Zintl-phase Eu2ZnSb2: A promising thermoelectric material with ultralow thermal conductivity

Thermoelectric Performance Enhancement in SnS Polycrystals Owing to Hole Doping Combined with Textured Microstructures

Recently, the earth-abundant tin sulfide (SnS) has emerged as a promising thermoelectric material due to its phonon and electron structure similar to that of tin selenide (SnSe). However, compared with SnSe, limited progress has been achieved in the thermoelectric property enhancement of SnS. Textured SnS polycrystals with an enhanced thermoelectric performance have been developed in this work. The high carrier mobility benefited from the enhanced texture through the repressing strategy of spark plasma sintering, improving the electrical conductivity. In addition, Sn atom deficiencies in the texture sample led to an increased hole concentration, further boosting the electrical conductivity and power factor. The power factor exceeded 4.10 μW/cm·K2 at 423 K and 5.50 μW/cm·K2 at 850 K. The phonon scattering was strengthened by adjusting the multiscale microstructures including dislocations, defect clusters, etc., leading to an ultralow lattice thermal conductivity of 0.23 W/m·K at 850 K. A figure of merit zT > 1.3 at 850 K and an average zTave of 0.58 in the temperature range 373-850 K were achieved in the SnS polycrystal.

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Global band convergence design for high-performance thermoelectric power generation in Zintls

Electronic band convergence can have a beneficial impact on thermoelectric performance, but finding the right band-converged compositions is still time-consuming. We propose a method for designing a series of compositions with simultaneous band convergence in the high-entropy YbxCa1-xMgyZn2-ySb2 material by zeroing the weighted sum of crystal-field splitting energies of the parent compounds. We found that so-designed compositions have both larger power factors and lower thermal conductivities and that one of these compositions exhibits a large thermoelectric figure of merit value in comparison with to other p-type Zintls. Our material shows high stability both thermally and temporally. We then assembled an all-Zintl single-stage module, nontoxic and free of tellurium, that demonstrates an exceptional heat-to-electricity conversion efficiency exceeding 10% at a temperature difference of 475 kelvin.

Improvement of the Thermoelectric Properties of p-Type Mg3Sb2 by Mg-Site Double Substitution

A P-type Mg3Sb2-based Zintl phase compound has been considered a promising candidate for thermoelectric applications. Alloying, which introduces a high concentration of point defects, is particularly effective in scattering phonons and reducing lattice thermal conductivity. Herein, alloying in p-type Mg2.995Na0.005Sb2 via the introduction of elements like Yb, Eu, Ca, and Ba was realized, and the room-temperature lattice thermal conductivity has been effectively reduced to ∼1.1 W m-1 K-1. To further intensify the phonon scattering, two groups of elements (Eu and Cd, and Yb and Cd) were chosen for heavy alloying at the Mg site, and the lattice thermal conductivity of Mg1.49Eu0.5Cd1Na0.01Sb2 was further reduced to ∼0.45 W m-1 K-1. Eventually, a peak zT as high as ∼1.0 was achieved at 773 K, and the compound outperforms the previously reported p-type Mg3Sb2 compounds.

Vacancies tailoring lattice anharmonicity of Zintl-type thermoelectrics

While phonon anharmonicity affects lattice thermal conductivity intrinsically and is difficult to be modified, controllable lattice defects routinely function only by scattering phonons extrinsically. Here, through a comprehensive study of crystal structure and lattice dynamics of Zintl-type Sr(Cu,Ag,Zn)Sb thermoelectric compounds using neutron scattering techniques and theoretical simulations, we show that the role of vacancies in suppressing lattice thermal conductivity could extend beyond defect scattering. The vacancies in Sr2ZnSb2 significantly enhance lattice anharmonicity, causing a giant softening and broadening of the entire phonon spectrum and, together with defect scattering, leading to a ~ 86% decrease in the maximum lattice thermal conductivity compared to SrCuSb. We show that this huge lattice change arises from charge density reconstruction, which undermines both interlayer and intralayer atomic bonding strength in the hierarchical structure. These microscopic insights demonstrate a promise of artificially tailoring phonon anharmonicity through lattice defect engineering to manipulate lattice thermal conductivity in the design of energy conversion materials.

