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In thermoelectrics, entropy engineering as an emerging paradigm-shifting strategy can simultaneously enhance the crystal symmetry, increase the solubility limit of specific elements, and reduce the lattice thermal conductivity. However, the severe lattice distortion in high-entropy materials blocks the carrier transport and hence results in an extremely low carrier mobility. Herein, the design principle for selecting alloying species is introduced as an effective strategy to compensate for the deterioration of carrier mobility in GeTe-based alloys. It demonstrates that high configurational entropy via progressive MnZnCdTe3 and Sb co-alloying can promote the rhombohedral-cubic phase transition temperature toward room temperature, which thus contributes to the enhanced density-of-states effective mass. Combined with the reduced carrier concentration via the suppressed Ge vacancies by high-entropy effect and Sb donor doping, a large Seebeck coefficient is attained. Meanwhile, the severe lattice distortions and micron-sized Zn0.6Cd0.4Te precipitations restrain the lattice thermal conductivity approaching to the theoretical minimum value. Finally, the maximum zT of Ge0.82Sb0.08Te0.90(MnZnCdTe3)0.10 reaches 1.24 at 723 K via the trade-off between the degraded carrier mobility and the improved Seebeck coefficient, as well as the depressed lattice thermal conductivity. These results provide a reference for the implementation of entropy engineering in GeTe and other thermoelectric materials. [ABSTRACT FROM AUTHOR]

The configurational entropy is an emerging descriptor in the functional materials genome. In thermoelectric materials, the configurational entropy helps tune the delicate trade‐off between carrier mobility and lattice thermal conductivity, as well as the structural phase transition, if any. Taking GeTe as an example, low‐entropy GeTe generally have high carrier mobility and distinguished zT > 2, but the rhombohedral‐cubic phase transition restricts the applications. In contrast, despite cubic structure and ultralow lattice thermal conductivity, the degraded carrier mobility leads to a low zT in high‐entropy GeTe. Herein, medium‐entropy alloying is implemented to suppress the phase transition and achieve the cubic GeTe with ultralow lattice thermal conductivity yet decent carrier mobility. In addition, co‐alloying of (Mn, Pb, Sb, Cd) facilitates multivalence bands convergence and band flattening, thereby yielding good Seebeck coefficients and compensating for decreased carrier mobility. For the first time, a state‐of‐the‐art zT of 2.1 at 873 K and average zTave of 1.3 between 300 and 873 K are attained in cubic phased Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te. Moreover, a record‐high Vickers hardness of 270 is attained. These results not only promote GeTe materials for practical applications, but also present a breakthrough in the burgeoning field of entropy engineering. [ABSTRACT FROM AUTHOR]

Besides the energy-conversion efficiency and production cost, the thermal stability is also a key parameter for the wide-spread use of a thermoelectric material in device-level. The rare-earth-element Sm content is introduced into the (GeTe) 0.85 (Pb 1- x Sm x Te) 0.15 alloys to tune the thermal stability and structural properties of these alloys, especially the spinodal decomposition. The Sm content shows suppression effect on the generation of secondary-phase PbTe-rich domains, and it also tend to stabilize the thermodynamically balanced state with stabilized microstructures and thermoelectric performance. The reduction of the lattice thermal conductivity by Sm alloying is mainly ascribed to the grain refining and the designing of all-scale hierarchical architectures for full-spectrum phonon scattering. By thermal annealing, the ZT values can increase by 25% and 76% for the (GeTe) 0.85 (Pb 1- x Sm x Te) 0.15 alloys with x  = 0.2 and x  = 0.28 respectively. The synergetic strategy of rare-earth element doping and thermal annealing in this study opens up a potential pathway to enhance the thermoelectric performance and thermal stability. [ABSTRACT FROM AUTHOR]

