Intrinsic Low Lattice Thermal Conductivity Research Articles

High cooling and power generation performance of α-MgAgSb with intrinsic low lattice thermal conductivity

α-MgAgSb is a promising near-room temperature thermoelectric material, characterized by its intrinsically low lattice thermal conductivity, a feature attributed to the significant atomic mass contrast and complex crystal structure. In this work, we achieved respective zTavg values of 0.58 in the temperature range of 150–300 K and 1.22 in the range of 300–550 K for α-MgAgSb, indicating exceptional potential for both cooling and power generation applications. Additionally, through the reduction of cross-sectional size, the stability of MgAgSb/Ag interface was enhanced under high temperature, which is crucial for the practical application of thermoelectric module. To verify the property of α-MgAgSb material, a 7-pair MgAgSb/Bi2Te3 module was fabricated, demonstrating a maximum cooling temperature difference ΔTmax of 60 K at hot-side temperature of 300 K and a power generation efficiency ηmax of 7.2 % with ΔT of 275 K. This work paves the way for the application of Mg-based thermoelectric materials.

Ultralow Thermal Conductivity of a Chalcogenide System Pt3Bi4Q9 (Q = S, Se) Driven by the Hierarchy of Rigid [Pt6Q12]12- Clusters Embedded in Soft Bi-Q Sublattice.

Knowledge of structure-property relationships in solids with intrinsic low thermal conductivity is crucial for fields such as thermoelectrics, thermal barrier coatings, and refractories. Herein, we propose a new "rigidness in softness" structural scheme for intrinsic low lattice thermal conductivity (κL), which embeds rigid clusters into the soft matrix to induce large lattice anharmonicity, and accordingly discover a new series of chalcogenides Pt3Bi4Q9 (Q = S, Se). Pt3Bi4S9-xSex (x = 3, 6) achieved an intrinsic ultralow κL down to 0.39 W/(m K) at 773 K, which is considerably low among the Bi chalcogenide thermoelectric materials. Pt3Bi4Q9 contains the rigid cubic [Pt6Q12]12- clusters embedded in the soft Bi-Q sublattice, involving multiple bonding interactions and vibration hierarchy. The hierarchical structure yields a large lattice anharmonicity with high Grüneisen parameters (γ) 1.97 of Pt3Bi4Q9, as verified by the effective scatter of low-lying optical phonons toward heat-carrying acoustic phonons. Consequently, the rigid-soft coupling significantly inhibits heat propagation, exhibiting low acoustic phonon frequencies (∼25 cm-1) and Debye temperatures (ΘD = 170.4 K) in Pt3Bi4Se9. Owing to the suppressed κL and considerable power factor (PF), the ZT value of Pt3Bi4S6Se3 can reach 0.56 at 773 K without heavy carrier doping, which is competitive among the pristine Bi chalcogenides. Theoretical calculations predicted a large potential for performance improvement via proper doping, indicating the great potential of this structure type for promising thermoelectric materials.

Promising thermoelectric performance in p-type AgBiSe2 doping with alkaline-earth metals

P-type AgBiSe2 is a promising thermoelectric material because of its intrinsic low lattice thermal conductivity and multiple valence band behavior. However, realizing high performance in p-type AgBiSe2 is challenging due to the doping bottleneck. In this work, the enhancement of thermoelectric performance for p-type AgBiSe2 has been successfully realized by adding an excess amount of Se and doping with alkaline-earth metals. We show that Ca or Mg doping at the Bi site effectively increases the room-temperature hole concentration and improves the electrical transport properties of p-type AgBiSe2. Combined with the intrinsic lower thermal conductivity, the maximum zT values of ∼0.74 and ∼0.63 at 448 K were achieved in AgBi0.98Ca0.02Se2 and AgBi0.98Mg0.02Se2, respectively. This work demonstrates a crucial strategy for improving the thermoelectric properties of p-type AgBiSe2-based materials.

N-Type Thermoelectric AgBiPbS3 with Nanoprecipitates and Low Thermal Conductivity.