Alloying-Induced Structural Transition in the Promising Thermoelectric Compound CaAgSb

AMX Zintl compounds, crystallizing in several closely related layered structures, have recently garnered attention due to their exciting thermoelectric properties. In this study, we show that orthorhombic CaAgSb can be alloyed with hexagonal CaAgBi to achieve a solid solution with a structural transformation at x ∼ 0.8. This transition can be seen as a switch from three-dimensional (3D) to two-dimensional (2D) covalent bonding in which the interlayer M-X bond distances expand while the in-plane M-X distances contract. Measurements of the elastic moduli reveal that CaAgSb1-xBix becomes softer with increasing Bi content, with the exception of a steplike 10% stiffening observed at the 3D-to-2D phase transition. Thermoelectric transport measurements reveal promising Hall mobility and a peak zT of 0.47 at 620 K for intrinsic CaAgSb, which is higher than those in previous reports for unmodified CaAgSb. However, alloying with Bi was found to increase the hole concentration beyond the optimal value, effectively lowering the zT. Interestingly, analysis of the thermal conductivity and electrical conductivity suggests that the Bi-rich alloys are low Lorenz-number (L) materials, with estimated values of L well below the nondegenerate limit of L = 1.5 × 10-8 W Ω K-2, in spite of the metallic-like transport properties. A low Lorenz number decouples lattice and electronic thermal conductivities, providing greater flexibility for enhancing thermoelectric properties.

Experiment and Theory in Concert To Unravel the Remarkable Electronic Properties of Na-Doped Eu11Zn4Sn2As12: A Layered Zintl Phase

Low-dimensional materials have unique optical, electronic, mechanical, and chemical properties that make them desirable for a wide range of applications. Nano-scaling materials to confine transport in at least one direction is a common method of designing materials with low-dimensional electronic structures. However, bulk materials give rise to low-dimensional electronic structures when bonding is highly anisotropic. Layered Zintl phases are excellent candidates for investigation due to their directional bonding, structural variety, and tunability. However, the complexity of the structure and composition of many layered Zintl phases poses a challenge for producing phase-pure bulk samples to characterize. Eu11Zn4Sn2As12 is a layered Zintl phase of significant complexity that is of interest for its magnetic, electronic, and thermoelectric properties. To prepare phase-pure Eu11-xNaxZn4Sn2As12, a binary EuAs phase was employed as a precursor, along with NaH. Experimental measurements reveal low thermal conductivity and a high Seebeck coefficient, while theoretical electronic structure calculations reveal a transition from a 3D to 2D electronic structure with increasing carrier concentration. Simulated thermoelectric properties also indicate anisotropic transport, and thermoelectric property measurements confirm the nonparabolicity of the relevant bands near the Fermi energy. Thermoelectric efficiency is known to improve as the dimensionality of the electronic structure is decreased, making this a promising material for further optimization and opening the door to further exploitation of layered Zintl phases with low-dimensional electronic structures for thermoelectric applications.

Realizing Excellent Structural and Thermoelectric Performance in Mg3Sb2-Based Alloys by Manipulating Mg Intrinsic Migration Kinetics with Interstitial Ni Doping

N-type Mg₃Sb₂ is attracting increasing focus for its outstanding room-temperature (RT) thermoelectric (TE) performance; however, achieving reliable n-type conduction remains challenging due to negatively charged Mg vacancies. Doping with compensation charges is generally used but does not fundamentally resolve the high intrinsic activity and easy formation of Mg vacancies. Herein, a robust structural and thermoelectric performance is obtained by manipulating Mg intrinsic migration activity by precisely incorporating Ni at the interstitial site. Density functional theory (DFT) indicates that a strong performance originates from a significant thermodynamic preference for Ni occupying the interstitial site across the complete Mg-poor to -rich window, which dramatically promotes the Mg migration barrier and kinetically immobilizes Mg. As a result, the detrimental vacancy-associated ionized scattering is eliminated with a leading room-temperature ZT up to 0.85. This work reveals that interstitial occupation in Mg₃Sb₂-based materials is a novel approach promoting both structural and thermoelectric performance.

