15][16][17] or development of novel strategies to decrease k lat such as introduction of bonding hierarchy, [18,19] and ferroelectric instability [20] are important in designing high performance TE materials.…”

Enhancement in thermoelectric performance of n-type Pb-deficit Pb-Sb-Te alloys

“…Advances in recent times show that it is feasible to enhance zT by a number of approaches [1][2][3][4]. One approach is solid solution alloying, which enables acoustic phonon scattering leading to a decreased latt [5][6][7][8][9][10].…”