High Thermoelectric Performance in Supersaturated Solid Solutions and Nanostructured n‐Type PbTe–GeTe

Sb‐doped and GeTe‐alloyed n‐type thermoelectric materials that show an excellent figure of merit ZT in the intermediate temperature range (400–800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as single‐phase solid solution (SS) or two‐phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the density‐of‐states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (κ_lat). Specifically, the Seebeck coefficient of Pb_0.988Sb_0.012Te–13%GeTe–Nano approaches ?280 µV K^?1 at 673 K with a low κ_lat of 0.56 W m^?1 K^?1 at 573 K. Consequently, a peak ZT value of 1.38 is achieved at 623 K. Moreover, a high average ZT_avg value of ≈1.04 is obtained in the temperature range from 300 to 773 K for n‐type Pb_0.988Sb_0.012Te–13%GeTe–Nano.     

High Thermoelectric Performance of ZnO by Coherent Phonon Scattering and Optimized Charge Transport

ZnO is identified as a potentially attractive n-type oxide thermoelectric material due to its abundance, nontoxicity, and a high degree of stability. However, working with ZnO is challenging due to its high thermal conductivity from its strong ionic bonds and low electrical conductivity due to its low charge concentrations. Here, it is demonstrated that the electrical and thermal transport properties of ZnO can be simultaneously improved via the successful doping of Al and ZnS coating. The ZnS coating in Al-doped ZnO is observed and analyzed through microstructure and spectroscopic studies. The power factor for 1% ZnS-coated Zn_0.98Al_0.02O is increased to ≈0.75 mW m⁻¹ K⁻² at 1073 K, 161% higher than pure ZnO. This enhancement in the power factor can be explained by the aliovalent Al³⁺ doping and modifications in intrinsic defects, leading to an increased carrier concentration. Interestingly, ZnS coating significantly reduces lattice thermal conductivity to ≈2.31 W m⁻¹ K⁻¹ at 1073 K for 2% ZnS-coated Zn_0.98Al_0.02O, a 62% decrease over pure ZnO. This large reduction in lattice thermal conductivity can be elucidated based on coherent phonon scattering via Callaway's model. Overall, the figure of merit, zT, increases to 0.2 in 2% ZnS-coated Zn_0.98Al_0.02O, which is 272% higher than pure ZnO at 1073 K.     

Analysis of Nanostructuring in High Figure‐of‐Merit Ag1–xPbmSbTe2+m Thermoelectric Materials

Thermoelectric materials based on quaternary compounds Ag_1?xPb_mSbTe_2+m exhibit high dimensionless figure‐of‐merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag_0.53Pb₁₈Sb_1.2Te₂₀ that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag₂Te phase based on the Ti₂Ni type structure. In‐situ high‐temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K.     

UV–Vis–Infrared Light Driven Thermocatalytic Activity of Octahedral Layered Birnessite Nanoflowers Enhanced by a Novel Photoactivation

Nanoflowers of octahedral layered birnessite (OL‐NF) were synthesized and characterized with a variety of techniques. Remarkably, OL‐NF exhibits a catalytic activity with a high efficiency for CO oxidation under irradiation of the full solar spectrum, visible‐infrared, or infrared light. This highly efficient catalytic activity under solar‐light irradiation originates from solar‐light‐driven thermocatalysis related to the efficient photothermal conversion and thermocatalytic activity of OL‐NF. A conceptually novel photoactivation effect is found to significantly improve the activity of the lattice oxygen of OL‐NF, thus considerably increasing the thermocatalytic activity of OL‐NF.     

The Zintl Compound Ca5Al2Sb6 for Low‐Cost Thermoelectric Power Generation

Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca₅Al₂Sb₆ is a charge‐balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ～2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca₅Al₂Sb₆ possesses an extremely low lattice thermal conductivity (0.6 W mK^?1 at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_4.75Na_0.25Al₂Sb₆ possesses a near‐optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca₅Al₂Sb₆ possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.     