Cation Substitution and Size Effects in Ca2ZnSb2 and Yb2MnSb2: Crystal and Electronic Structures and Thermoelectric Properties

Zintl compounds often feature complex structural fragments and small band gaps, favoring promising thermoelectric properties. In this work, a new phase Ca₂ZnSb₂ is synthesized and characterized to be a LiGaGe-type structure. It is isotypic to Yb₂MnSb₂ with half vacancies at transition metal sites and undergoes a phase transition to Ca₉Zn_4+xSb₉ after annealing. Interestingly, Ca₂ZnSb₂ and Yb₂MnSb₂ are amenable to diverse doping mechanisms at different sites. Here, by substituting smaller Li on cation sites, two novel layered compounds Ca_1.84(1)Li_0.16(1)Zn_0.84(1)Sb₂ and Yb_1.82(1)Li_0.18(1)Mn_0.96(1)Sb₂ with the P6₃/mmc space group are discovered, which can be viewed as derivatives of LiGaGe type. Despite having lower occupancy, the structural stability is improved compared with the prototype compounds owing to the reduced interlayered distances. Besides, the band structure analyses demonstrate that the bands near the Fermi level are mainly governed by the interlayered interaction. Due to the highly disordered structure, Yb_1.82Li_0.18Mn_0.96Sb₂ features ultralow thermal conductivity from 0.79 to 0.47 W·m^-1·K^-1 among the testing range; in addition, a remarkable Seebeck coefficient of 270.77 μV·K^-1 at 723 K is observed. The discovery of the Ca₂ZnSb₂ phase enriches the 2-1-2 map, and the size effect induced by cations provides new ideas for material designing.

Thermoelectric Properties of Ba2-xEuxZnSb2, a Zintl Phase with One-Dimensional Covalent Chains

The compound Ba₂ZnSb₂ has been predicted to be a promising thermoelectric material, potentially achieving zT > 2 at 900 K due to its one-dimensional chains of edge-shared [ZnSb_4/2]^4- tetrahedra and interspersed Ba cations. However, the high air sensitivity of this material makes it difficult to measure its thermoelectric properties. In this work, isovalent substitution of Eu for Ba was carried out to make Ba_2-xEu_xZnSb₂ in order to improve the stability of the material in air and to allow characterization of thermal and electronic properties of three different compositions (x = 0.2, 0.3, and 0.4). Polycrystalline samples were synthesized using binary precursors via ball milling and annealing, and their thermoelectric properties were measured. Samples showed low thermal conductivity (<0.8 W/m K), a high Seebeck coefficient (350-550 μV/K), and high charge carrier mobility (20-35 cm²/V) from 300 to 500 K, consistent with predictions of high thermoelectric efficiency. Evaluation of the thermoelectric quality factor suggests that a higher zT can be attained if the carrier concentration can be increased via doping.

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.

Advanced hematite nanomaterials for newly emerging applications

Because of the combined merits of rich physicochemical properties, abundance, low toxicity, etc., hematite (α-Fe2O3), one of the most chemically stable compounds based on the transition metal element iron, is endowed with multifunctionalities and has steadily been a research hotspot for decades. Very recently, advanced α-Fe2O3 materials have also been developed for applications in some cutting-edge fields. To reflect this trend, the latest progress in developing α-Fe2O3 materials for newly emerging applications is reviewed with a particular focus on the relationship between composition/nanostructure-induced electronic structure modulation and practical performance. Moreover, perspectives on the critical challenges as well as opportunities for future development of diverse functionalities are also discussed. We believe that this timely review will not only stimulate further increasing interest in α-Fe2O3 materials but also provide a profound understanding and insight into the rational design of other materials based on transition metal elements for various applications.

