Nature of Thermal Hysteresis of Thermoelectric Properties in Ag2TexS1-x Compounds.

Ag2TexS1-x usually undergo various phase structures upon heating or cooling processes; however, the correlation between the heat treatment, the phase structure, and the physical properties is still a controversy. Herein, three different phases are realized for Ag2TexS1-x (0.35 ≤ x ≤ 0.65) samples during the heat treatment, including the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase. The metastable amorphous phase is an intermediate phase formed during transition from the high-temperature cubic phase to the low-temperature crystalline phase upon cooling via a solid-state conversion rather than the conventional liquid quenching process. The relative content of these three phases is highly sensitive to the heat treatment process. This as-formed low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase convert into the low-temperature crystalline phase and high-temperature cubic phase through long-time dwelling at the temperature below or above the transition temperature around 567 K, respectively. The status of the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase significantly affects the thermoelectric properties, resulting in the thermal hysteresis of thermoelectric properties. Below the phase transition temperature (TM), the electrical conductivity of the amorphous phase surpasses that of the low-temperature crystalline phase, which shows a growth of 112% for the Ag2Te0.60S0.40 sample annealed at 823 K in comparison with that of the sample annealed at 473 K. For Ag2Te0.50S0.50 samples annealed at 473 K, the maximum ZT value reaches 1.02 at 623 K during the initial test, while the maximum ZT value is improved to 1.34 at 523 K in the second-round test.

Balanced High Thermoelectric Performance in n-Type and p-Type CuAgSe Realized through Vacancy Manipulation.

As a liquid-like material, CuAgSe has high carrier mobility and ultralow lattice thermal conductivity. It undergoes an n-p conduction-type transition during ß- to α-phase transition with increasing temperature. Moreover, optimization of the thermoelectric performance of CuAgSe is rather difficult, owing to the two-carrier conduction in this material. In this work, we reported the free tuning of the conduction type and thermoelectric performance of CuAgSe by manipulating the cation vacancies. Positron annihilation measurements reveal that the increase in CuAg content can effectively suppress the cation vacancies and reduce the hole carrier concentration, resulting in n-type conduction at high temperatures. Doping with Zn at the Cu sublattice in the CuAg-excessive CuAgSe can further decrease the number of vacancies, leading to a significant decrease in hole carrier concentration. Furthermore, the reduction of vacancies leads to weakening of carrier scattering. As a result, carrier mobility is also enhanced, thus improving the thermoelectric performance of n-type CuAgSe. On the other hand, high-performance p-type CuAgSe can be achieved by decreasing the CuAg content to introduce more cation vacancies. Ultimately, both n-type and p-type CuAgSe with superb thermoelectric performance are obtained, with a zTmax of 0.84 in Cu1.01Ag1.02Zn0.01Se (n-type) and 1.05 in (CuAg)0.96Se (p-type) at 600 K and average zT of 0.77 and 0.94 between 470 and 630 K for n-type and p-type, respectively.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 µW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Excellent Stability of Ga-Doped Garnet Electrolyte against Li Metal Anode via Eliminating LiGaO2 Precipitates for Advanced All-Solid-State Batteries.

Ga-doped garnet-type Li7La3Zr2O12 (Ga-LLZO) ceramics have long been recognized as ideal electrolyte candidates for all-solid-state lithium batteries (ASSLBs). However, in this study, it is shown that Ga-LLZO easily and promptly cracks in contact with molten lithium during the ASSLB assembly. This can be mainly ascribed to two aspects: (i) lithium captures O atoms and reduces Ga ions of the Ga-LLZO matrix, leading to a band-gap closure from >5 to <2 eV and a structural collapse from cubic to tetrahedral; and (ii) the in situ-formed LiGaO2 impurity phase has severe side reactions with lithium, resulting in huge stress release along the grain boundaries. It is also revealed that, while the former process consumes hours to take effect, the latter one is immediate and accounts for the crack propagation of Ga-LLZO electrolytes. A minute SiO2 is preadded during the synthesis of Ga-LLZO and found effective in eliminating the LiGaO2 impurity phase. The SiO2-modified Ga-LLZO solid electrolytes display excellent thermomechanical and electrochemical stabilities against lithium metals and well-reserved ionic conductivities, which was further confirmed by half-cells and full batteries. This study contributes to the understanding of the stability of garnet electrolytes and promotes their potential commercial applications in ASSLBs.