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS3, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag2S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m-1 K-1, owing to the intrinsic low lattice thermal conductivity of Ag2S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS2, AgBiPbS3 demonstrates a significantly improved weighted mobility of >16 cm2 V-1 s-1 at 300 K, leading to an enhanced PF of 1.6 μW cm-1 K-2 at 300 K. The superior electrical transport in AgBiPbS3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS3, leading to further enhancement of PF with a PFave of 2.7 μW cm-1 K-2 in the temperature range of 300-823 K. Consequently, a relatively high ZTave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS3, highlighting the potential of AgBiPbS3 as an n-type thermoelectric sulfide.

Realizing high thermoelectric performance in CeCl3-doped n-type SnSe polycrystals

SnSe is a promising thermoelectric material because of its intrinsic low lattice thermal conductivity. However, the inferior electrical transport properties restrict the thermoelectric performance of n-type SnSe polycrystals especially at lower temperature range. In this work, an effective strategy to enhance the performance of n-type SnSe polycrystals by CeCl3 doping is reported. CeCl3 doping increased both the carrier concentration and electrical conductivity. The high weighted mobility contributed to the high power factor over the whole temperature range. Combined with the reduced thermal conductivity, a peak zT of ∼1.3 at 823 K was obtained in SnSe0.92+1 mol% CeCl3 with an average zT of ∼0.36 in the temperature range of 300–773 K. This work suggests that CeCl3 doping is an effective way to optimize the thermoelectric properties of n-type SnSe polycrystals.

Room temperature high thermoelectric performance of Bi-based full-Heusler compounds CsxRb[formula omitted]Bi with strong anharmonicity

Bismuth-based compounds have been identified as a class of promising candidates for the realization of high-performance thermoelectrics, derived from their exceptional electrical conductivity and intrinsic low lattice thermal conductivity κL. Here, we employed first-principles calculations, self-consistent phonon theory, and compressive sensing technique in conjunction with the Boltzmann transport equation to investigate the anharmonic thermoelectric properties of full-heusler bismuth compounds CsxRb3−xBi. As the Cs content increases, the κL of CsxRb3−xBi decreases due to impurity scattering effects, and then increases. Among them, at 300 K, Rb3Bi has the largest κL of 0.69 Wm−1K−1, while Cs2RbBi has the smallest κL of 0.45 Wm−1K−1, both of which are lower than the κL of traditional bismuth-based thermoelectric materials. The ultralow κL originates from the strong lattice anharmonicity. In addition, the coexistence of high dispersion and flat band edges leads to high thermoelectric power factor at optimal doping concentration. As a result, the largest thermoelectric figure of merit ZT values of 2.59 (4.19) at 300 (500) K were obtained for CsRb2Bi at optimal hole doping level. These findings suggest that CsxRb3−xBi compounds are superior for thermal management and thermoelectric applications.

Enhancement of Thermoelectric Performance for CuCl Doped P-Type Cu2Sn0.7Co0.3S3

Cu2SnS3 (CSS) has gained great attraction due to its constitutive earth-abundant elements and intrinsic low lattice thermal conductivity, κl, potentially providing high quality factor, B, and high zT value. However, the lack of band convergence is the bottleneck to enhancing the thermoelectric performance of Cu2SnS3 when performing the band engineering. To study the doping effect on the band structure and the thermoelectric performance, the composite Cu2Sn0.7Co0.3S3-xCuCl (x = 0, 0.1, 0.2, 0.3) (CSCS-xCuCl) has been investigated for the first time. The samples showed excellent data repeatability at high temperatures of up to 700 K. It was found that CuCl could compensate the Cu loss, enhance the phonon scattering and minimize the adverse effect on the power factor, PF. The ultralow lattice thermal conductivity could reach 0.38 W m-1 K-1 for the nominal composition of CSCS-0.3CuCl at 700 K. A peak zT of 0.56 (evaluated with no cold finger effect) was realized at 700 K when x = 0.3, which is almost double the performance of pristine samples.