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba_1.01 AgSb shows a high room-temperature hole mobility of 344 cm² V^-1 S^-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Contradicting Influence of Zn Alloying on Electronic and Thermal Properties of a YbCd2 Sb2 -Based Zintl Phase at 700 K

Zintl compounds are promising thermoelectric materials for power generation as their electronic and thermal transport properties can be simultaneously engineered with anion/cation alloying. Recently, a peak thermoelectric figure-of-merit, zT, of 1.4 was achieved in a (Yb0.9 Mg0.1 )Cd1.2 Mg0.4 Zn0.4 Sb2 Zintl phase at 700 K. Although the effects of alloying Zn in lattice thermal conductivity had been studied thoroughly, how the Zn alloying affects its electronic transport properties has not yet been fully investigated. This study evaluates how the Zn alloying at Cd sites alters the band parameters of (Yb0.9 Mg0.1 )Cd1.6-x Mg0.4 Znx Sb2 (x=0-0.6) using the Single Parabolic Band model at 700 K. The Zn alloying increased the density-of-states effective mass (md * ) from 0.87 to 0.97 m0 . Among Zn-alloyed samples, the md * of the x=0.4 sample was the lowest (0.93 m0 ). The Zn alloying decreased the non-degenerate mobility (μ0 ) from 71 to 57 cm2  s-1  V-1 . Regardless of Zn alloying content, the μ0 of the Zn-alloyed samples were similar (∼57 cm2  s-1  V-1 ). Consequently, the x=0.4 with the highest zT exhibited the lowest weighted mobility (μW ). The lowest μW represents the lowest theoretical electronic transport properties among other x. The highest zT at x=0.4 despite the lowest μW was explained with a significant lattice thermal conductivity reduction achieved with Zn alloying with x=0.4, which outweighed the deteriorated electronic transport properties also due to the alloying.

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×10¹⁸  cm^-3 and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 μW cm^-1  K^-2 at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd-Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.

Novel insights into lattice thermal transport in nanocrystalline Mg3Sb2 from first principles: the crucial role of higher-order phonon scattering

Zintl phase Mg₃Sb₂, which has ultra-low thermal conductivity, is a promising anisotropic thermoelectric material. It is worth noting that the prediction and experiment value of lattice thermal conductivity (κ) maintain a remarkable difference, troubling the development and application. Thus, we firstly included the four-phonon scattering processes effect and performed the Peierls-Boltzmann transport equation (PBTE) combined with the first-principles lattice dynamics to study the lattice thermal transport in Mg₃Sb₂. The results showed that our theoretically predicted κ is consistent with the experimentally measured, breaking through the limitations of the traditional calculation methods. The prominent four-phonon scatterings decreased phonon lifetime, leading to the κ of Mg₃Sb₂ at 300 K from 2.45 (2.58) W m^-1 K^-1 to 1.94 (2.19) W m^-1 K^-1 along the in (cross)-plane directions, respectively, and calculation accuracy increased by 20%. This study successfully explains the lattice thermal transport behind mechanism in Mg₃Sb₂ and implies guidance to advance the prediction accuracy of thermoelectric materials.

Intrinsic nanostructure induced ultralow thermal conductivity yields enhanced thermoelectric performance in Zintl phase Eu2ZnSb2

The Zintl thermoelectric phase Eu₂ZnSb₂ has a remarkable combination of high mobility and low thermal conductivity that leads to good thermoelectric performance. The key feature of this compound is a crystal structure that has a Zn-site with a 50% occupancy. Here we use comparison of experimental thermal conductivity measurements and first principles thermal conductivity calculations to characterize the thermal conductivity reduction. We find that partial ordering, characterized by local order, but Zn-site disorder on longer scales, leads to an intrinsic nanostructuring induced reduction in thermal conductivity, while retaining electron mobility. This provides a direction for identifying Zintl compounds with ultralow lattice thermal conductivity and good electrical conductivity.