High-Performance Thermoelectric α-Ag9 Ga1-x Te6 Compounds with Ultralow Lattice Thermal Conductivity Originating from Ag9 Te2 Motifs.

We have determined the complex atomic structure of high-temperature α-Ag9 GaTe6 phase with a hexagonal lattice (P63 mc space group, a=b=8.2766 Å, c=13.4349 Å). The structure has outer [GaTe4 ]5- tetrahedrons and inner [Ag9 Te2 ]5+ clusters. All of the Ag ions are disorderly distributed in the lattice. Seven types of the Ag atoms constitute the cage-like [Ag9 Te2 ]5+ clusters. The highly disordered Ag ions vibrate in-harmonically, producing strong coupling between low frequency optical phonons and acoustic phonons. This in conjunction with a low sound velocity of 1354 m s-1 leads to an ultralow thermal conductivity of 0.20 W m-1 K-1 at 673 K. Meanwhile, the deficiency of Ga in Ag9 Ga1-x Te6 compounds effectively optimizes the electronic transport properties. Ag9 Ga0.91 Te6 attains a highest power factor of 0.40 mW m-1 K-2 at 673 K. All these contribute to a much-improved ZT value of 1.13 at 623 K for Ag9 Ga0.95 Te6 , which is 41 % higher than that of the pristine Ag9 GaTe6 sample.

2D Homologous Series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1) and Commensurately Modulated Sr2F2Bi2/3S2.

We report three new mixed-anion two-dimensional (2D) compounds: SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2. Their structures as well as the parent compound SrFBiS2 were refined using single-crystal X-ray diffraction data, with the sequence of SrFBiS2, SrFPbBiS3, and SrFAg0.5Bi1.5S3 defining the new homologous series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1). Sr2F2Bi2/3S2 has a different structure, which is modulated with a q vector of 1/3b* and was refined in superspace group X2/m(0ß0)00 as well as in the 1 × 3 × 1 superstructure with space group C2/m (with similar results). Sr2F2Bi2/3S2 features hexagonal layers of alternating [Sr2F2]2+ and [Bi2/3S2]2-, and the modulated structure arises from the unique ordering pattern of Sr2+ cations. SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2 are semiconductors with band gaps of 1.31, 1.21, and 1.85 eV, respectively. The latter compound exhibits room temperature red photoluminescence at ∼700 nm.

Electrically Tunable Antiferroelectric to Paraelectric Switching in a Semiconductor.

The monoclinic α-Cu2Se phase is the first multipolar antiferroelectric semiconductor identified recently by electron microscopy. As a semiconductor, although there are no delocalized electrons to form a static macroscopic polarization, a spontaneous and localized antiferroelectric polarization was found along multiple directions. In conventional ferroelectrics, the polarity can be switched by an applied electric field, and a ferroelectric to paraelectric transition can be modulated by temperature. Here, we show that a reversible and robust antiferroelectric to paraelectric switching in a Cu2Se semiconductor can be tuned electrically by low-voltage and high-frequency electric pulses, and the structural transformations are imaged directly by transmission electron microscopy (TEM). The atomic mechanism of the transformation was assigned to an electrically triggered cation rearrangement with a low-energy barrier. Due to differences of the antiferroelectric and paraelectric phases regarding their electrical, mechanical, and optical properties, such an electrically tunable transformation has a great potential in various applications in microelectronics.

Enhanced Thermoelectric Properties of Cu2SnSe3-Based Materials with Ag2Se Addition.

