Zinc Doping Induces Enhanced Thermoelectric Performance of Solvothermal SnTe

The creation of hierarchical nanostructures can effectively strengthen phonon scattering to reduce lattice thermal conductivity for improving thermoelectric properties in inorganic solids. Here, we use Zn doping to induce a remarkable reduction in the lattice thermal conductivity in SnTe, approaching the theoretical minimum limit. Microstructure analysis reveals that ZnTe nanoprecipitates can embed within SnTe grains beyond the solubility limit of Zn in the Zn alloyed SnTe. These nanoprecipitates result in a substantial decrease of the lattice thermal conductivity in SnTe, leading to an ultralow lattice thermal conductivity of 0.50 W m-1 K-1 at 773 K and a peak ZT of ~0.48 at 773 K, marking an approximately 45 % enhancement compared to pristine SnTe. This study underscores the effectiveness of incorporating ZnTe nanoprecipitates in boosting the thermoelectric performance of SnTe-based materials.

Achieving High Isotropic Figure of Merit in Cd and in Codoped Polycrystalline SnSe

Here, we combined Cd and In codoping with a simple hydrothermal synthesis method to prepare SnSe powders composed of nanorod-like flowers. After spark plasma sintering, its internal grains inherited well the morphological features of the precursor, and the multiscale microstructure included nanorod-shaped grains, high-density dislocations, and stacking faults, as well as abundant nanoprecipitates, resulting in an ultralow thermal conductivity of 0.15 W m-1 K-1 for the synthesized material. At the same time, Cd and In synergistically regulated the electrical conductivity and Seebeck coefficient of SnSe, leading to an enhanced power factor. Among them, Sn0.94Cd0.03In0.03Se achieved a peak ZT of 1.50 parallel to the pressing direction, representing an 87.5% improvement compared with pure SnSe. Notably, the material possesses isotropic ZT values parallel and perpendicular to the pressing direction, overcoming the characteristic anisotropy in thermal performance observed in previous polycrystalline SnSe-based materials. Our results provide a new strategy for optimizing the performance of thermoelectric materials through structural engineering.

Strain induced modulations in the thermoelectric properties of 2D SiH and GeH monolayers: insights from first-principle calculations

The present paper is primarily focused to understand the strain driven alterations in thermoelectric properties of two-dimensional SiH and GeH monolayers from first-principle calculations. Electronic band structures and the associated thermoelectric properties of the compounds under ambient and external strains have been critically unveiled in terms of Seebeck coefficients, electrical conductivities, power factors and electronic thermal conductivities. The phonon dispersion relations have also been investigated to estimate the lattice thermal conductivities of the systems. The thermoelectric figure of merits of SiH and GeH monolayers under ambient and external strains have been explored from the collective effects of their Seebeck coefficients, electrical conductivities, electronic and lattice thermal conductivities. The present study will be helpful in exploring the strain induced thermoelectric responses of SiH and GeH compounds which in turn may bear potential applications in clean and global energy conservation.

Hydrogenation driven ultra-low lattice thermal conductivity inβ12borophene

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestandingβ12borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at differentβ12lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenatedβ12borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m-1 K-1(along armchair direction) and from 4.42 to 0.07 W m-1 K-1(along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenatedβ12nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestandingβ12borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

All-Scale Hierarchical Structuring, Optimized Carrier Concentration, and Band Manipulation Lead to Ultra-High Thermoelectric Performance in Eco-Friendly MnTe

MnTe emerges as an enormous potential for medium-temperature thermoelectric applications due to its lead-free nature, high content of Mn in the earth's crust, and superior mechanical properties. Here, it is demonstrate that multiple valence band convergence can be realized through Pb and Ag incorporations, producing large Seebeck coefficient. Furthermore, the carrier concentration can be obviously enhance by Pb and Ag codoping, contributing to significant enhancement of power factor. Moreover, microstructural characterizations reveal that PbTe nanorods can be introduced into MnTe matrix by alloying Pb. This can modify the microstructure into all-scale hierarchical architectures (including PbTe nanorods, enhances point-defect scattering, dense dislocations and stacking faults), strongly lowering lattice thermal conductivity to a record low value of 0.376 W m-1 K-1 in MnTe system. As a result, an ultra-high ZT of 1.5 can be achieved in MnTe thermoelectric through all-scale hierarchical structuring, optimized carrier concentration, and valence band convergence, outperforming most of MnTe-based thermoelectric materials.

Improved Thermoelectric Performance of p-Type PbTe by Entropy Engineering and Temperature-Dependent Precipitates

Entropy engineering is aneffective scheme to reduce the thermal conductivity of thermoelectric materials, but it inevitably deteriorates the carrier mobility. Here, we report the optimization of thermoelectric performance of PbTe by combining entropy engineering and nanoprecipitates. In the continuously tuned compounds of Pb0.98Na0.02Te(1-2x)SxSex, we show that the x = 0.05 sample exhibits an exceptionally low thermal conductivity relative to its configuration entropy. By introducing Mn doping, the produced temperature-dependent nanoprecipitates of MnSe cause the high-temperature thermal conductivity to be further reduced. A very low lattice thermal conductivity of 0.38 W m-1 K-1 is achieved at 825 K. Meanwhile, the carrier mobility of the samples is only slightly influenced, owing to the well-controlled configuration entropy and the size of nanoprecipitates. Finally, a high peak zT of ∼2.1 at 825 K is obtained in the Pb0.9Na0.04Mn0.06Te0.9S0.05Se0.05 alloy.

Strain-Induced Defect Evolution for the Construction of Porous Cu2-xSe with Enhanced Thermoelectric Performance

Superionic Cu2-xSe, with disordered and even liquid-like Cu ions, has been extensively studied as a high efficiency thermoelectric material. However, the relationship between lattice stability and microstructure evolution in Cu2-xSe under strain, which is crucial for its application, has seldom been explored in previous research. In this study, we investigate the impacts of hydrostatic compression strain on the microstructural evolution and, consequently, its implications for thermoelectric performance. Molecular dynamics (MD) simulations show that high hydrostatic compression strain could induce local diffusion of Cu ions and Se twin evolution, resulting in the breaking and reforming of Cu-Se dynamic bonds and the unstable Se sublattice. The subsequent annealing process of the destabilized structure promoted Se evaporation from the sublattice and resulted in lotus-seedpod-like pores. The reduced sound velocity and intensified phonon scattering, due to pores, lead to a reduction in the lattice thermal conductivity from 0.44 W m-1 K-1 to 0.24 W m-1 K-1 at 800 K, a decrease of approximately 45%, in the porous Cu1.92Se sample. These findings reveal the relationship between stability and defect evolution in Cu2-xSe under high hydrostatic compression, offering a straightforward strategy of defect engineering for designing unique microstructures by leveraging the instability in superionic conductor materials.