Enhanced Thermoelectric Performance in 18‐Electron Nb0.8CoSb Half‐Heusler Compound with Intrinsic Nb Vacancies

Typical 18‐electron half‐Heusler compounds, ZrNiSn and NbFeSb, are identified as promising high‐temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, is recently reported to also exhibit thermoelectric properties of a heavily doped n‐type semiconductor. Here for the first time, it is experimentally demonstrated that the nominal 19‐electron NbCoSb is actually the composite of 18‐electron Nb_0.8+δCoSb (0 ≤ δ < 0.05) and impurity phases. Single‐phase Nb_0.8+δCoSb with intrinsic Nb vacancies, following the 18‐electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ≈100% and a maximum zT of 0.9 is achieved in Nb_0.83CoSb at 1123 K. This work expands the family of half‐Heusler thermoelectric materials and opens a new avenue for searching for nominal 19‐electron half‐Heusler compounds with intrinsic vacancies as promising thermoelectric materials.     

Jahn–Teller Driven Electronic Instability in Thermoelectric Tetrahedrite

Tetrahedrite, Cu₁₂Sb₄S₁₃, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal‐to‐semiconductor transition (MST), remains largely unknown. This work presents the first determination of the low‐temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, the results show that the trigonal–planar copper cations remain in planar coordination below the MST. The atomic displacement parameters of the trigonal–planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the MST. The phase transition is a consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found in the cubic phase. This study reveals that a Jahn–Teller electronic instability leads to the formation of “molecular‐like” Cu₅⁷⁺ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper–copper interactions. First principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. These results provide insights on the interplay between phonon transport, electronic properties, and crystal structure in mixed‐valence compounds.     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.     

Enhancement of Thermopower of TAGS‐85 High‐Performance Thermoelectric Material by Doping with the Rare Earth Dy

Enhancement of thermopower is achieved by doping the narrow‐band semiconductor Ag_6.52Sb_6.52Ge_36.96Te₅₀ (acronym TAGS‐85), one of the best p‐type thermoelectric materials, with 1 or 2% of the rare earth dysprosium (Dy). Evidence for the incorporation of Dy into the lattice is provided by X‐ray diffraction and increased orientation‐dependent local fields detected by ¹²⁵Te NMR spectroscopy. Since Dy has a stable electronic configuration, the enhancement cannot be attributed to 4f‐electron states formed near the Fermi level. It is likely that the enhancement is due to a small reduction in the carrier concentration, detected by ¹²⁵Te NMR spectroscopy, but mostly due to energy filtering of the carriers by potential barriers formed in the lattice by Dy, which has large both atomic size and localized magnetic moment. The interplay between the thermopower, the electrical resistivity, and the thermal conductivity of TAGS‐85 doped with Dy results in an enhancement of the power factor (PF) and the thermoelectric figure of merit (ZT) at 730 K, from PF = 28 μW cm^?1 K^?2 and ZT ≤ 1.3 in TAGS‐85 to PF = 35 μW cm^?1 K^?2 and ZT ≥ 1.5 in TAGS‐85 doped with 1 or 2% Dy for Ge. This makes TAGS‐85 doped with Dy a promising material for thermoelectric power generation.     

Exceptional Performance Driven by Planar Honeycomb Structure in a New High Temperature Thermoelectric Material BaAgAs

The discovery of new, high-performing thermoelectrics is of vital importance to promoting thermal energy conversion efficiency. Herein, a new p-type thermoelectric material BaAgAs with an exceptional figure of merit (zT) surpassing 1.1 at 970 K is present as a promising candidate for high-temperature applications. Verified by comprehensive experimental and theoretical investigations, BaAgAs possesses two intrinsic features in favoring zT: i) low lattice thermal conductivity, ascribed to the heavy element Ba in a loose mono-hexagonal layer, the large mass fluctuation in the Ag-As honeycomb layer, and the alternately interlayer stacking between mono-hexagonal and honeycomb layers; ii) good electrical properties contributed by multiple band transport, due to the small band offset between two valence band extremums and the strong anisotropic band effective mass. With enhanced phonon–phonon scattering via Sb/Bi substitution on the As sites, the lattice thermal conductivity is minimized, which results in significantly enhanced zT values. Additionally, an inspiring prediction via the first-principles calculation suggests that n-type BaAgAs can potentially outperform its p-type counterpart due to its higher conducting band degeneracy. This study will stimulate intense interests in the exploration of compounds with planar honeycomb structures as new high-performance thermoelectric materials.     