Revealing the promising near-room-temperature thermoelectric performance in Ag2Se single crystals

With high carrier mobility and intrinsic low lattice thermal conductivity, Ag2Se compounds have attracted increasing attention for thermoelectric application near room temperature. Due to its phase transition at ∼406 K and resulting thermal volume expansion, the growth and thermoelectric properties of large-sized Ag2Se single crystals have seldom been reported so far. In this work, the vertical Bridgeman method was used for growing bulk Ag2Se single crystal, with an orientation preference along low-symmetric (201) plane. The Hall mobility as high as 2 000 cm2/(V·s) and weak electron-phonon coupling contributes to a high electronic quality BE of ∼7.0 in near-room-temperature β-Ag2Se single crystals, which is superior to the high-temperature phase α-Ag2Se. The observed low lattice thermal conductivity of 0.8 W/(m·K) at 300 K is due to the low group speeds and strong anharmonicity. A promising peak zT of 0.66 at 375 K and an average zT of 0.65 at 300–375 K were realized in β-Ag2Se crystals. The low Vickers hardness and good ductile properties were confirmed by experiment and theoretical analysis. This work not only synthesized large-sized and highly-orientated Ag2Se crystals, but also revealed its great potential of thermoelectric performance and mechanical properties for various applications near room temperature.

Thermoelectric properties of ductile Ag2S0.7Te0.3 prepared via zone melting method

Recently, the discovery of ductility inorganic semiconductors Ag2S-based alloys have drawn significant interest in hetero-shaped thermoelectric applications. In this work, Ag2S0.7Te0.3 inorganic thermoelectric compound was prepared using the zone melting technique. The crystal structure of Ag2S0.7Te0.3 is similar to that of the cubic-structured Ag2S compound, in which S and Te atoms are randomly occupied at the anionic sites. The observed high hall carrier mobility of ∼410 cm2V−1s−1 and intrinsic low lattice thermal conductivity of ∼0.2 Wm−1K−1 lead to a maximum zT of 0.30 at 600 K for Ag2S0.7Te0.3. In addition, its mechanical properties including microhardness Hv and fracture toughness Kc are measured, showing great potential for thermoelectric application.

Janus γ-GeSSe Monolayer as a High-Performance Material for Photocatalysis and Thermoelectricity

Two-dimensional Janus materials have attracted increasing attention in recent years because of their potential as materials for energy applications, such as photocatalysis and thermoelectricity (TE). Here, for the first time, we propose a monolayer Janus structure with the Mexican-hat band, γ-GeSSe, by replacing the S atoms on one side of the synthesized γ-GeS with Se atoms. Using first-principles calculations based on density functional theory (DFT), we show that γ-GeSSe is mechanically, dynamically, and thermally stable. The mechanical, electronic, optical, and transport properties of monolayer Janus γ-GeSSe are also investigated in the DFT calculations. We find that the ideal strengths of the Janus γ-GeSSe are 6.08, 8.6, and 10.08 GPa for the armchair, zigzag, and biaxial directions, respectively. The Janus γ-GeSSe is an indirect-gap semiconductor with a band gap of 1.131 eV. Our results show the Janus γ-GeSSe as a photocatalysis material for water splitting with a high solar-to-hydrogen efficiency of up to 28.78%. This is because the combination of features includes an intrinsic electric field, high optical absorption coefficient, and carrier mobility. In addition, with the γ-structure, the Janus γ-GeSSe shows an intrinsic low lattice thermal conductivity of 3.33 W/mK at room temperature, which will contribute to obtaining high TE efficiency. By using the Boltzmann transport theory, we find that the maximum TE power factor and figure of merit of the Janus γ-GeSSe reach 55–85 mW/mK2 and 0.7–0.9 in the temperature range from 300 to 900 K, respectively. Thus, our findings not only contribute to proposing a Janus material but also suggest that it can be used as an energy material with high-performance photocatalysis and TE.