Lattice Thermal Conductivity of Mg3(Bi,Sb)2 Nanocomposites: A First-Principles Study

Mg3(BixSb1-x)2 (0 ≤ x ≤ 1) nanocomposites are a highly appealing class of thermoelectric materials that hold great potential for solid-state cooling applications. Tuning of the lattice thermal conductivity is crucial for improving the thermoelectric properties of these materials. Hereby, we investigated the lattice thermal conductivity of Mg3(BixSb1-x)2 nanocomposites with varying Bi content (x = 0.0, 0.25, 0.5, 0.75, and 1.0) using first-principles calculations. This study reveals that the lattice thermal conductivity follows a classical inverse temperature-dependent relationship. There is a significant decrease in the lattice thermal conductivity when the Bi content increases from 0 to 0.25 or decreases from 1.0 to 0.75 at 300 K. In contrast, when the Bi content increases from 0.25 to 0.75, the lattice thermal conductivity experiences a gradual decrease and reaches a plateau. For the nanohybrids (x = 0.25, 0.5, and 0.75), the distribution patterns of the phonon group velocity and phonon lifetime are similar, with consistent distribution intervals. Consequently, the change in lattice thermal conductivity is not pronounced. However, the phonon group speed and phonon lifetime are generally lower compared to those of the pristine components with x = 0 and x = 1.0. Our results suggest that the lattice thermal conductivity is sensitive to impurities but not to concentrations. This research provides valuable theoretical insights for adjusting the lattice thermal conductivity of Mg3(BixSb1-x)2 nanocomposites.

Extremely Large Response of Phonon Coherence in Twisted Penta-NiN2 Bilayer

Twisting has recently been demonstrated as an effective strategy for tuning the interactions between particles or quasi-particles in layered materials. Motivated by the recent experimental synthesis of pentagonal NiN2 sheet [ACS Nano 2021, 15, 13539], for the first time, the response of phonon coherence to twisting in bilayer penta-NiN2 , going beyond the particle-like phonon transport is studied. By using the unified theory of phonon transport and high order lattice anharmonicity, together with the self-consistent phonon theory, it is found that the lattice thermal conductivity is reduced by 80.6% from 33.35 to 6.47 W m-1 K-1 at 300 K when the layers are twisted. In particular, the contribution of phonon coherence is increased sharply by an order of magnitude, from 0.21 to 2.40 W m-1 K-1 , due to the reduced differences between the phonon frequencies and enhanced anharmonicity after the introduction of twist. The work provides a fundamental understanding of the phonon interaction in twisted pentagonal sheets.

Enhancing Thermoelectric Performance of CuInTe2 via Trace Ag Doping at Indium Sites

Thermoelectric technology can be utilized to directly convert waste heat into electricity, aiming at energy harvesting in an environmentally friendly manner. As a promising p-type thermoelectric material, CuInTe2 possesses a high inherent lattice thermal conductivity, which limits the practical implementation in the field of thermoelectricity. Herein, through the combination of vacuum melting and annealing along with hot-pressure sintering techniques, we demonstrated that CuIn0.95Ag0.05Te2 thermoelectric materials with trace Ag doping can exhibit a notably high Seebeck coefficient of 614 μV/K, arising from the high density-of-states effective mass and reduced carrier concentration. Owing to the diminished lattice thermal conductivity derived from Umklapp scattering induced by point defects and dislocation, stemming from the trace Ag doping at In sites rather than Cu sites, CuIn0.95Ag0.05Te2 exhibited a maximum figure of merit (ZT) of 1.38 at 823 K, an 18% enhancement over pristine CuInTe2, leading to a maximum average ZT of 0.67 across temperatures ranging from 303 to 823 K. In essence, our work underscores the efficacy of doping engineering and point defects in tailoring the thermoelectric performance of CuInTe2-based materials. This study not only contributes to advancing the fundamental understanding of thermoelectric enhancement but also lays out a practical pathway toward the realization of high-performance CuInTe2-based thermoelectric materials.

Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

Strained Lamellar Structures Leading to Improved Thermoelectric Performance in Mg3Sb1.5Bi0.5

Mg3Sb2-xBix solid-solutions represent an important class of thermoelectric (TE) materials due to their high efficiency and variable operating temperature range. Of particular significance for midtemperature applications is the Mg3Sb1.5Bi0.5 composition whose superior thermoelectric (TE) performance is attributed to the complex conduction band edge in conjunction with alloy dominated phonon scattering. In this work, we show that microstructure also plays a significant role in lowering the lattice thermal conductivity which in turn affects the overall TE performance (change in peak zT values between 1.1 and 1.4 have been observed). Temperature dependent TE properties of Mg3+xSb1.5Bi0.5 compositions with varying nominal Mg content (x = 0.2, 0.3, 0.4) have been studied. A marked reduction of the lattice thermal conductivity (κL) is observed in compositions with low nominal Mg content (x = 0.2), which is due to the presence of lamellar structures within the grains. These lamellar regions are isostructural to the matrix with a low misfit angle and represent compositional fluctuations in the Bi to Sb ratio. Both the size (200 nm-500 nm) and the interfacial strain contribute to the enhanced phonon scattering. A quantitative estimate of κL reduction due to these structures have been carried out using a mean free path (MFP) spectrum analysis which reveal a good match with experiments at room temperature. Further, the electrical properties are not influenced by these lamellar structures as observed from the similar power-factor (S2σ) and weighted mobilities in all of the compositions. This is due to their similar orientation to the adjacent matrix region. Thus, the zT parameter in the various compositions with similar carrier concentration can be significantly altered (∼25%) by adjusting the nominal Mg content. The results demonstrate that preferential phonon scattering by microstructure modification can be a new route for property improvement in Mg3+xSb2-yBiy solid-solutions.