Stabilizing the Optimal Carrier Concentration in Al/Sb-Codoped GeTe for High Thermoelectric Performance

GeTe is a promising thermoelectric material and has attracted growing research interest recently. In this study, the effect of Al doping and Al&Sb codoping on the thermoelectric properties of GeTe was investigated. Due to the presence of a high concentration of intrinsic Ge vacancies, pristine GeTe exhibited a very high hole concentration and unpromising thermoelectric performance. By Sb doping in GeTe, the hole concentration can be effectively reduced, thus improving the thermoelectric performance. Aluminum, as a p-type dopant in GeTe, will increase the hole concentration and lattice thermal conductivity; thus, it has long been considered as an unfavorable dopant for the optimization of GeTe-based materials. However, when Al and Sb were codoped into GeTe, the hole concentration was effectively suppressed, and the lattice thermal conductivity can be reduced. Eventually, a maximum zT of ∼2.0 at 773 K was achieved in Al&Sb-codoped Al_0.01Sb_0.1Ge_0.89Te.

Two Polymorphs of BaZn2P2: Crystal Structures, Phase Transition, and Transport Properties

The novel α-BaZn₂P₂ structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu₂S₂ structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn₂P₂ being previously identified as belonging to the ThCr₂Si₂ family and with the precedent of structural phase transitions between the α-BaCu₂S₂ type and the ThCr₂Si₂ type, the potential for the pattern to be extended to the two different structural forms of BaZn₂P₂ was explored. Thermal analysis suggests that a first-order phase transition occurs at ∼1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-BaZn₂P₂, the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn₂P₂ is a p-type, narrow-gap semiconductor with a direct bandgap of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the β-BaZn₂P₂ phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 μV/K to 184 μV/K at 600 K. The electrical resistivity (ρ) of α-BaZn₂P₂ is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.

Solid Solution Yb2-xCaxCdSb2: Structure, Thermoelectric Properties, and Quality Factor

Solid solutions of Yb_2-xA_xCdSb₂ (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb_2-xCa_xCdSb₂ has end members with different crystal structures. Yb₂CdSb₂ crystallizes in the polar space group Cmc2₁, whereas Ca₂CdSb₂ crystallizes in the centrosymmetric space group Pnma. Other solid solutions, Yb_2-xA_xCdSb₂ (A = Sr, Eu), crystallize in the polar space group for x ≤ 1, and compositions with x ≥ 1 have not been reported. Both structure types are composed of corner-sharing CdSb₄ tetrahedra condensed into sheets that differ by the stacking of the layers. Single crystals of the solid solution Yb_2-xCa_xCdSb₂ (x = 0-1) were studied to elucidate the structural transition between the Yb₂CdSb₂ and Ca₂CdSb₂ structure types. For x ≤ 1, the structures remain in the polar space group Cmc2₁. As the Ca content is increased, a positional disorder arises in the intralayer cation sites (Yb2/Ca2) and the Cd site, resulting in inversion of the CdSb₄ tetrahedral chain. This phenomenon could be indicative of an intergrowth of the opposing space group. The TE properties of polycrystalline samples of Yb_2-xCa_xCdSb₂ (x ≤ 1) were measured from 300 to 525 K. The lattice thermal conductivity is extremely low (0.3-0.4 W/m·K) and the Seebeck coefficients are high (100-180 μV/K) across the temperature range. First-principles calculations show a minimum in the thermal conductivity for the x = 0.3 composition, in good agreement with experimental data. The low thermal conductivity stems from the acoustic branches being confined to low frequencies and a large number of phonon scattering channels provided by the localized optical branches. The TE quality factor of the Yb_1.7A_0.3CdSb₂ (A = Ca, Sr, Eu) series has been calculated and predicts that the A = Ca and Sr solid solutions may not improve with carrier concentration optimization but that the Eu series is worthy of additional modifications. Overall, the x = 0.3 compositions provide the highest zT because they provide the best electronic properties with the lowest thermal conductivity.