In this work, (Ag, In)-co-doped Cu2SnSe3-based compounds are prepared using a self-propagating high-temperature synthesis process. Ag2Se and as-synthesized (Ag, In)-co-doped Cu2SnSe3-based powders are mixed in a proportion according to the formula of Cu1.85Ag0.15Sn0.91In0.09Se3/x Ag2Se (x = 0, 3, 4, and 5%), which is followed by a subsequent plasma-activated sintering (PAS) to obtain consolidated composite bulks. A sandwich experiment is designed to reveal the evolution of the microstructure and phase composition of the composite samples during the PAS process. We investigate the reaction mechanism between Ag2Se and Cu2SnSe3-based matrix as well as the influence of Ag2Se on the phase composition, microstructure, and thermoelectric transport properties of the composites. Ag2Se addition is proven to be effective to improve Ag solubility in the Cu1.85Ag0.15Sn0.91In0.09Se3 matrix and introduce a CuSe secondary phase and an Ag-rich phase at grain boundaries. The electrical conductivity of Cu1.85Ag0.15Sn0.91In0.09Se3/x Ag2Se (x = 0, 3, 4, and 5%) composites decreases while the Seebeck coefficient increases with increasing Ag2Se addition, resulting in an optimized power factor. Moreover, benefiting from the collective phonon scattering at various defects induced by Ag2Se addition, the composite samples exhibit significantly suppressed lattice thermal conductivity, which reaches as low as 0.11 W m-1 K-1 at 700 K for the x = 5% sample. A peak figure-of-merit (ZT) of 1.26 at 750 K and an average ZT of 0.75 at 300-800 K are obtained for Cu1.85Ag0.15Sn0.91In0.09Se3/5% Ag2Se. This work provides an efficient way to improve average ZT values of Cu2SnSe3-based compounds for promising power generation at intermediate temperatures.

Enhancing Thermoelectric Performance of AgSbTe2-Based Compounds via Microstructure Modulation Combining with Entropy Engineering.

Modulation of the microstructure and configurational entropy tuning are the core stratagem for improving thermoelectric performance. However, the correlation of evolution among the preparation methods, chemical composition, structural defects, configurational entropy, and thermoelectric properties is still unclear. Herein, two series of AgSbTe2-based compounds were synthesized by an equilibrium melting-slow-cooling method and a nonequilibrium melting-quenching-spark plasma sintering (SPS) method, respectively. The equilibrium method results in coarse grains with a size of >300 µm in the samples and a lower defect concentration, leading to higher carrier mobility of 10.66 cm2 V-1 s-1 for (Ag2Te)0.41(Sb2Te3)0.59 compared to the sample synthesized by nonequilibrium preparation of 1.83 cm2 V-1 s-1. Moreover, tuning the chemical composition of nonstoichiometric AgSbTe2 effectively improves the configurational entropy and creates a large number of cation vacancies, which evolve into dense dislocations in the samples. Owing to all of these in conjunction with the strong inharmonic vibration of lattice, an ultralow thermal conductivity of 0.51 W m-1 K-1 at room temperature is achieved for the (Ag2Te)0.42(Sb2Te3)0.58 sample synthesized by the equilibrium preparation method. Due to the enhanced carrier mobility, optimized carrier concentration, and low thermal conductivity, the (Ag2Te)0.42(Sb2Te3)0.58 sample synthesized by the equilibrium preparation method possesses the highest ZT of 1.04 at 500 K, more than 60% higher than 0.64 at 500 K of the same composition synthesized by nonequilibrium preparation.

Doping Achieves High Thermoelectric Performance in SnS: A First-Principles Study.