AgCl Addition to Chalcopyrite Compound for Ultra-Low Thermal Conductivity in Realizing High ZT Thermoelectric Materials

Optimizing the performance of thermoelectric materials by reducing its thermal conductivity is crucial to enhance its thermoelectric efficiency. Novel thermoelectric materials like the CuGaTe₂ compound are hindered by high intrinsic thermal conductivity, which negatively impacts its thermoelectric performance. In this paper, we report that the introduction of AgCl by the solid-phase melting method will influence the thermal conductivity of CuGaTe₂. The generated multiple scattering mechanisms are expected to reduce the lattice thermal conductivity while maintaining sufficient good electrical properties. The experimental results were supported by first-principles calculations confirming that the doping of the Ag will decrease the elastic constants, bulk modulus, and shear modulus of CuGaTe₂, which makes the mean sound velocity and Debye temperature of Ag-doped samples lower than those of CuGaTe₂, indicating the lower lattice thermal conductivity. In addition, the Cl elements within the CuGaTe₂ matrix escaping during the sintering process will create holes of various sizes within the sample. These combined effects of holes and impurities will induce phonon scattering, which further reduces the lattice thermal conductivity. Based on our findings, we conclude that the introduction of AgCl into CuGaTe₂ has shown a lower thermal conductivity without compromising the electrical performance, resulting in an ultra-high ZT value of 1.4 in the (CuGaTe₂)_0.96(AgCl)_0.04 sample at 823 K.

Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu₄TiS₄ and Cu₄TiSe₄ were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu₄TiSe₄ compared to that in Cu₄TiS₄, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu₄TiSe₄. In addition, the strong lattice anharmonicity in Cu₄TiSe₄ enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ_L) of 0.11 W m^-1 K^-1 at room temperature in Cu₄TiSe₄ compared with that of 0.58 W m^-1 K^-1 in Cu₄TiS₄. Owing to the suitable band gaps of Cu₄TiS₄ and Cu₄TiSe₄, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu₄TiSe₄ are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu₄TiS₄, due to its low κ_L, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu₄TiSe₄ demonstrates its great potential for applications in thermoelectric conversion.

Bi-Deficiency Leading to High-Performance in Mg3 (Sb,Bi)2 -Based Thermoelectric Materials

Mg₃ (Sb,Bi)₂ is a potential nearly-room temperature thermoelectric compound composed of earth-abundant elements. However, complex defect tuning and exceptional microstructural control are required. Prior studies have confirmed the detrimental effect of Mg vacancies (V_Mg ) in Mg₃ (Sb,Bi)₂ . This study proposes an approach to mitigating the negative scattering effect of V_Mg by Bi deficiency, synergistically modulating the electrical and thermal transport properties to enhance the thermoelectric performance. Positron annihilation spectrometry and C_s -corrected scanning transmission electron microscopy analyses indicated that the V_Mg tends to coalesce due to the introduced Bi vacancies (V_Bi ). The defects created by Bi deficiency effectively weaken the scattering of electrons from the intrinsic V_Mg and enhance phonon scattering. A peak zT of 1.82 at 773 K and high conversion efficiency of 11.3% at ∆T = 473 K are achieved in the optimized composition of Mg₃ (Sb,Bi)₂ by tuning the defect combination. This work demonstrates a feasible and effective approach to improving the performance of Mg₃ (Sb,Bi)₂ as an emerging thermoelectric material.

Ab initiolattice thermal conductivity of (Mg,Fe)O ferropericlase at the Earth's lower mantle pressure and temperature

The effects of iron (Fe) incorporation on the lattice thermal conductivity (κlat) of MgO are investigated under the Earth's lower mantle pressure (P) and temperature (T) condition (P> ∼20 GPa,T> ∼2000 K) based on the density-functional theory combined with the anharmonic lattice dynamics theory. Theκlatof ferropericlase (FP) is determined combining the internally consistent LDA +Umethod and self-consistent approach to solve the phonon Boltzmann transport equation. The calculatedκlatare well fitted to the extended Slack model which is proposed in this study to representκlatin a wide volume andTrange. Results demonstrate that theκlatof MgO decreases strongly by Fe incorporation. This strong negative effect is found due to decreases in phonon group velocity and lifetime. Consequently, theκlatof MgO at the core-mantle boundary condition (P∼ 136 GPa,T∼ 4000 K) is substantially reduced from ∼40 to ∼10 W m^-1K^-1by the incorporation of Fe (12.5 mol%). The effect of Fe incorporation on theκlatof MgO is found to be insensitive toPandT, and at highT, theκlatof FP obeys a well-establishedTinverse relation unlike the experimental observations.

Multiple Valence Bands Convergence and Localized Lattice Engineering Lead to Superhigh Thermoelectric Figure of Merit in MnTe

MnTe has been considered a promising candidate for lead-free mid-temperature range thermoelectric clean energy conversions. However, the widespread use of this technology is constrained by the relatively low-cost performance of materials. Developing environmentally friendly thermoelectrics with high performance and earth-abundant elements is thus an urgent task. MnTe is a candidate, yet a peak ZT of 1.4 achieved so far is less satisfactory. Here, a remarkably high ZT of 1.6 at 873 K in MnTe system is realized by facilitating multiple valence band convergence and localized lattice engineering. It is demonstrated that SbGe incorporation promotes the convergence of multiple electronic valence bands in MnTe. Simultaneously, the carrier concentration can be optimized by SbGeS alloying, which significantly enhances the power factor. Simultaneously, MnS nanorods combined with dislocations and lattice distortions lead to strong phonon scattering, resulting in a markedly low lattice thermal conductivity(κ_lat ) of 0.54 W m K^-1 , quite close to the amorphous limit. As a consequence, extraordinary thermoelectric performance is achieved by decoupling electron and phonon transport. The vast increase in ZT promotes MnTe as an emerging Pb-free thermoelectric compound for a wide range of applications in waste heat recovery and power generation.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κ_L in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κ_L by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m^-1 K^-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 μW cm^-1 K^-2 in Ge_0.85Bi_0.09Ca_0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Insight into the intrinsic microstructures of polycrystalline SnSe based compounds

SnSe based compounds have attracted much attention due to the ultra-low lattice thermal conductivity and excellent thermoelectric properties. The origin of the low thermal conductivity has been ascribed to the strong phonon anharmonicity. Generally, the microstructures are also effective in scattering the phonons and further reducing the lattice thermal conductivity. In this work, the microstructures of undoped SnSe and Bi-doped Sn_0.97SeBi_0.03have been investigated by transmission electron microscopy. A characteristic microstructure of lath-like grains has been observed in SnSe based compounds from perpendicular to the pressure direction. In addition, there exist a large quantity of low-angle grain boundaries and a high concentration of edge dislocations and stacking faults in the grains. All these microstructures result in lattice mismatch and distortion and can act as the phonon scattering centers, which broaden the understanding of the low thermal conductivity of SnSe based compounds.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 from ab initio Lattice Dynamics

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN₃ displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κ_L ) and a non-standard κ_L ∝T^-0.491 dependence. At high T, the departure from the standard κ_L ∝T^-1 law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κ_L =2.98 W mK^-1 arises mainly from the conventional particle-like propagation of phonons, there is also a significant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κ_L with a weak T-dependence points to a new utility for LaWN₃ in energy technology applications, and motivates synthesis and exploration of nitride perovskites.