Ternary Zinc Antimonides Unlocked Using Hydride Synthesis

Three new sodium zinc antimonides Na₁₁Zn₂Sb₅, Na₄Zn₉Sb₉, and NaZn₃Sb₃ were synthesized utilizing sodium hydride NaH as a reactive sodium source. In comparison to the synthesis using sodium metal, salt-like NaH can be ball-milled, leading to the easy and uniform mixing of precursors in the desired stoichiometric ratios. Such comprehensive compositional control enables a fast screening of the Na-Zn-Sb system and identification of new compounds, followed by their preparation in bulk with high purity. Na₁₁Zn₂Sb₅ crystallizes in the triclinic P1 space group (No. 2, Z = 2, a = 8.8739(6) Å, b = 10.6407(7) Å, c = 11.4282(8) Å, α = 103.453(2)°, β = 96.997(2)°, γ = 107.517(2)°) and features polyanionic [Zn₂Sb₅]^11- clusters with unusual 3-coordinated Zn atoms. Both Na₄Zn₉Sb₉ (Z = 4, a = 28.4794(4) Å, b = 4.47189(5) Å, c = 17.2704(2) Å, β = 98.3363(6)°) and NaZn₃Sb₃ (Z = 8, a = 32.1790(1) Å, b = 4.51549(1) Å, c = 9.64569(2) Å, β = 98.4618(1)°) crystallize in the monoclinic C2/m space group (No. 12) and have complex new structure types. For both compounds, their frameworks are built from ZnSb₄ distorted tetrahedra, which are linked via edge-, vertex-sharing, or both, while Na cations fill in the framework channels. Due to the complex structures, Na₄Zn₉Sb₉ and NaZn₃Sb₃ compounds exhibit low thermal conductivities (0.97-1.26 W·m^-1 K^-1) at room temperature, positive Seebeck coefficients (19-32 μV/K) suggestive of holes as charge carriers, and semimetallic electrical resistivities (∼1.0-2.3 × 10^-4 Ω·m). Na₄Zn₉Sb₉ and NaZn₃Sb₃ decompose into the equiatomic NaZnSb above ∼800 K, as determined by in situ synchrotron powder X-ray diffraction. The discovery of multiple ternary compounds highlights the importance of judicious choice of the synthetic method.

All-Scale Hierarchical Structure Contributing to Ultralow Thermal Conductivity of Zintl Phase CaAg0.2Zn0.4Sb

TiNiSi-type Zintl phase CaAgSb can transform into LiGaGe-type Zintl phase CaAg x Zn(1- x )/2Sb when some of the Ag atoms are substituted by Zn atoms, leading to an ultralow thermal conductivity of ≈0.4 W m-1 K-1 in the whole measured temperature range of CaAg0.2Zn0.4Sb. The microstructure is then investigated by spherical aberration-corrected electron microscopy on an atomic scale, which reveals an all-scale hierarchical structure that can scatter the phonons in a wide frequency range. There exist a large quantity of CaAgSb nanometer precipitates as well as quite a lot of edge dislocations close to these nanometer precipitates, thus releasing the stress caused by the mismatch between the precipitates and the parent phase. Many twin boundaries also exist around the CaAgSb precipitates. High-density point defects contain the randomly dispersed Ag vacancies and Zn atoms substituted for the Ag atoms. All these widely distributed multidimensional defects contribute to the decrease of lattice thermal conductivity in a wide temperature range.

Enhanced Thermoelectric Performance of LiZnSb-Alloyed CaZn0.4Ag0.2Sb by Band Engineering

LiZnSb is a Zintl phase that has been predicted to be a good material in thermoelectric applications for a long time. However, experimental work indicated that the synthesized LiZnSb materials were p type, and their maximum zT value is only 0.08 at 525 K. CaZn_0.4Ag_0.2Sb, which belongs to the LiGaGe structure type and is also closely associated with the LiZnSb structure, did show high zT plateaus in a wide range of temperature, with the mixed transition metal Zn/Ag sites regulated. By comparing their crystallographic and electronic band structures, it is evident that the interlayered distances in both compounds have a great effect on the regulation of the corresponding electrical transport properties. When alloying CaZn_0.4Ag_0.2Sb with LiZnSb, solid solutions form within a specific range, which led to a marked enhancement in the Seebeck coefficient through the orbital alignment and carrier concentration optimization. In addition, a low thermal conductivity was obtained owing to the reduced electronic component. With the above optimization, a maximum zT value of ∼1.3 can be realized for (CaZn_0.4Ag_0.2Sb)_0.87(LiZnSb)_0.13 at 873 K, more than twice that of the pristine CaZn_0.4Ag_0.2Sb and about 10-fold compared to that of LiZnSb. This work may shed new light on the optimization of thermoelectric properties based on Zintl phases, for which the crystal structures are usually very complicated and a direct correlation between the structures and properties is difficult to make.