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

AbstractLayer‐structured GeSb2Te4 is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge1–xInxSb2Te4 single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c‐axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c‐axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb2Te4 and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323–773 K are obtained in Ge0.93In0.07Sb2Te4 along the c‐axis direction, which are 54% and 79% higher than that of the pristine GeSb2Te4 single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb2Te4, and shed light on the performance optimization of other layered thermoelectric materials.

Synergistically optimized electron and phonon transport of p-type BiCuSeO oxyselenides via Pb dopant and Te composite

Layer-structured BiCuSeO oxyselenides with intrinsic low lattice thermal conductivity, non-toxicity, non-pollution and outstanding thermal stability is regarded as a promising thermoelectric material, but its application is limited by the poor electrical conductivity. In this paper, Pb-doped BiCuSeO (Bi1-xPbxCuSeO) was fabricated via a fast preparation method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS). Although Pb doping could greatly improve the electrical conductivity of BiCuSeO, the thermal conductivity increased as well, and it limits the thermoelectric performance. To further optimize the electron and phonon transport properties, Te powders and Te nanowires were introduced in Bi0·9Pb0·1CuSeO by ball milling, respectively. As a result, the thermal conductivity largely decreased with a maintained electrical performance. Ultimately, a maximum ZT value of 1.0 at 873 K was achieved for the 5 vol% Te nanowire + Bi0·9Pb0·1CuSeO sample, which is about 2.5 times as large as that for pristine BiCuSeO. Our work paves an alternative strategy for tuning the thermoelectric performance of BiCuSeO through Bi site doping combined low-dimension nanocomposites.

Preparation and thermoelectric transport properties of BiSe and Sb-doped BiSe single crystals

<p indent="0mm">Exploring high thermoelectric performance in single crystal materials with intrinsic low lattice thermal conductivity has become an important field for thermoelectric research. BiSe, as a member of (Bi₂Se₃)_{<italic>m</italic>}(Bi₂)_{<italic>n</italic>} series superlattice compounds, has a very low intrinsic thermal conductivity due to the existence of Bi₂ bilayer in its layered crystal structure. Moreover, it is a new and promising thermoelectric material due to the non-toxic and environmentally friendly chemical compositions. The special layered structure of BiSe makes its thermoelectric transport properties significantly anisotropic. In this paper, BiSe single crystal was prepared by Bridgman method, and its anisotropic thermoelectric transport properties were investigated. The results show that the sample has higher conductivity and thermal conductivity in the in-plane direction, while the lower Seebeck coefficient makes the thermoelectric figure of merit <italic>ZT</italic> value in this direction lower than that in the interlamellar direction. The maximum values of <italic>ZT</italic> in the two directions are 0.11 <sc>(573 K)</sc> and 0.16 <sc>(623 K),</sc> respectively. Compared with polycrystalline BiSe, the single crystal sample exhibits lower electric transport performance, which can be attributed to the difference of scattering coefficient. The carrier concentration of BiSe single crystal is regulated by Sb doping, which effectively improves the Seebeck coefficient of the samples and optimizes the electrical transport performance. In the meanwhile, Sb doping can produce lattice distortion and reduce the lattice thermal conductivity. The final <italic>ZT</italic> value of Bi_0.8Sb_0.2Se single crystal increases by about 1 fold, reaching 0.34 <sc>(573 K)</sc> in the interlamellar direction and 0.20 <sc>(573 K)</sc> in the in-plane direction.

Enhanced average thermoelectric properties of n‑type Mg3Sb2 based materials by mixed-valence Ni doping