Doping Copper Selenide for Tuning the Crystal Structure and Thermoelectric Performance of Germanium Telluride-Based Materials

Germanium telluride (GeTe) compounds exhibit excellent thermoelectric performance. In this study, copper selenide (Cu₂Se) was used to tune the crystal structure and carrier concentration (n_H) of GeTe materials. The zT of the 1% Cu₂Se-doped GeTe sample reaches 1.32, which is 52% higher than that of the pure phase. The results show that Cu₂Se tunes the GeTe crystal structure and carrier concentration to achieve promising enhancements to the thermoelectric performance. Meanwhile, a herringbone-like crystal structure that reduces the lattice thermal conductivity was observed. However, because the directional movement of Cu ions at high temperatures leads to an increase in electrical conductivity, the electronic thermal conductivity also increased. This study focuses on crystal engineering strategies for the study of nontoxic thermoelectric materials.

Enhanced Thermoelectric Performance in Black Phosphorene via Tunable Interlayer Twist

Twist-angle two-dimensional (2D) systems are attractive in their exotic and tunable properties by the formation of the moiré superlattices, allowing easy access to manipulating intrinsic electrical and thermal properties. Here, the angle-dependent thermoelectric properties of twisted bilayer black phosphorene (tbBP) by first-principles calculations are reported. The simulations show that significantly enhanced Seebeck coefficient and power factor can be achieved in p-type tbBP due to merging of the multi-valley electronic states and flat moiré bands. Moreover, the twisted layers bring in a strong anharmonic phonon scattering and thus very low lattice thermal conductivity of 4.51 W m^-1  K^-1 at 300 K. Consequently, a maximal ZT value can be achieved in p-type 10.11° tbBP along the armchair direction up to 0.57 and 1.06 at 300 and 500 K, respectively. The room-temperature ZT value along the zigzag direction is also significantly increased by almost 40 times compared to pristine BP when the twist angle is close to 70.68°. This work demonstrates a platform to manipulate thermoelectric performance in 2D materials by creating moiré patterns, leading tbBP as a promising eco-friendly candidate for thermoelectric applications.

Giant Thermoelectric Efficiency of Single-Filled Skutterudite Nanocomposites: Role of Interface Carrier Filtering

The advantage of secondary-phase induced carrier filtering on the thermoelectric properties has paved the way for developing cost-effective, highly efficient thermoelectric materials. Here, we report a very high thermoelectric figure-of-merit of skutterudite nanocomposites achieved by tailoring interface carrier filtering. The single-filled skutterudite nanocomposites are prepared by dispersing rare-earth oxides nanoparticles (Yb₂O₃, Sm₂O₃, La₂O₃) in the skutterudite (Dy_0.4Co_3.2Ni_0.8Sb₁₂) matrix. The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly enhancing the Seebeck coefficient without compromising the electrical conductivity. As a result, the highest power factor of ∼6.5 W/mK² is achieved in the skutterudite nanocomposites. The nonuniform strain distribution near the nanoparticles due to the local lattice misfit and concentration fluctuations affect the heat carriers and thereby reduce the lattice thermal conductivity. Moreover, the three-dimensional atom probe analysis reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the reduction in lattice thermal conductivity and the enhancement of the power factor, lead to an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the temperature range from 523 to 723 K, the highest among the single-filled skutterudites reported so far.

Anharmonic lattice dynamics and structural phase transition of SnTe monolayer from first principles

In this work we investigate the role of quartic anharmonicity on the lattice- and thermo-dynamic properties of rectangular (γ) and square (β) phases of two-dimensional (2D) SnTe monolayer (ML) by using self-consistent phonon (SCP) theory, based on the first-principles calculations. For both phases, as compared to the usual harmonic approximation (HA), the renormalized phonon frequency at the optical modes (4-10) is found to be increased upon the inclusion of quartic anharmonicity via the SCP method, where the effects of cubic anharmonicity are neglected. At the experimentally observed transition temperature (T_c= 270 K), the difference in the vibrational free-energy between the square and rectangular phases of SnTe ML, calculated by using the anharmonic SCP correction is found to be much closer to the structural energy gain as compared to that obtained by using only the quasi-harmonic contribution. This validates the significance of SCP approach over the HA to explain the lattice dynamics properties and predict theT_cfor SnTe ML and similar 2D compounds. The calculated lattice thermal conductivity of square SnTe ML (e.g. 10.67 W m^-1K^-1at 300 K) is higher than that of the rectangular SnTe ML (e.g. 6.72 W m^-1K^-1at 300 K) due to the relatively higher corresponding thermodynamic parameters: specific heat capacity, group velocity, and phonon lifetime obtained for the square SnTe ML. Particularly, the low energy phonon modes are found to transport most of the heat in the system and, hence, played major role to the total lattice thermal conductivity.

Comprehensive Insight into p-Type Bi2Te3-Based Thermoelectrics near Room Temperature

Bi2Te3 is a well-recognized material for its unique properties in diverse thermoelectric applications near room temperature. The considerable efforts on Bi2Te3-based alloys have been extremely extensive in recent years, and thus the latest breakthroughs in high-performance p-type (Bi, Sb)2Te3 alloys are comprehensively reviewed to further implement applications. Effective strategies to further improve the thermoelectric performance are summarized from the perspective of enhancing the power factor and minimizing the lattice thermal conductivity. Furthermore, the surface states of topological insulators are investigated to provide a possibility of advancing (Bi, Sb)2Te3 thermoelectrics. Finally, future challenges and outlooks are overviewed. This review will provide potential guidance toward designing and developing high-efficient Bi2Te3-based and other thermoelectrics.