Deconvoluting the Magnetic Structure of the Commensurately Modulated Quinary Zintl Phase Eu11-xSrxZn4Sn2As12

The structure, magnetic properties, and ¹⁵¹Eu and ¹¹⁹Sn Mössbauer spectra of the solid-solution Eu_11-xSr_xZn₄Sn₂As₁₂ are presented. A new commensurately modulated structure is described for Eu₁₁Zn₄Sn₂As₁₂ (R3m space group, average structure) that closely resembles the original structural description in the monoclinic C2/c space group with layers of Eu, puckered hexagonal Zn₂As₃ sheets, and Zn₂As₆ ethane-like isolated pillars. The solid-solution Eu_11-xSr_xZn₄Sn₂As₁₂ (0 < x < 10) is found to crystallize in the commensurately modulated R3 space group, related to the parent phase but lacking the mirror symmetry. Eu₁₁Zn₄Sn₂As₁₂ orders with a saturation plateau at 1 T for 7 of the 11 Eu²⁺ cations ferromagnetically coupled (5 K) and shows colossal magnetoresistance at 15 K. The magnetic properties of Eu₁₁Zn₄Sn₂As₁₂ are investigated at higher fields, and the ferromagnetic saturation of all 11 Eu²⁺ cations occurs at ∼8 T. The temperature-dependent magnetic properties of the solid solution were investigated, and a nontrivial structure-magnetization correlation is revealed. The temperature-dependent ¹⁵¹Eu and ¹¹⁹Sn Mössbauer spectra confirm that the europium atoms in the structure are all Eu²⁺ and that the tin is consistent with an oxidation state of less than four in the intermetallic region. The spectral areas of both Eu(II) and Sn increase at the magnetic transition, indicating a magnetoelastic effect upon magnetic ordering.

Vacancy ordering induced topological electronic transition in bulk Eu2ZnSb2

Metal-semiconductor transitions from changes in edge chirality from zigzag to armchair were observed in many nanoribbon materials, especially those based on honeycomb lattices. Here, this is generalized to bulk complex Zintl semiconductors, exemplified by Eu₂ZnSb₂ where the Zn vacancy ordering plays an essential role. Five Eu₂ZnSb₂ structural models are proposed to guide transmission electron microscopy imaging. Zigzag vacancy ordering models show clear metallicity, while the armchair models are semiconducting with indirect bandgaps that monotonously increase with the relative distances between neighboring ZnSb₂ chains. Topological electronic structure changes based on cation ordering in a Zintl compound point toward tunable and possibly switchable topological behavior, since cations in these are often mobile. Thus, their orderings can often be adjusted by temperature, minor alloying, and other approaches. We explain the electronic structure of an interesting thermoelectric and point the way to previously unidentified types of topological electronic transitions in Zintl compounds.

Revealing the origin of dislocations in Pb1-xSb2x/3Se (0 < x ≤ 0.07)

Defect engineering is an effective route to improve the performance of thermoelectric materials, including Sb doped PbSe, but the formation mechanism of defects remains unclear. In the thermoelectric material Pb1-xSb2x/3Se (0 < x ≤ 0.07), a large number of dislocations have been reported, and they enhance intermediate-frequency phonon scattering, thereby improving the zT value. However, the microstructural origin of dislocations remains unclear. In this paper, via a combination of atomic resolution scanning transmission electron microscopy and density functional theory, we successfully revealed the microstructure of Pb1-xSb2x/3Se (x = 0-0.07) for in-depth understanding of the formation mechanism of dislocations. Plenty of zinc blende (ZB) nanostructures are found in the PbSe matrix with a rock salt (RS) structure, and the theoretical calculations confirm its viability from the point of view of formation energy. A similar ZB structure is identified in the dislocation cores of Sb-doped materials as well, and thus the formation mechanism of dislocations is discussed for this PbSe system. This result provides important guidance to understand the structural evolution in compounds with a RS structure, especially in high-performance lead chalcogenide thermoelectric materials.