Among the thermoelectrics discovered in the past few decades, SnS stands out as a promising candidate for its inexpensive, earth-abundant, and environment-friendly merits. However, with emerging research on optimizing the thermoelectric performance of SnS, there are not many theoretical studies giving explicit analysis about the underlying mechanism of charge and heat transport in the system. In this work, we find an abnormal optical-phonon-dominated κL in SnS with heat-carrying optical phonons showing higher group velocity than acoustic phonons. These high-velocity phonon modes are contributed by "antiphase" movements in the adjacent Sn-S sublayers. Meanwhile, we calculate the electrical properties with a nonempirical carrier lifetime and discover that the optical phonon also plays an essential role in the charge transport process, limiting the carrier mobility dominantly. Our calculation results suggest that p-type SnS can achieve a maximal ZT of 1.68 at 850 K and a hole concentration of 5.5 × 1019 cm-3 even without band engineering. We further investigate 11 possible dopants and screen out 4 candidates (Na, K, Tl, and Ag) that effectively boost the hole concentration in SnS. Defect calculations reveal that Na is the best dopant for SnS, while we also suggest K and Tl as potential candidates, for they can also help SnS achieve its optimal hole concentration. To ensure that each dopant reaches its best doping effect, we suggest that doped SnS samples be synthesized under sulfur-excess circumstances and the synthesis temperature be higher than 1353 K. Our findings provide insight into the charge and heat transport process of SnS and pave the way for the rational design of high-performance SnS-based thermoelectric materials.

Enhancing the Thermoelectric and Mechanical Properties of Bi0.5Sb1.5Te3 Modulated by the Texture and Dense Dislocation Networks.

Bi2Te3-based materials are dominating thermoelectrics for almost all of the room-temperature applications. To meet the future demands, both their thermoelectric (TE) and mechanical properties need to be further improved, which are the requisite for efficient TE modules applied in areas such as reliable micro-cooling. The conventional zone melting (ZM) and powder metallurgy (PM) methods fall short in preparing Bi2Te3-based alloys, which have both a highly textured structure for high TE properties and a fine-grained microstructure for high mechanical properties. Herein, a mechanical exfoliation combined with spark plasma sintering (ME-SPS) method is developed to prepare Bi0.5Sb1.5Te3 with highly improved mechanical properties (correlated mainly to the dislocation networks), as well as significantly improved thermoelectric properties (correlated mainly to the texture structure). In the method, both the dislocation density and the orientation factor (F) can be tuned by the sintering pressure. At a sintering pressure of 20 MPa, an exceptional F of up to 0.8 is retained, leading to an excellent power factor of 4.8 mW m-1 K-2 that is much higher than that of the PM polycrystalline. Meanwhile, the method can readily induce high-density dislocations (up to ∼1010 cm-2), improving the mechanical properties and reducing the lattice thermal conductivity as compared to the ZM ingot. In the exfoliated and then sintered (20 MPa) sample, the figure-of-merit ZT = 1.2 (at 350 K), which has increased by about ∼20%, and the compressive strength has also increased by ∼20%, compared to those of the ZM ingot, respectively. These results demonstrate that the ME-SPS method is highly effective in preparing high-performance Bi2Te3-based alloys, which are critical for TE modules in commercial applications at near-room temperature.

Electroresistance in multipolar antiferroelectric Cu2Se semiconductor.

Electric field-induced changes in the electrical resistance of a material are considered essential and enabling processes for future efficient large-scale computations. However, the underlying physical mechanisms of electroresistance are currently remain largely unknown. Herein, an electrically reversible resistance change has been observed in the thermoelectric α-Cu2Se. The spontaneous electric dipoles formed by Cu+ ions displaced from their positions at the centers of Se-tetrahedrons in the ordered α-Cu2Se phase are examined, and α-Cu2Se phase is identified to be a multipolar antiferroelectric semiconductor. When exposed to the applied voltage, a reversible switching of crystalline domains aligned parallel to the polar axis results in an observed reversible resistance change. The study expands on opportunities for semiconductors with localized polar symmetry as the hardware basis for future computational architectures.

Removing the Oxygen-Induced Donor-like Effect for High Thermoelectric Performance in n-Type Bi2Te3-Based Compounds.