Electronic and transport properties of semimetal ZrBeSi crystal: a first-principles study

In recent years, semimetals have aroused people's research interest. Here, we systematically study phonon and electronic transport properties of the ZrBeSi with semimetal character by using the first-principles calculations together with the Boltzmann transport theory. Calculated lattice thermal conductivities of the ZrBeSi alongaandcaxes are 31.3 W (m · K)^-1and 56.0 W (m · K)^-1at room temperature, respectively, which are larger than the most semiconductors and semimetals. By comparing with other semimetals, we find that the larger lattice thermal conductivity of ZrBeSi is due to its smaller Grüneisen parameter, which indicates the weaker phonon scattering. Main contributions to the lattice thermal conductivities alongaandcaxis come from the acoustic branches, and conversely, the contributions of optical branches are very small. In addition, we calculate the Seebeck coefficient and the electron thermal conductivity of ZrBeSi based on the relaxation time approximation. The electronic transport properties of ZrBeSi exhibit strong anisotropy in bothaandbdirections. Calculated electronic thermal conductivities of pristine ZrBeSi alongaandcaxes are 8.8 W (m · K)^-1and 9.7 W (m · K)^-1at room temperature, respectively. Furthermore, we also obtain the figure of meritZTon the basis of phonon and electron transport. The obtainedZTalongcaxis reaches a maximum of 0.11 at 900 K, demonstrating that ZrBeSi has a generalZT, but it has good heat conduction ability. Our research will help to understand the transport properties of semimetals and expand the application of semimetals to heat conduction devices. At the same time, it also provides some reference for the future experimental work.

Band Structure and Phonon Transport Engineering Realizing Remarkable Improvement in Thermoelectric Performance of Cu2SnSe4 Incorporated with In2Te3

Cu₂SnSe₄ (CTS) ternary chalcogenides have potential applications in thermoelectrics for they crystallize in a high-symmetry cubic structure and consist of earth-abundant and eco-friendly elements. However, the pristine CTS does not have optimal thermoelectric (TE) performance (ZT = 0.35 at ∼700 K), so further investigation is required in this regard. In this work, we propose an incorporation of In₂Te₃ with a defect zinc-blende cubic structure into CTS, aiming to regulate the electronic and phonon transport mechanism simultaneously. The first-principles calculation reveals that the element In favors the residing at a vacancy site as an interstitial atom while Te at the Se site, which leads to band convergence and degeneracy, respectively. As a result, the electrical property improves with a 22% increase in the power factor (PF), and at the same time, the lattice thermal conductivity (κ_L) reduces to 0.31 W K^-1 m^-1 at 718 K. Synergistic engineering realizes a remarkable improvement in TE performance with the highest figure of merit (ZT) of 0.92 at 718 K. This value is ∼3 times that of the pristine CTS and stands among the highest in the Cu₂SnSe₄ family so far, which proves that the incorporation of In₂Te₃ into CTS is a good proposal.

Synergistically Optimized Carrier and Phonon Transport Properties in Bi-Cu2S Coalloyed GeTe

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu₂S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu⁺ ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu₂S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m^-1 K^-1. Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge_0.94Bi_0.06Te)_0.988(Cu₂S)_0.012 sample, which is very promising for mid-low temperature range waste heat harvest.

Cu2Te Incorporation-Induced High Average Thermoelectric Performance in p-Type Bi2Te3 Alloys

p-Type (Bi, Sb)₂Te₃ alloys are attractive materials for near-room-temperature thermoelectric applications due to their high atomic masses and large spin-orbit interactions. However, their narrow band gaps originating from spin-orbit interactions lead to bipolar excitation, thereby limiting average thermoelectrics within a local temperature region (300-400 K). Here, we introduce Cu₂Te into the Bi_0.3Sb_1.7Te₃ (BST) lattice to implement high thermoelectrics over a wide temperature range. The carrier concentration is synergistically modulated via Cu substitution and the evolution of intrinsic point defects (antisites and vacancies). Furthermore, the chain effect caused by Cu₂Te incorporation in BST is reflected in the improvement of the weighted mobility μ_W, thereby enhancing the power factor in the whole temperature range. Extrinsic and intrinsic defects due to the incorporation of Cu₂Te lead to a significant reduction in the lattice thermal conductivity κ_L, which is further demonstrated by Raman spectroscopy. Combining κ_L and μ_W, the quantity factor B increases from 0.5 to 1 with increasing Cu₂Te content due to not only the reduction of κ_L but also a significant improvement in electrical properties. Eventually, a peak figure of merit (zT) of ∼1.15 at 423 K is achieved in BST-Cu₂Te samples, and an average figure of merit (zT_ave) of ∼1.12 (350-500 K) surpasses other excellent p-type Bi₂Te₃-based thermoelectrics. Such a synergistic effect can facilitate near-room-temperature thermoelectric applications of Bi₂Te₃-based alloys and provide chances for the technology space in thermoelectrics.

Electronic Structure-, Phonon Spectrum-, and Effective Mass- Related Thermoelectric Properties of PdXSn (X = Zr, Hf) Half Heuslers

We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.

Understanding Micro and Atomic Structures of Secondary Phases in Cu-Doped SnTe

Highly efficient thermoelectric materials require, including point defects within the host matrix, secondary phases generating positive effects on lowering lattice thermal conductivity (κ_L ). Amongst effective dopants for a functional thermoelectric material, SnTe, Cu doping realizes the ultra-low κ_L approaching the SnTe amorphous limit. Such effective κ_L reduction is first attributed to strong phonon scattering by substitutional Cu atoms at Sn sites and interstitial defects in the host SnTe. However, other crystallographic defects in secondary phases have been unfocused. Here, this work reports micro- to atomic-scale characterization on secondary phases of Cu-doped SnTe using advanced microscopes. It is found that Cu-rich secondary phases begin precipitation ≈1.7 at% Cu (x = 0.034 where Sn_{1- x} Cu_x Te). The Cu-rich secondary phases encapsulate two distinct solids: Cu₂ SnTe₃ ( F 4 ¯ 3 m $F\bar{4}3m$ ) has semi-coherent interfaces with SnTe ( F m 3 ¯ m $Fm\bar{3}{\rm{m}}$ ) such that they minimize lattice mismatch to favor the thermoelectric transport; the other resembles a stoichiometric Cu₂ Te model, yet is so meta-stable that it demonstrates not only various defects such as dislocation cores and ordered/disordered Cu vacancies, but also dynamic grain-boundary migration with heating and a subsequent phase transition ≈350 °C. The atomic-scale analysis on the Cu-rich secondary phases offers viable strategies for reducing κ_L through Cu addition to SnTe.