Ternary Compounds Cu3RTe3 (R = Y, Sm, and Dy): A Family of New Thermoelectric Materials with Trigonal Structures

In this work, we report a series of Cu₃RTe₃ (R = Y, Sm, and Dy) ternary compounds with a trigonal structure (R3̅) as a family of new thermoelectric materials. First-principles calculations show that Cu₃RTe₃ (R = Y, Sm, and Dy) compounds are semiconductors with similar band structures and moderate band gaps (0.69-0.82 eV). The synthesized polycrystalline Cu₃RTe₃ (R = Y, Sm, and Dy) compounds possess moderate carrier concentrations (0.8-2.2 × 10²⁰ cm^-3) and density-of-state effective masses (around 1.1 m_e), yielding decent electrical transport performance. Furthermore, intrinsically low lattice thermal conductivities, below 1 W m^-1 K^-1 at 300-900 K, originating from the heavy average atomic masses and large number of atoms in the unit cell, are observed for Cu₃RTe₃ (R = Y, Sm, and Dy). Finally, Cu₃DyTe₃ demonstrates a peak dimensionless figure of merit of 0.9 at 900 K, which is among the highest reported for the Cu/Ag-based tellurides.

Enhanced Average Thermoelectric Figure of Merit of p-Type Zintl Phase Mg2ZnSb2 via Zn Vacancy Tuning and Hole Doping

Isoelectronic Zn substitution at the Mg site has been proved to be effective in regulating the carrier concentration of p-type Mg₃Sb₂ Zintl phase. However, the reported thermoelectric performance is still unsatisfactory compared with that of n-type Mg₃Sb₂ due to the poor electrical transport properties. Here, we report an enhanced average ZT through improving low-temperature ZTs by introducing Zn vacancy followed suppressing the bipolar effect by doping. First, the Zn vacancy simultaneously increases the power factor and decreases the thermal conductivity, leading to a peak ZT value of ∼0.52 at 773 K in Mg₂Zn_0.98Sb₂. Additionally, doping Li or Ag at the Mg site is identified as a high-efficiency strategy for further increasing the carrier concentration and hence suppressing the bipolar effect. Finally, a peak ZT of ∼0.73 at 773 K and an average ZT of ∼0.46 between 300 and 773 K were obtained in Mg_1.98Li_0.02Zn_0.98Sb₂.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Good stability and high thermoelectric performance of Fe doped Cu1.80S

By doping Fe in Cu_1.80S, good stability and a high thermoelectric performance were simultaneously obtained in the intermediate temperature range.

Enhanced Thermoelectric Performance of Zintl Phase Ca9Zn4+xSb9 by Beneficial Disorder on the Selective Cationic Site

Zintl phase compounds Ca₉Zn_4+xSb₉ have promising thermoelectric properties due to their complex crystal structure and tunable interstitial Zn. In this work, we prepared nominal Ca₉Zn_4+xSb₉ (x = 0.5, 0.6, 0.7, and 0.8) using ball milling and hot pressing. Further decreased lattice thermal conductivity was obtained by isoelectronic substitution of Eu on the selective Ca site, which is farther away from the framework of [Zn_4+xSb₉]^δ- for the smaller disturbance of carrier transport. Together with the intensively enhanced carrier mobility, which is attributed to the decreased effective mass and the increased interstitial Zn by inclusion of Eu, an increased peak ZT value to ∼1.05 at 773 K and an enhanced average ZT value to ∼0.73 from 300 to 823 K were achieved in Ca_6.75Eu_2.25Zn_4.7Sb₉.