High symmetry favors high power factor by virtue of the balanced Seebeck coefficient and carrier mobility, nonetheless, the role of crystal symmetry in enhancing the material's thermoelectric performance is abstruse. Here, we employ the interplay between crystal symmetry and native point defects towards high zT of IV-VI semiconductor GeSe, which is scarce in thermoelectric study. Pristine orthorhombic GeSe has a low zT ~ 0.05 due to the high formation energy of Ge vacancy and thus the low carrier concentration (~ 1016 cm−3). Alloying GeSe with MnCdTe 2 stabilizes higher-symmetry rhombohedral structure at ambient conditions, thereby effectively lowering the formation energy of Ge vacancy and raising the carrier concentration by four orders of magnitude. Meanwhile, compared to orthorhombic GeSe, the rhombohedral Ge 1- y Bi y Se(MnCdTe 2) x own higher valley degeneracy and smaller band effective mass, rendering the decent Seebeck coefficient and larger carrier mobility, respectively. Moreover, the generated multiscale microstructures in rhombohedral Ge 0.96 Bi 0.04 Se(MnCdTe 2) 0.10 , including atomic-scale native Ge vacancies and substitution point defects, nanoscale domain structures, and micron-sized secondary phases effectively depress the lattice thermal conductivity. As a result, a state-of-the-art zT ~ 1.0 at 723 K is achieved in Ge 0.96 Bi 0.04 Se(MnCdTe 2) 0.10. These results attest to the efficacy of the interplay between crystal symmetry and native point defects towards high performance GeSe-based and other thermoelectric materials. [Display omitted] • Synergy of crystal symmetry and native point defect enables high figure of merit zT. • MnCdTe 2 alloying stabilizes rhombohedral GeSe and optimizes carrier concentration. • Crystal symmetry regulates band structure and thereby increases the power factor. • The induced multiscale microstructures restrain the lattice thermal conductivity. • A state-of-the-art zT value of ~ 1.0 at 723 K is achieved. [ABSTRACT FROM AUTHOR]

The phase-transition behavior in GeTe-based alloys has been intensively studied recently for enhancing their thermoelectric performance. In this study, thermal annealing induced phase transition and corresponding effects on the thermoelectric properties of Pb-doped GeTe-Bi 2 Te 3 alloys were systematically studied. By Pb doping, the lattice thermal conductivity of (Ge x Pb 1-x Te) 0.93 (Bi 2 Te 3) 0.07 alloys were significantly reduced due to the strong phonon scattering from point defects and stacking faults, resulting in a peak thermoelectric figure of merit ZT of ∼1.47 at 673 K for the (Ge 0.84 Pb 0.16 Te) 0.93 (Bi 2 Te 3) 0.07. By thermal annealing, the structure of GeTe-Bi 2 Te 3 alloys can be shifted from pseudo-cubic to rhombohedral phase, and corresponding induced lattice distortion and microstructures can further reduce the lattice thermal conductivity. The thermal annealing can also lead to the increase of hole concentration and carrier mobility of the GeTe-based alloys. Our results proved the significance of thermal annealing treatment for tuning the microstructures and thermoelectric properties in the GeTe based alloys with phase-transition behavior. Image 1 • Thermal annealing can induce phase transition in the GeTe-Bi 2 Te 3 alloys. • Point defects and stacking faults are introduced to reduce the thermal conductivity. • Hole concentration and carrier mobility are increased by thermal annealing. • The thermoelectric performance is enhanced by Pb doping. [ABSTRACT FROM AUTHOR]

The core effects of high entropy alloys distinguish high entropy alloying from ordinary multielement doping, allowing for a synergy of band structure and microstructure engineering. Here, a systematic synthesis, structural, theoretical, and thermoelectric study of multi‐principal‐element‐alloyed SnTe is reported. Toward high thermoelectric performance, the entropy of mixing needs to be high enough to make good use of the core effects, yet low enough to minimize the degradation of carrier mobility. It is demonstrated that high entropy of mixing extends the solubility limit of Mn while retaining the lattice symmetry, the enhanced Mn content elicits multiscale microstructures. The resulting ultralow lattice thermal conductivity of ≈0.32 W m−1 K−1 at 900 K in (Sn0.7Ge0.2Pb0.1)0.75Mn0.275Te is not only lower than the amorphous limit of SnTe but also comparable to those thermoelectric materials with complex crystal structures and strong anharmonicity. Co‐alloying of (Sn,Ge,Pb,Mn) also enhances band convergence and band effective mass, yielding good power factors. Further tuning of the Sn excess yields a state‐of‐the‐art zT ≈1.42 at 900 K in (Sn0.74Ge0.2Pb0.1)0.75Mn0.275Te. In view of the simple face‐centered‐cubic structure of SnTe‐based alloys, these results attest to the efficacy of entropy engineering toward a new paradigm of high entropy thermoelecrics. The core effects of high‐entropy alloying are utilized to elicit a synergy of all‐scale hierarchical microstructures and band structure optimization in (Sn,Ge,Pb,Mn)Te alloys. The resultant ultralow lattice thermal conductivity ≈0.32 W m−1 K−1 and state‐of‐the‐art zT of 1.42 at 900 K amount to a breakthrough in the burgeoning field of entropy engineering toward a paradigm‐shifting "high‐entropy thermoelectrics." [ABSTRACT FROM AUTHOR]