Enhancement of the Thermoelectric Performance of Cu2GeSe3 via Isoelectronic (Ag, S)-co-substitution

Recently, ternary Cu-based Cu-IV-Se (IV = Sb, Ge, and Sn) compounds have received extensive attention in the thermoelectric field. Compared with Cu-Sb-Se and Cu-Sn-Se, Cu-Ge-Se compounds have been less studied due to its poor Seebeck coefficient and high thermal conductivity. Here, the Cu₂GeSe₃ material with high electrical conductivity was first prepared, and then, its effective mass was increased by doping with S, which led to the Seebeck coefficient of the doped sample being 1.93 times higher than that of pristine Cu₂GeSe₃ at room temperature. Moreover, alloying Ag at the Cu site in the Cu₂GeSe_2.96S_0.04 sample could further cause a 5.16 times increase in the Seebeck coefficient at room temperature, and the lattice thermal conductivity was remarkably decreased because of the introduction of the dislocations in the Cu₂GeSe₃ sample. Finally, benefitted from the high Seebeck coefficient and low thermal conductivity, a record high ZT = 0.9 at 723 K was obtained for the Cu_1.85Ag_0.15GeSe_2.96S_0.04 sample, which increased 345% in comparison with the pristine Cu₂GeSe₃, and it is among the highest reported values for Cu₂GeSe₃-based thermoelectric.

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports

Guided by the concept of "phonon-liquid electron-crystal", many n-type argyrodite compounds have been developed as candidates for thermoelectric (TE) materials. In recent years, the p-type Cu₈GeSe₆ (CGS) compound has attracted some attention in TEs due to the presence of very strong atomic vibrational arharmonicity inside the sublattice, which is caused by the weak bonding between Cu ions and [GeSe₆]^8-. However, its TE performance is still poor, with a ZT value of only 0.2 at 623 K. Therefore, in this work, we propose to engineer both the electronic and phonon transports in CGS by incorporating the species In₂Te₃. This strategy tunes the carrier concentration and at the same time increases the phonon scattering on the point defects (In_Ge, In_interstitial, and Te_Se) and randomly distributed tetrahedra ([InSe₄]^5- and [GeTeSe₃]^4-). As a result, the phase transformation at 329 K in CGS is eliminated, and the peak ZT value is enhanced from 0.27 for CGS to ∼0.92 for (Cu₈SnSe₆)_0.9(In₂Te₃)_0.1 at 774 K; this thus proves that the incorporation of In₂Te₃ in CGS is an effective way of regulating its TE performance.

Synergistically Optimized Thermal Conductivity and Carrier Concentration in GeTe by Bi-Se Codoping

The GeTe compound has been revealed to be an outstanding thermoelectric compound, while its inherent high thermal conductivity restricts further improvement in its performance. Herein, we report a study on the synergistic optimization of the thermoelectric performance of GeTe by Bi-Se codoping. It is shown that the introduction of Bi decreases the carrier concentration and increases the structural parameter of the interaxial angle. With Se doping in the Te site, the lattice thermal conductivity is markedly reduced, while the carrier mobility is slightly influenced. Compared with the singly Se-doped GeTe, the Ge_1-xBi_xTe_1-ySe_y samples are more closed to a cubic phase, as indicated by the larger interaxial angle. On account of the reduction of carrier concentration and thermal conductivity, a ZT_max of 1.80 at 665 K and a high ZT_ave of 1.39 (400-800 K) are obtained in Ge_0.94Bi_0.06Te_0.85Se_0.15. This work reveals that the interaxial angle is vital to the performance optimization of rhombohedral GeTe.

Remarkable decrease in lattice thermal conductivity of transition metals borides TiB2by dimensional reduction

Two-dimensional transition metals borides Ti_xB_xhave excellent magnetic and electronic properties and great potential in metal-ion batteries and energy storage. The thermal management is important for the safety and stability in these applications. We investigated the lattice dynamical and thermal transport properties of bulk-TiB₂and its two-dimensional (2D) counterparts based on density functional theory combined with solving phonon Boltzmann transport equation. The Poisson's ratio of bulk-TiB₂is positive while it changes to negative for monolayer TiB₂. We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease, which is opposite the trend of MoS₂, MoSe₂, WSe₂and SnSe. Additionally, the room temperature thermal conductivity of mono-TiB₂is only one sixth of that for bulk-TiB₂. It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-TiB₂. The bulk-TiB₂has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts. On the other hand, the room temperature lattice thermal conductivity of mono-Ti₂B₂is two times higher than that of mono-TiB₂, which is due to three-phonon selection rule caused by the horizontal mirror symmetry.

Stability, optoelectronic and thermal properties of two-dimensional Janusα-Te2S

Motivated by recent progress in the two-dimensional (2D) materials of group VI elements and their experimental fabrication, we have investigated the stability, optoelectronic and thermal properties of Janusα-Te₂S monolayer using first-principles calculations. The phonon dispersion and MD simulations confirm its dynamical and thermal stability. The moderate band gap (∼1.5 eV), ultrahigh carrier mobility (∼10³cm²V^-1s^-1), small exciton binding energy (0.26 eV), broad optical absorption range and charge carrier separation ability due to potential difference (ΔV = 1.07 eV) on two surfaces of Janusα-Te₂S monolayer makes it a promising candidate for solar energy conversion. We propose various type-II heterostructures consisting of Janusα-Te₂S and other transition metal dichalcogenides for solar cell applications. The calculated power conversion efficiencies of the proposed heterostructures, i.e.α-Te₂S/T-PdS₂,α-Te₂S/BP andα-Te₂S/H-MoS₂are ∼21%, ∼19% and 18%, respectively. Also, the ultralow value of lattice thermal conductivity (1.16 W m^-1K^-1) of Janusα-Te₂S makes it a promising material for the fabrication of next-generation thermal energy conversion devices.

Improved Thermoelectric Performance of P-type SnTe through Synergistic Engineering of Electronic and Phonon Transports

SnTe has been regarded as a potential alternative to PbTe in thermoelectrics because of its environmentally friendly features. However, it is a challenge to optimize its thermoelectric (TE) performance as it has an inherent high hole concentration (n_H∼2 × 10²⁰ cm^-3) and low mobility (μ_H∼18 cm² V^-1 s^-1) at room temperature (RT), arising from a high intrinsic Sn vacancy concentration and large energy separation between its light and heavy valence bands. Therefore, its TE figure of merit is only 0.38 at ∼900 K. Herein, both the electronic and phonon transports of SnTe were engineered by alloying species Ag_0.5Bi_0.5Se and ZnO in succession, thus increasing the Seebeck coefficient and, at the same time, reducing the thermal conductivity. As a result, the TE performance improves significantly with the peak ZT value of ∼1.2 at ∼870 K for the sample (SnGe_0.03Te)_0.9(Ag_0.5Bi_0.5Se)_0.1 + 1.0 wt % ZnO. This result proves that synergistic engineering of the electronic and phonon transports in SnTe is a good approach to improve its TE performance.