Bismuth telluride-based alloys are the best performing thermoelectric materials near room temperature. Grain size refinement and nanostructuring are the core stratagems for improving thermoelectric and mechanical properties. However, the donor-like effect induced by grain size refinement strongly restricts the thermoelectric properties especially in the vicinity of room temperature. In this study, the formation mechanism for the donor-like effect in Bi2Te3-based compounds was revealed by synthesizing five batches of polycrystalline samples. We demonstrate that the donor-like effect in Bi2Te3-based compounds is strongly related to the vacancy defects (VBi‴ and VTe···) induced by the fracturing process and oxygen in air for the first time. The oxygen-induced donor-like effect dramatically increases the carrier concentration from 2.5 × 1019 cm-3 for the zone melting ingot and bulks sintered with powders ground under an inert atmosphere to 7.5 × 1019 cm-3, which is largely beyond the optimum carrier concentration and seriously deteriorates the thermoelectric performance. Moreover, it is found that both avoiding exposure to air and eliminating the thermal vacancy defects (VBi‴ and VTe···) via heat treatment before exposure to air can effectively remove the donor-like effect, producing almost the same carrier concentration and Seebeck coefficient as those of the zone melting ingot for these samples. Therefore, a defect equation of oxygen-induced donor-like effect was proposed and was further explicitly corroborated by positron annihilation measurement. With the removal of donor-like effect and improved texturing via multiple hot deformation (HD) processes, a maximum power factor of 3.5 mW m-1 K-2 and a reproducible maximum ZT value of 1.01 near room temperature are achieved. This newly proposed defect equation of the oxygen-induced donor-like effect will provide a guideline for developing higher-performance V2VI3 polycrystalline materials for near-room-temperature applications.

Synergistically Enhanced Thermoelectric Performance of Cu2SnSe3-Based Composites via Ag Doping Balance.

As an ecofriendly and low-cost thermoelectric material, Cu2SnSe3 has recently drawn much attention. In this work, the thermoelectric properties of Cu2SnSe3-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu1.85Ag0.15SnSe3. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu1.85Ag0.15Sn1-yInySe3 samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3 sample. Considering that Ag2S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu1.85Ag0.15Sn0.91In0.09Se3 bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 µW cm-1 K-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m-1 K-1 at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite, which is nearly an increase of 204% compared to that of Cu2SnSe3. This work provides an effective solution to optimize the conflicting material properties for Cu2SnSe3-based thermoelectric materials.

Fast ion transport for synthesis and stabilization of ß-Zn4Sb3.

Mobile ion-enabled phenomena make ß-Zn4Sb3 a promising material in terms of the re-entry phase instability behavior, mixed electronic ionic conduction, and thermoelectric performance. Here, we utilize the fast Zn2+ migration under a sawtooth waveform electric field and a dynamical growth of 3-dimensional ionic conduction network to achieve ultra-fast synthesis of ß-Zn4Sb3. Moreover, the interplay between the mobile ions, electric field, and temperature field gives rise to exquisite core-shell crystalline-amorphous microstructures that self-adaptively stabilize ß-Zn4Sb3. Doping Cd or Ge on the Zn site as steric hindrance further stabilizes ß-Zn4Sb3 by restricting long-range Zn2+ migration and extends the operation temperature range of high thermoelectric performance. These results provide insight into the development of mixed-conduction thermoelectric materials, batteries, and other functional materials.

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe.

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m-1 K-1 ), a high power factor (11.6 µW cm-1 K-2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Mechanical Properties and Thermal Stability of the High-Thermoelectric-Performance Cu2Se Compound.