Due to their high band degeneracy and intrinsic low lattice thermal conductivity, n-type Mg3Sb2-based thermoelectric materials have drawn a lot of attention in thermoelectric applications in recent years. By using simple and efficient fast induction melting and hot pressing, the n-type Mg3.5−xNixSbBi0.96Te0.04 (x = 0, 0.015, 0.02, 0.03, 0.04, 0.05, and 0.06) samples have been successfully synthesized. The structure of Mg3.5−xNixSbBi0.96Te0.04 samples is examined, as well as their thermoelectric properties. The findings demonstrate that the mixed valence of Ni2+ and Ni3+ that occupied distinct Mg sites occurred as the Ni doping level grew in the Mg3.5−xNixSbBi0.96Te0.04 samples. The trivalent Ni3+ increases carrier concentrations dramatically, while the divalent Ni2+ increases the effective mass. The mixed valence of Ni causes lattice disorder, which enhances phonon scattering and reduces lattice thermal conductivity. As a result, the Mg3.485Ni0.015SbBi0.96Te0.04 sample has a high ZT value of 1.58 at 743 K and an average ZT value of 1.22 from 323 to 743 K. This work suggests that the mixed valence of cationic dopant effectively improve the thermoelectric performance of n-type Mg3Sb2-based compounds.

Band engineering and improved thermoelectric performance in p-type SmMg2Sb2: A first-principles study

The thermoelectric properties of p-type SmMg2Sb2 are investigated by using the density functional theory and the Boltzmann transport equation method. It is found that SmMg2Sb2 shows a narrow band gap characteristic, and the crystal field splitting and spin-orbital coupling effect result in a singly degenerate valence band maximum. Meanwhile, SmMg2Sb2 exhibits an intrinsic low lattice thermal conductivity owing to the obvious phonon softening in the low-frequency acoustic branches. Based on a careful analysis of the band structure, we propose a scheme of metallic dopant at Sm site to promote the valence band convergence and improve the Seebeck coefficient. The improved thermoelectric properties are numerically confirmed in the Ba-doped p-type SmMg2Sb2, where a significantly improved ZT of 1.6 at 750 K is predicted with the synergistical effects of band engineering and enhanced phonon scattering.

Phonon Symphony of Stacked Multilayers and Weak Bonds Lowers Lattice Thermal Conductivity.

Controlling lattice vibrations to obtain intrinsic low thermal conductivity play a critical role in thermal management of electronic and photonic devices, energy converters, and thermal insulation, which necessitates exploring new compounds and a thorough understanding of their chemical structure, bonding, and lattice dynamics. Herein, a new chalcogenide, Ga6 Cr5 Se16 , shows intrinsic low lattice thermal conductivity κlat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe4 layers (g'), close-packed Cr3+ Se6 layers (c), GaSe4 layers (g) and loosely-stacked Cr2+ Se6 layers (c') along the c-axis. In this structure, a wide variety of chemical bonding is arranged in each layer, such as covalent Ga-Se, covalent Cr3+ -Se, and weaker Cr2+ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga6 Cr5 Se16 realizes an intrinsic low κlat of 0.79 W m- 1 K- 1 at 323 K, which is almost four times, or twice lower than that of Cr3 Se4 (2.95 W m- 1 K- 1 ), or Cr2 Se3 (1.56 W m- 1 K- 1 ), Ga2 Se3 (1.36 W m- 1 K- 1 ) at 323 K, respectively. These insights will offer comprehensive understanding of the phonon propagation in complex layered chalcogenides, andalso shed useful light on future design of low-κlat solids.

Ag rearrangement induced metal-insulator phase transition in thermoelectric MgAgSb

In recent years, materials with metal-to-insulator phase transitions have attracted great attention due to their property variations before and after the transition. As nearly all good thermoelectric materials are small band-gap semiconductors, a metal-insulator transition makes a profound difference in thermoelectric performance. Revealing the microscopic reasons for these variations is critical to understanding the structure-property relation in these materials. MgAgSb is an important thermoelectric material around room temperature, due to its high zT value and intrinsic low lattice thermal conductivity in the semiconducting α-phase. However, the thermoelectric properties of mid-temperature β-phase MgAgSb significantly deteriorate because the β-phase has no band gap. In this work, we reveal that the Ag atomic rearrangement between the two phases is the major reason responsible for the metal-insulator transition and deterioration of thermoelectric properties. In MgAgSb, there are strong interactions between Mg and Sb, as well as Ag and Sb, leading to bands of mixed character around the Fermi Level. In α-MgAgSb, the Mg–Sb bands in the conduction band minimum separate well with the bands around the valence band maximum, and form a finite band gap. Due to the Ag rearrangement and the formation of a quasi-two-dimensional structure of β-MgAgSb, the interactions among Mg–Ag–Sb atoms form multiple bands crossing the Fermi level, and realize the “two-dimensional metallization”. In addition, the structural variations in the β-phase results in higher phonon velocities and removal of additional low-frequency optical phonons as only shown in the α-phase, both of which leading to the relatively high κL of the β-phase. Our work provides new perspectives for the performance research and application of materials with metal-insulator phase transitions.