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 Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/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 Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ matrix and introduce a CuSe secondary phase and an Ag-rich phase at grain boundaries. The electrical conductivity of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/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 Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/5% Ag₂Se. This work provides an efficient way to improve average ZT values of Cu₂SnSe₃-based compounds for promising power generation at intermediate temperatures.

Realizing High Thermoelectric Performance in p-Type SnSe Crystals via Convergence of Multiple Electronic Valence Bands

SnSe crystals have gained considerable interest for their outstanding thermoelectric performance. Here, we achieve excellent thermoelectric properties in Sn_0.99-xPb_xZn_0.01Se crystals via valence band convergence and point-defect engineering strategies. We demonstrate that Pb and Zn codoping converges the energy offset between multiple valence bands by significantly modifying the band structure, contributing to the enhancement of the Seebeck coefficient. The carrier concentration and electrical conductivity can be optimized, leading to an enhanced power factor. The dual-atom point-defect effect created by the substitution of Pb and Zn in the SnSe lattice introduces strong phonon scattering, significantly reducing the lattice thermal conductivity to as low as 0.284 W m^-1 K^-1. As a result, a maximum ZT value of 1.9 at 773 K is achieved in Sn_0.93Pb_0.06Zn_0.01Se crystals along the bc-plane direction. This study highlights the crucial role of manipulating multiple electronic valence bands in further improving SnSe thermoelectrics.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm² V^-1 s^-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge_1.04-x-yIn_xCu_yTe series. Moreover, we introduce Cu₂Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu₂Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m^-1 K^-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge_0.9In_0.015Cu_0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

Biaxial Tensile Strain-Induced Enhancement of Thermoelectric Efficiency of α-Phase Se2Te and SeTe2 Monolayers

Thermoelectric (TE) materials can convert waste heat into electrical energy, which has attracted great interest in recent years. In this paper, the effect of biaxial-tensile strain on the electronic properties, lattice thermal conductivity, and thermoelectric performance of α-phase Se₂Te and SeTe₂ monolayers are calculated based on density-functional theory and the semiclassical Boltzmann theory. The calculated results show that the tensile strain reduces the bandgap because the bond length between atoms enlarges. Moreover, the tensile strain strengthens the scatting rate while it weakens the group velocity and softens the phonon model, leading to lower lattice thermal conductivity k_l. Simultaneously, combined with the weakened k_l, the tensile strain can also effectively modulate the electronic transport coefficients, such as the electronic conductivity, Seebeck coefficient, and electronic thermal conductivity, to greatly enhance the ZT value. In particular, the maximum n-type doping ZT under 1% and 3% strain increases up to six and five times higher than the corresponding ZT without strain for the Se₂Te and SeTe₂ monolayers, respectively. Our calculations indicated that the tensile strain can effectively enhance the thermoelectric efficiency of Se₂Te and SeTe₂ monolayers and they have great potential as TE materials.

Defect Engineering Boosted Ultrahigh Thermoelectric Power Conversion Efficiency in Polycrystalline SnSe

Two-dimensional (2D)-layered atomic arrangement with ultralow lattice thermal conductivity and ultrahigh figure of merit in single-crystalline SnSe drew significant attention among all thermoelectric materials. However, the processing of polycrystalline SnSe with equivalent thermoelectric performance as single-crystal SnSe will have great technological significance. Herein, we demonstrate a high zT of 2.4 at 800 K through the optimization of intrinsic defects in polycrystalline SnSe via controlled alpha irradiation. Through a detailed theoretical calculation of defect formation energies and lattice dynamic phonon dispersion studies, we demonstrate that the presence of intrinsically charged Sn vacancies can enhance the power factor and distort the lattice thermal conductivity by phonon-defect scattering. Supporting our theoretical calculations, the experimental enhancement in the electrical conductivity leads to a massive power factor of 0.9 mW/mK² and an ultralow lattice thermal conductivity of 0.22 W/mK through the vacancy-phonon scattering effect on polycrystalline SnSe. The strategy of intrinsic defect engineering of polycrystalline thermoelectric materials can increase the practical implementation of low-cost and high-performance thermoelectric generators.

Lattice Thermal Conductivity: An Accelerated Discovery Guided by Machine Learning

In the present work, we used machine learning (ML) techniques to build a crystal-based model that can predict the lattice thermal conductivity (LTC) of crystalline materials. To achieve this, first, LTCs of 119 compounds at various temperatures (100-1000 K) were obtained based on density functional theory (DFT) and phonon calculations, and then, these data were employed in the next learning process to build a predictive model using various ML algorithms. The ML results showed that the model built based on the random forest (RF) algorithm with an R² score of 0.957 was the most accurate compared with the models built using other algorithms. Additionally, the accuracy of this model was validated using new cases of four compounds, which was not seen for the model before, where a good matching between calculated and predicted LTCs of the new compounds was found. To find candidates with ultralow LTCs (<1 W m^-1 K^-1) at room temperature, the model was used to screen compounds (32116) in the Inorganic Crystal Structure Database. From the screened compounds, Cs₂SnI₆ and SrS were selected to validate the ML prediction using the counterpart theoretical calculations (DFT and phonon), and it was found that the outcome behaviors by both methods (ML prediction and DFT/phonon calculations) are fairly consistent. Considering the type of employed feature, the prime novelty in this work is that the built model can credibly predict the LTC-temperature behaviors of new compounds that are constructed based on prototype structures and chemical compositions, without the use of any DFT-relaxed structure parameters. Accordingly, using the periodic table, prototype structures, and the RF-based model, the LTC-temperature behavior of a huge number of compounds can be predicated.

Synergistic Optimization of the Electronic and Phonon Transports of N-Type Argyrodite Ag8Sn1-xGaxSe6 (x = 0-0.6) through Entropy Engineering

The argyrodite compound, Ag₈SnSe₆ (ATS), which is one of the promising thermoelectric (TE) candidates, is receiving growing attention in thermoelectrics recently. However, its TE performance is still low and phases are unstable as the temperature varies. In this work, inspired by entropy engineering, we eliminate the β/γ phase transformation at ∼355 K via alloying Ga, thus extending its high-temperature cubic phase from 320 to 730 K. In the meantime, the power factor (PF) enhances by 10% and lattice thermal conductivity (κ_L) reduces by 40% at 723 K. As a result, the ZT value is boosted to ∼1.15 for Ag₈Sn_0.5Ga_0.5Se₆, which stands high among the ATS systems. This proves that the entropy engineering is an effective approach to extend the high-temperature range for the cubic γ-phase and improve its TE performance simultaneously.