Design of High‐Performance Disordered Half‐Heusler Thermoelectric Materials Using 18‐Electron Rule

Ternary half‐Heusler (HH) alloys display intriguing functionalities ranging from thermoelectric to magnetic and topological properties. For thermoelectric applications, stable HH alloys with a nominal valence electron count (VEC) of 18 per formula or defective HH alloys with a VEC of 17 or 19 are assumed to be promising candidates. Inspired by the pioneering efforts to design a TiFe_0.5Ni_0.5Sb double HH alloy by combining 17‐electron TiFeSb and 19‐electron TiNiSb HH alloys, both high‐performance n‐type and p‐type materials based on the same parent TiFe_0.5Ni_0.5Sb are developed. First‐principles calculation results demonstrate their beneficial band structure having a high band degeneracy that contributes to their large effective mass and thereby maintains their high Seebeck coefficient values. Due to the strong Fe/Ni disorder effect, TiFe_0.5Ni_0.5Sb exhibits a much lower lattice thermal conductivity than does TiCoSb, consistent with very recently reported results. Furthermore, tuning the ratio of Fe and Ni leads to achieving both p‐ and n‐types, and alloying Ti by Hf further enhances the thermoelectric performance significantly. A peak ZT of ≈1 and ≈0.7 at 973 K are achieved in the p‐type and n‐type based on the same parent, respectively, which are beneficial and promising for real applications.     

XBi4S7 (X = Mn,Fe): New Cost‐Efficient Layered n‐Type Thermoelectric Sulfides with Ultralow Thermal Conductivity

A new class of cost‐efficient n‐type thermoelectric sulfides with a layered structure is reported, namely MnBi₄S₇ and FeBi₄S₇. Theoretical calculations combined with synchrotron X‐ray/neutron diffraction analyses reveal the origin of their electronic and thermal properties. The complex low‐symmetry monoclinic crystal structure generates an electronic band structure with a mixture of heavy and light bands near the conduction band edge, as well as vibrational properties favorable for high thermoelectric performance. The low thermal conductivity can be attributed to the complex layered crystal structure and to the existence of the lone pair of electrons in Bi³⁺. This feature combined with the relatively high power factor lead to a figure of merit as high as 0.21 (700 K) in undoped MnBi₄S₇, making this material a promising n‐type candidate for low‐ and intermediate‐temperature thermoelectric applications.     

Thermal Conductivity and Phonon Scattering Processes of ALD Grown PbTe–PbSe Thermoelectric Thin Films

This work studies the thermal conductivity and phonon scattering processes in a series of n‐type lead telluride‐lead selenide (PbTe–PbSe) nanostructured thin films grown by atomic layer deposition (ALD). The ALD growth of the PbTe–PbSe samples in this work results in nonepitaxial films grown directly on native oxide/Si substrates, where the Volmer–Weber mode of growth promotes grains with a preferred columnar orientation. The ALD growth of these lead‐rich PbTe, PbSe, and PbTe–PbSe thin films results in secondary oxide phases, along with an increase microstructural quality with increased film thickness. The compositional variation and resulting point and planar defects in the PbTe–PbSe nanostructures give rise to additional phonon scattering events that reduce the thermal conductivity below that of the corresponding ALD‐grown control PbTe and PbSe films. Temperature‐dependent thermal conductivity measurements show that the phonon scattering in these ALD‐grown PbTe–PbSe nanostructured materials, along with ALD‐grown PbTe and PbSe thin films, are driven by extrinsic defect scattering processes as opposed to phonon–phonon scattering processes intrinsic to the PbTe or PbSe phonon spectra. The implication of this work is that polycrystalline, nanostructured ALD composites of thermoelectric PbTe–PbSe films are effective in reducing the phonon thermal conductivity, and represent a pathway for further improvement of the figure of merit (ZT), enhancing their thermoelectric application potential.     

Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

Understanding the lattice dynamics and phonon transport from the perspective of chemical bonds is essential for improving and finding high‐efficiency thermoelectric materials and for many applications. Here, the coexistence of global and local weak chemical bonds is elucidated as the origin of the intrinsically low lattice thermal conductivity of non‐caged structure Nowotny–Juza compound, α‐MgAgSb, which is identified as a new type of promising thermoelectric material in the temperature range of 300–550 K. The global weak bonds of the compound lead to a low sound velocity. The unique three‐centered Mg? Ag? Sb bonds in α‐MgAgSb vibrate locally and induce low‐frequency optical phonons, resulting in “rattling‐like” thermal damping to further reduce the lattice thermal conductivity. The hierarchical chemical bonds originate from the low valence electron count of α‐MgAgSb, with the feature shared by Nowotny–Juza compounds. Low lattice thermal conductivities are therefore highly possible in this series of compounds, which is verified by phonon and bulk modulus calculations on some of the compositions.     

High-Density Frenkel Defects as Origin of N-Type Thermoelectric Performance and Low Thermal Conductivity in Mg3Sb2-Based Materials

Mg₃Sb₂-based intermetallic compounds with exceptionally high thermoelectric performance exhibit unconventional n-type dopability and anomalously low thermal conductivity, attracting much attention to the underlying mechanisms. To date, investigations have been limited to first-principle calculations and thermodynamic analysis of defect formation, and detailed experimental analysis on crystal structure and phonon modes has not been achieved. Here, a synchrotron X-ray diffraction study clarifies that, against a previous view of a simple crystal structure with a small unit cell, Mg₃Sb₂ is inherently a heavily disordered material with Frenkel defects, charge-neutral defect complexes of cation vacancies and interstitials. Ionic charge neutrality preserved in Mg₃Sb₂ is responsible for exotic n-type dopability, which is unachievable for other Zintl phase materials. The thermal conductivity of Mg₃Sb₂ exhibits deviation from the standard T⁻¹ temperature dependency with strongly limited phonon transport due to a strain field. Inelastic X-ray scattering measurement reveals enhanced phonon scattering induced by disorder. The results will draw renewed attention to crystal defects and disorder as means to explore new high-performance thermoelectric materials.     

Controlling the Thermoelectric Properties of Organometallic Coordination Polymers via Ligand Design

Organometallic coordination polymers (OMCPs) are a promising class of thermoelectric materials with high electrical conductivities and thermal resistivities. The design criteria for these materials, however, remain elusive and so far material modifications have been focused primarily on the nature of the metal cation to tune the thermoelectric properties. Herein, an alternative approach is described by synthesizing new organic ligands for OMCPs, allowing modulation of the thermoelectric properties of the novel OMCP materials over several orders of magnitude, as well as controlling the polarity of the Seebeck coefficient. Extensive material purification combined with spectroscopy experiments and calculations furthermore reveal the charge‐neutral character of the polymer backbones. In the absence of counter‐cations, the OMCP backbones are composed of air‐stable, ligand‐centered radicals. The findings open up new synthetic possibilities for OMCPs by removing structural constraints and putting significant emphasis on the molecular structure of the organic ligands in OMCP materials to tune their thermoelectric properties.     

All‐Layered 2D Optoelectronics: A High‐Performance UV–vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes

Nanoelectronics is in urgent demand of exceptional device architecture with ultrathin thickness below 10 nm and dangling‐bond‐free surface to break through current physical bottleneck and achieve new record of integration level. The advance in 2D van der Waals materials endows scientists with new accessibility. This study reports an all‐layered 2D Bi₂Te₃‐SnSe‐Bi₂Te₃ photodetector, and the broadband photoresponse of the device from ultraviolet (370 nm) to near‐infrared (808 nm) is demonstrated. In addition, the optimized responsivity reaches 5.5 A W^?1, with the corresponding eternal quantum efficiency of 1833% and detectivity of 6 × 10¹⁰ cm Hz^1/2 W^?1. These figures‐of‐merits are among the best values of the reported all‐layered 2D photodetectors, which are several orders of magnitude higher than those of the previous SnSe photodetectors. The superior device performance is attributed to the synergy of highly conductive surface state of Bi₂Te₃ topological insulator, perfect band alignment between Bi₂Te₃ and SnSe as well as small interface potential fluctuation. Meanwhile, the all‐layered 2D device is further constructed onto flexible mica substrate and its photoresponse is maintained roughly unchanged upon 60 bending cycles. The findings represent a fundamental scenario for advancement of the next generation high performance and high integration level flexible optoelectronics.     