The Cu2Se compound possesses extraordinary thermoelectric performance at high temperatures and shows great potential for the application of waste heat recycling. However, a thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. Therefore, mechanical properties are of equal importance as thermoelectric performance. However, the mechanical performance and stability of the Cu2Se compound during long-term service at high temperatures have rarely been reported. In this study, we systematically investigated the mechanical properties of Cu2Se compounds synthesized by three varied methods (melting (M), self-propagating high-temperature synthesis (SHS), and a combination of SHS and ultrasonic treatment (UT)) and investigated the thermal stability of the SHS-UT compound under different annealing temperatures. The SHS-UT process effectively refines the grain size from 19 µm for the melting sample to 5 µm for the SHS-UT sample. The high density of grain boundaries in the SHS-UT sample effectively dissipates the energy of crack propagation; thus, the mechanical properties are greatly improved. The compressive strength, bending strength, and Vickers hardness of the SHS-UT sample are 147 MPa, 52.6 MPa, and 0.46 GPa, respectively, which are 21.5, 16.6, and 35.3% higher than those of the melting sample, respectively. Moreover, excellent thermal stability is achieved in the compound prepared by SHS and ultrasonication at a temperature below 873 K. After annealing at temperatures up to 873 K for 7 days, the excellent thermoelectric performance of the Cu2Se compound is well maintained with a ZT value exceeding 1.80 at 873 K. However, with further increasing the annealing temperature to 973 K, the volatilization of Se and the precipitation of Cu result in the instability and significantly deteriorated thermoelectric performance of the material. This work provides an avenue for boosting the mechanical properties and commercial application of Cu2Se.

Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds.

Although defect engineering is the core strategy to improve thermoelectric properties, there are limited methods to effectively modulate the designed defects. Herein, we demonstrate that a high ZT value of 1.36 at 775 K and a high average ZT value of 0.99 in the temperature range from 300 to 825 K are realized in Zn-containing PbTe by designing complex defects. By combining first-principles calculations and experiments, we show that Zn atoms occupy both Pb sites and interstitial sites in PbTe and couple with each other. The contraction stress induced via substitutional Zn on Pb sites alleviates the swelling stress by Zn atoms occupying the interstitial sites and promotes the solubility of interstitial Zn atoms in the structure of PbTe. The stabilization of Zn impurity as a complex defect extends the region of PbTe phase stability toward Pb0.995Zn0.02Te, while the solid solution region in the other direction of the ternary phase diagram is much smaller. The evolution of defects in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and positron annihilation measurement. The Zn atoms compensate the Pb vacancies (VPb) and Zn interstitials (Zni) significantly improve the electron concentration, producing a high carrier mobility of 1467.7 cm2 V-1 s-1 for the Pb0.995Zn0.02Te sample. A high power factor of 4.11 mW m-1 K-2 is achieved for the Pb0.995Zn0.02Te sample at 306 K. This work provides new insights into understanding the nature and evolution of the defects in n-type PbTe as well as improving the electronic and thermal transport properties toward higher thermoelectric performance.

Strong Anisotropy and Bipolar Conduction-Dominated Thermoelectric Transport Properties in the Polycrystalline Topological Phase of ZrTe5.

ZrTe5 has unique features of a temperature-dependent topological electronic structure and anisotropic crystal structure and has obtained intensive attention from the thermoelectric community. This work revealed that the sintered polycrystalline bulk ZrTe5 possesses both (020) and (041) preferred orientations. The transport properties of polycrystalline bulk p-type ZrTe5 exhibits an obvious anisotropic characteristic, that is, the room-temperature resistivity and thermal conductivity, possessing anisotropy ratios of 0.71 and 1.49 perpendicular and parallel to the pressing direction, respectively. The polycrystalline ZrTe5 obtained higher ZT values in the direction perpendicular to the pressing direction, as compared to that in the other direction. The highest ZT value of 0.11 is achieved at 350 K. Depending on the temperature-dependent topological electronic structure, the electronic transport of p-type ZrTe5 is dominated by high-mobility electrons from linear bands and low-mobility holes from the valence band, which, however, are merely influenced by valence band holes at around room temperature. Furthermore, external magnetic fields are detrimental to thermoelectric properties of our ZrTe5, mainly arising from the more prominent negative effects of electrons under fields. This research is instructive to understand the transport features of ZrTe5 and paves the way for further optimizing their ZTs.