Enhanced Thermoelectric Performance by Strong Phonon Scattering at the Heterogeneous Interfaces of the Mg2Sn/Mg3Sb2 High-Content Nanocomposite

Nano approaches are practical strategies to boost the thermoelectric figure of merit due to the strong phonon scattering from the grain boundaries and nanoinclusions. Here, we have reported a strong phonon scattering at the heterogeneous interfaces of Mg₂Sn/Mg₃Sb₂ high-content nanocomposites (HCnCs). As a result, a significantly reduced lattice thermal conductivity of 1.09 W m^-1 K^-1 was observed in the equimolar Mg₂Sn/Mg₃Sb₂ HCnC, 80% lower than pure Mg₂Sn and 25% lower than pure Mg₃Sb₂. As a result, a high ZT ∼ 1.13 at 773 K was achieved in the Mg₂Sn/Mg₃Sb₂ HCnC. Furthermore, various defects, including solid solutions, nanoinclusions, and misfit dislocations, were observed in both the Mg₃Sb₂ phase and the Mg₂Sn phase through the microstructure characterization.

Self-Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-Type Half-Heuslers

Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence electron count, namely, ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising thermoelectric properties. Exemplarily, it has been experimentally observed that a slight change in the content of Y site atoms (by ∼3-12.5% i.e., m = 0.03 and 0.125 in ZrY_1+mZ) leads to a drastic decrease in lattice thermal conductivity κ_L by more than 65-80% in many of these compounds. The present work aims at exploring the possibility of maximizing the electronic transport scenario after achieving the low κ_L limit in these compounds. By taking into account the full anharmonicity of the lattice dynamics, Boltzmann transport calculations are performed under the framework of density functional theory. Our results show that these excess atoms present in the vacant lattice site induce scattering either by acting as a rattling mode or by hybridizing with the acoustic modes of the host depending upon their mass and bonding chemistry, respectively. Furthermore, the introduction of these scattering centers may lead either to the formation of a defect midgap state in the electronic band structure (detrimental for electronic transport) or to light doping of the host compound. The latter is found to be particularly conducive for attaining synergy in both thermal and electronic transport.

Large exciton binding energy, superior mechanical flexibility, and ultra-low lattice thermal conductivity in BiI3monolayer

The exciton binding energy, mechanical properties, and lattice thermal conductivity of monolayer BiI₃are investigated on the basis of first principle calculation. The excitation energy of monolayer BiI₃is predicted to be 1.02 eV, which is larger than that of bulk BiI₃(0.224 eV). This condition is due to the reduced dielectric screening in systems. The monolayer can withstand biaxial tensile strain up to 30% with ideal tensile strength of 2.60 GPa. Compared with graphene and MoS₂, BiI₃possesses superior flexibility and ductility due to its large Poisson's ratio and smaller Young's modulus by two orders of magnitude. The predicted lattice thermal conductivityk_Lof monolayer BiI₃is 0.247 W m^-1 K^-1at room temperature, which is lower than most reported values for other 2D materials. Such ultralowk_Lresults from the scattering between acoustic and optical phonon modes, heavy atomic mass, and relatively weak chemical bond.

Excellent Room-Temperature Thermoelectricity of 2D GeP3: Mexican-Hat-Shaped Band Dispersion and Ultralow Lattice Thermal Conductivity

Although some atomically thin 2D semiconductors have been found to possess good thermoelectric performance due to the quantum confinement effect, most of their behaviors occur at a higher temperature. Searching for promising thermoelectric materials at room temperature is meaningful and challenging. Inspired by the finding of moderate band gap and high carrier mobility in monolayer GeP_3, we investigated the thermoelectric properties by using semi-classical Boltzmann transport theory and first-principles calculations. The results show that the room-temperature lattice thermal conductivity of monolayer GeP₃ is only 0.43 Wm⁻¹K⁻¹ because of the low group velocity and the strong anharmonic phonon scattering resulting from the disordered phonon vibrations with out-of-plane and in-plane directions. Simultaneously, the Mexican-hat-shaped dispersion and the orbital degeneracy of the valence bands result in a large p-type power factor. Combining this superior power factor with the ultralow lattice thermal conductivity, a high p-type thermoelectric figure of merit of 3.33 is achieved with a moderate carrier concentration at 300 K. The present work highlights the potential applications of 2D GeP₃ as an excellent room-temperature thermoelectric material.

The Electrical and Thermal Transport Properties of La-Doped SrTiO3 with Sc2O3 Composite

Donor-doped strontium titanate (SrTiO₃) is one of the most promising n-type oxide thermoelectric materials. Routine doping of La at Sr site can change the charge scattering mechanism, and meanwhile can significantly increase the power factor in the temperature range of 423–773 K. In addition, the introduction of Sc partially substitutes Sr, thus further increasing the electron concentration and optimizing the electrical transport properties. Moreover, the excess Sc in the form of Sc₂O₃ composite suppresses multifrequency phonon transport, leading to low thermal conductivity of κ = 3.78 W·m⁻¹·K⁻¹ at 773 K for sample Sr_0.88La_0.06Sc_0.06TiO₃ with the highest doping content. Thus, the thermoelectric performance of SrTiO₃ can be significantly enhanced by synergistic optimization of electrical transport and thermal transport properties via cation doping and composite engineering.

Lattice Thermal Transport in the Homogeneous Cage-Like Compounds Cu3 VSe4 and Cu3 NbSe4 : Interplay between Phonon-Phase Space, Anharmonicity, and Atomic Mass

Understanding the correlation between crystal structure and thermal conductivity in semiconductors is very important for designing heat-transport-related devices, such as high-performance thermoelectric materials and heat dissipation in micro-nano-scale devices. In this work, the lattice thermal conductivity ( κ L ) of the cage-like compounds Cu₃ VSe₄ and Cu₃ NbSe₄ was investigated by experimental measurements and first-principles calculations. The experimental κ L of Cu₃ NbSe₄ is approximately 25 % lower than that of Cu₃ VSe₄ at 300 K. The relevant important physical parameters, including the sound velocity, heat capacity, weighted phonon phase space (W), and third-order force constants along with atomic mass were theoretically analyzed. It is found that W is the dominant parameter in determining the κ L , and the other factors only play a minor role. The physical origin is the relatively "soft" lattice of Cu₃ NbSe₄ with heavier atomic mass. This research provides deep insight into the correlation between the thermal conductivity and crystal structure and paves the way for discovering high-performance thermal management device and thermoelectric materials with intrinsically low κ L .