Intrinsic point defect plays a core role in improving thermoelectric performance of V 2 VI 3 polycrystalline materials. However, the control of intrinsic point defect is challenging during powder refinement due to the donor-like effect. This paper shows a systematic approach to thermal control the donor-like effect via engineering spark plasma sintering and hot deformation temperature. Bi 2 Te 2.85 Se 0.15 polycrystalline alloy with high electron concentration was chosen as example to validate the method. The low Seebeck coefficient of n -type Bi 2 Te 2.85 Se 0.15 polycrystalline, arising from strong donor-like effect after powder refinement, was enhanced by recovery role induced reduction in electron concentration through proper tuning sintering or deforming temperature. Repetitive hot deformation was applied to further improve the Seebeck coefficient via the recovery role and a maximum zT of 1.0 is obtained at 410 K. The present results give insights into developing higher performance V 2 VI 3 polycrystalline materials through thermal control the donor-like effect. [ABSTRACT FROM AUTHOR]

[Display omitted] • Realizing a phase transition from orthorhombic to rhombohedral and then to cubic. • The enhanced crystal symmetry and off-stoichiometric doping manipulate Ge vacancy. • Achieving a delicate balance between carrier concentration and carrier mobility. • The introduction of multiple phonon scattering reduces the thermal conductivity. • The maximum zT value of 0.62 at 573 K is achieved in cubic GeSeTe 0.15 (InSnTe 2) 0.2. In thermoelectrics, the native point defect enables the delicate balance between carrier concentration and carrier mobility. The proper native point defect is prerequisite for the carrier concentration optimization. However, the carrier mobility is inevitable to be affected in case of the carrier mean free path approaches to the size of point defects. Herein, we reported the stepwise Ge vacancy manipulation as a point defect engineering to realize the zT enhancement in cubic GeSe. The pristine orthorhombic GeSe has the ultralow zT of 0.05 due to the inferior carrier concentration arising from the low Ge vacancy concentration, as well as the low carrier mean free path. Hence, we adopted InSnTe 2 alloying to stabilize the cubic phase at ambient conditions and significantly increase the Ge vacancy concentration due to the generation of Ge precipitates, thereby increasing the carrier concentration by several orders of magnitude but damaging the carrier mobility. Subsequently, the moderate Te doping was adopted to make the Ge-precipitations to be dissolved into the matrix, thus mitigating the strong carrier scattering deriving from the Ge vacancy and enhancing the carrier mobility to some extent. Combined with the restrained thermal conductivity originating from the multiscale defects and the reduced sound velocity, the maximum zT of 0.62 at 573 K was achieved in cubic GeSeTe 0.15 (InSnTe 2) 0.2. These results demonstrated that the stepwise point defect manipulation is an effective strategy for enhancing the figure of merit in thermoelectric materials with intrinsic low carrier mobility. [ABSTRACT FROM AUTHOR]

[Display omitted] • An ultralow lattice thermal conductivity of ∼ 0.21 Wm-1K−1 at 398 K is obtained. • The donor-like effect in n-type Bi 2 Te 3 is suppressed. • A peak figure of merit of ∼ 1.03 at 473 K is obtained. • The influence of graphene structures on the interfacial design is discussed. • A single-couple thermoelectric device is assembled. Solution-processed Bi 2 Te 3 -based nanocomposites usually have rich nanostructures and interfaces that show great promise for promoting the thermoelectric performance according to the theoretical effects of "quantum confinement" and "topological insulator". Here, an interfacial engineering strategy is developed to enhance the thermoelectric performance of n-type Bi 2 Te 3 -based nanocomposites upon synergistically introducing liquid-phase-sintering (LPS) process and adding graphene oxide (GO). Enhanced phonon scatterings by interfacial GO and various nanograins lead to an ultralow lattice thermal conductivity of ∼ 0.21 Wm-1K−1 at 398 K for the 1 wt% GO-added Bi 2 Te 2.5 Se 0.5. Excess Te activated LPS process can improve the interfacial connection for good electrical conductivity without increasing the lattice thermal conductivity. The excess Te and addition of GO can also suppress the donor-like effect in Bi 2 Te 3 for optimizing the carrier concentration. Due to these synergetic effects, a peak figure of merit (ZT) of ∼ 1.03 at 473 K and an average ZT of ∼ 0.85 within 300–473 K can be achieved in the 1 wt% GO-added sample. A single-couple TE device is also made using our n-type 1 wt% GO-added Bi 2 Te 2.5 Se 0.5 and the traditional p-type Bi 0.5 Sb 1.5 Te 3 , showing a maximum power density of ∼ 0.06 Wcm−2 at temperature difference of 154.8 K. Moreover, the difference between GO and nitrogen-doped graphene as additives is also systematically discussed, providing a guide for the rational use of graphene in interfacial design. [ABSTRACT FROM AUTHOR]