Synergistically enhanced thermoelectric properties in n-type Bi6Cu2Se4O6 through inducing resonant levels

Oxygen-containing semiconductors are considered to be promising thermoelectric materials because of the high physicochemical stability. New layered oxyselenide Bi6Cu2Se4O6 possesses intrinsic low lattice thermal conductivity and can be modulated to n-type semiconductor through halogen doping, which is considered as a potential n-type thermoelectric oxide. In this work, first-principles calculations are utilized to obtain the electronic and phonon band structures for deeply recognizing the transport features of intrinsic Bi6Cu2Se4O6. Furthermore, Nb is selected as donor dopant in Bi6Cu2Se3.6Cl0.4O6, realizing a significantly improved power factor of ∼ 4.3 μV cm−1 K−2, which originates from the simultaneously optimized carrier concentration by effective doping and strengthened effective mass by inducing resonant levels around Fermi level. Meanwhile, the enhancement of phonon scattering are introduced, leading to a low lattice thermal conductivity of ∼ 0.68 W m−1 K−1. Finally, thanks to the effective doping of Nb and the introduction of resonant levels, a maximum ZT ∼ 0.4 at 873 K and an average ZT ∼ 0.21 from 303 to 873 K can be obtained in n-type Bi5.91Nb0.09Cu2Se3.6Cl0.4O6. This study broadens the applicability of resonant levels to optimize performances of thermoelectric oxides, and promotes the potential application of n-type Bi6Cu2Se4O6-based thermoelectric materials in medium temperature range.

Flat phonon modes driven ultralow thermal conductivities in Sr3AlSb3 and Ba3AlSb3 Zintl compounds

Searching for compounds with intrinsic low lattice thermal conductivity has been proven a successful strategy for achieving high thermoelectric performance. Herein, employing density functional theory calculations combined with electron and phonon Boltzmann transport theories, we report that Sr3AlSb3 and Ba3AlSb3 within the Zintl 3–1–3 compositional family exhibit record low thermal conductivities of 0.78 and 0.55 W/mK at room temperature, respectively. These low thermal conductivities are rooted in low-energy optical phonon modes with strong anharmonicity and the emergence of high-energy flat optical phonon modes with zero contribution to the lattice thermal conductivity. Heavier cationic atoms are found to soften low-lying optical phonon modes, which enhance phonon scattering and, therefore, favor a lower thermal conductivity. These combined characteristics lead to high and balanced figure of merit values around 2.3 for Zintl Ba3AlSb3 at both optimal p-type and n-type doping and high temperature. Our work highlights the important role of flat optical phonon modes on designing promising thermoelectric materials with intrinsic low thermal conductivity.

Integrating band engineering with point defect scattering for high thermoelectric performance in Bi2Si2Te6

A layer-structure compound Bi2Si2Te6 is introduced as a promising thermoelectric material. The intrinsic low lattice thermal conductivity of Bi2Si2Te6 is because of the low phonon group velocities near the Γ point and the overlap of optical and acoustic phonon branches. A further decreased thermal conductivity is obtained by alloying Sb at the Bi site for the additional point-defect scattering of phonons. This Sb alloying also enhances the density-of-states near the valence band maximum for a larger carrier effective mass, and suppresses the bipolar effect by increasing the carrier concentration and widening the band gap. Benefitting from both the enhancement of power factor and the decrease of lattice thermal conductivity, the peak zT value is improved from ∼ 0.45 at 573 K for Bi2Si2Te6 to ∼ 1.2 at 773 K for BiSbSi2Te6.