Layered Materials in the Magnesium Ion Batteries: Development History,Materials Structure,and Energy Storage Mechanism

Layered crystal materials have blazed a promising trail in the design and optimization of electrodes for magnesium ion batteries (MIBs). The layered crystal materials effectively improve the migration kinetics of the Mg²⁺ storage process to deliver a high energy and power density. To meet the future demand for high-performance MIBs, significant work has been applied to layered crystal materials, including crystal modification, mechanism investigation, and micro/nanostructure design. Herein, this review presents a comprehensive overview of layered crystal materials applied to MIBs, from development history to current applications. It focuses on the relationship between the layered crystal structure and the energy storage mechanism. Meanwhile, recent achievements in the design principles of layered crystal materials and their application to electrodes are summarized. Finally, future perspectives on the application of layered materials in MIBs are presented. The overview of the development process and structural characteristics contributes to a thorough understanding of these materials, while a discussion of design strategies and practical applications can inspire further research. Therefore, this review provides guidance and assistance for constructing high-performance MIBs.     

Simultaneous Electrical and Thermoelectric Parameter Retrieval via Two Terminal Current–Voltage Measurements on Individual ZnO Nanowires

The thermoelectric parameters, in particular the thermal conductivity and dimensionless figure of merit ZT, of ZnO nanowires, are estimated via two terminal current–voltage measurements. The measurements are carried out in situ in a transmission electron microscope and negative differential conductance is observed on individually suspended ZnO nanowires. From the low bias region of the current–voltage curve, the electrical parameters, including carrier concentration and mobility, are obtained by fitting the experimental data using a metal–semiconductor–metal model. The thermal conductivity is extracted from the high bias region of the same current–voltage curve using a self‐consistent method, which combines the self‐heating thermal conduction and electrical transport properties of ZnO nanowires. It is shown that the thermal conductivity of ZnO nanowires is suppressed significantly in comparison with that of bulk ZnO, which is attributed to the strong surface scattering of phonons. The thermal conductivity is also found to decrease more steeply than the expected $ {1 \mathord{\left/{\vphantom {1 T}} \right.} T} $ trend, but does obey a $ {1 \mathord{\left/{\vphantom {1 {\left({\alpha T + \beta T^2} \right)}}} \right. } {\left({\alpha T + \beta T^2} \right)}} $ relation; this is shown to result from four‐phonon processes at high temperatures. The dimensionless figure of merit ZT is determined to be about 0.1 at 970 K. Finally, the thermoelectric properties of individual ZnO nanowires are also discussed, indicating that ZnO nanowires are promising high temperature thermoelectric materials.     

In Situ X‐Ray Diffraction Studies on Structural Changes of a P2 Layered Material during Electrochemical Desodiation/Sodiation

Sodium layered oxides with mixed transition metals have received significant attention as positive electrode candidates for sodium‐ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X‐ray absorption spectroscopy reveals the phase transition mechanism for the ternary transition metal system (Fe–Mn–Co) with P2 stacking. In situ synchrotron X‐ray diffraction using a capillary‐based microbattery cell shows a structural change from P2 to O2 in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ at the voltage plateau above 4.1 V on desodiation. The P2 structure is restored upon subsequent sodiation. The lattice parameter c in the O2 structure decreases significantly, resulting in a volumetric contraction of the lattice toward a fully charged state. Observations on the redox behavior of each transition metal in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ using X‐ray absorption spectroscopy indicate that all transition metals are involved in the reduction/oxidation process.