Concerted Rattling in CsAg5 Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

High throughput screening of semiconductors with low lattice thermal transport induced by long-range interactions

Semiconductors with long-range interactions (LRI) due to resonant bonding exhibit delocalized electronic states and low lattice thermal conductivity, contributing to the efficiency of heat-to-electricity conversion. Here, we build a descriptor for high-throughput screening of LRI materials from the second-order interaction force constants. We identify 75 semiconducting candidates from the binary compounds in the MatHub-3d database that contain LRI. By analyzing the bonding properties of LRI atoms, we classify LRI in materials into two categories: type I and type II. In the structural unit of type I LRI, the atoms have strong bond connections, while a weak bond exists between the two groups in the structural unit of type II LRI. We have identified atypical type I LRI formed by Sb-Sb and Mg-Mg pairs in the emerging thermoelectric material Mg3Sb2, resulting in the softening of TA1 phonons and large anharmonicity. For type II LRI, the LRI of Ge-Ge and Se-Se pairs in R3m-GeSe can cross different layers. Moreover, we observe a combination of type II LRI and rattling effect in BaSe2 to restrict thermal transport. This work is of great significance for understanding the relationship between LRI and thermal transport properties, and for designing new LRI-induced low lattice thermal conductivity materials.

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

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

Chemically Transformed Ag2 Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications

Flexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2 Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2 Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt-grown Ag2 Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n-type) of Ag2 Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc ) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Ba10In2Mn11Si3O12S18: First Hexanary Oxychalcogenide Containing an Infrequent Three-Dimensional Noncentrosysmmetric Framework

Noncentrosymmetric (NCS) oxychalcogenides have attracted great attention in recent years due to their immense potential as candidates for IR nonlinear-optical (NLO) applications. Despite notable advancements in this field, the discovery of oxychalcogenides with three-dimensional (3D) framework structures remains a formidable challenge. In this study, we report the discovery of the first hexanary oxychalcogenide, Ba10In2Mn11Si3O12S18, exhibiting second-order NLO activity, using a high-temperature solid-phase method. This compound showcases a novel structure type, featuring an uncommon NCS 3D [In2Mn11Si3O12S18]20- framework formed by vertex-sharing [(Mn/In)S6] octahedra, [(Mn/In)OS3] tetrahedra, and [SiO4] tetrahedra, with charge-balanced Ba2+ cations occupying the channels. Our study serves as a source of inspiration for researchers to further investigate the synthesis of novel NLO-active oxychalcogenides with 3D frameworks.

Functional Alkali Metal-Based Ternary Chalcogenides: Design, Properties, and Opportunities

The search for novel materials has recently brought research attention to alkali metal-based chalcogenides (ABZ) as a new class of semiconducting inorganic materials. Various theoretical and computational studies have highlighted many compositions of this class as ideal functional materials for application in energy conversion and storage devices. This Perspective discusses the expansive compositional landscape of ABZ compositions that inherently gives a wide spectrum of properties with great potential for application. In the present paper, we examine the technique of synthesizing this particular class of materials and explore their potential for compositional engineering in order to manipulate key functional properties. This study presents the notable findings that have been documented thus far in addition to outlining the potential avenues for implementation and the associated challenges they present. By fulfilling the sustainability requirements of being relativity earth-abundant, environmentally benign, and biocompatible, we anticipate a promising future for alkali metal chalcogenides. Through this Perspective, we aim to inspire continued research on this emerging class of materials, thereby enabling forthcoming breakthroughs in the realms of photovoltaics, thermoelectrics, and energy storage.

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.

A novel bifunctional thioarsenate based on unprecedented molecular [Cd4As8Se16(Se2)2]8- cluster anions

Exploring and developing new functional inorganic chalcogenides with unique structures is always one of the most important missions in solid-state chemistry, especially those with molecular structures. Herein, a novel quaternary thioarsenate, Cs2CdAsSe5, is found to be based on an unprecedented molecular (poly)chalcogenide cluster architecture, which has never been discovered in inorganic chalcogenide systems. This rare windmill-like [Cd4As8Se16(Se2)2]8- cluster is made of four [CdSe4] and [As(V)Se4] tetrahedra via corner-sharing Se atoms and Se-Se bonds. Specifically, Cs2CdAsSe5 exhibits a remarkable photocurrent response and a large computationally predicted birefringence, and the origin of the optoelectronic performance and optical anisotropy is confirmed by detailed theoretical investigation.

Lattice Dynamics and Thermal Transport in Semiconductors with Anti-Bonding Valence Bands

Achieving high thermoelectric performance requires efficient manipulation of thermal conductivity and a fundamental understanding of the microscopic mechanisms of phonon transport in crystalline solids. One of the major challenges in thermal transport is achieving ultralow lattice thermal conductivity. In this study, we use the anti-bonding character of the highest-occupied valence band as an efficient descriptor for discovering new materials with an ultralow thermal conductivity. We first examined the relationship between anti-bonding valence bands (ABVBs) and low lattice thermal conductivity in model systems PbTe and CsPbBr3. Then, we conducted a high-throughput search in the Materials Project database and identified over 600 experimentally stable binary semiconductors with an anti-bonding character in their valence bands. From our candidate list, we conducted a comprehensive analysis of the chemical bonds and the thermal transport in the XS family, where X = K, Rb, and Cs are alkaline metals. These materials all exhibit ultralow thermal conductivities less than 1 W/(m K) at room temperature despite simple structures. We attributed the ultralow thermal conductivity to the weakened bonds and increased phonon anharmonicity due to their ABVBs. Our results provide chemical intuitions to understand lattice dynamics in crystals and open up a convenient venue toward searching for materials with an intrinsically low lattice thermal conductivity.

A unified understanding of minimum lattice thermal conductivity

We propose a first-principles model of minimum lattice thermal conductivity ([Formula: see text]) based on a unified theoretical treatment of thermal transport in crystals and glasses. We apply this model to thousands of inorganic compounds and find a universal behavior of [Formula: see text] in crystals in the high-temperature limit: The isotropically averaged [Formula: see text] is independent of structural complexity and bounded within a range from ∼0.1 to ∼2.6 W/(m K), in striking contrast to the conventional phonon gas model which predicts no lower bound. We unveil the underlying physics by showing that for a given parent compound, [Formula: see text] is bounded from below by a value that is approximately insensitive to disorder, but the relative importance of different heat transport channels (phonon gas versus diffuson) depends strongly on the degree of disorder. Moreover, we propose that the diffuson-dominated [Formula: see text] in complex and disordered compounds might be effectively approximated by the phonon gas model for an ordered compound by averaging out disorder and applying phonon unfolding. With these insights, we further bridge the knowledge gap between our model and the well-known Cahill-Watson-Pohl (CWP) model, rationalizing the successes and limitations of the CWP model in the absence of heat transfer mediated by diffusons. Finally, we construct graph network and random forest machine learning models to extend our predictions to all compounds within the Inorganic Crystal Structure Database (ICSD), which were validated against thermoelectric materials possessing experimentally measured ultralow κL. Our work offers a unified understanding of [Formula: see text], which can guide the rational engineering of materials to achieve [Formula: see text].

Ultralow thermal conductivity and anisotropic thermoelectric performance in layered materials LaMOCh (M = Cu, Ag; Ch = S, Se)

In layered materials with the stacking axis perpendicular to the basal plane, anharmonicity strongly affects phonon propagation due to weak interlayer coupling, which is helpful to reduce the lattice thermal conductivity and improve the thermoelectric (TE) performance significantly. By combining first-principles calculations and the Boltzmann transport equation, we systematically analyzed and evaluated the lattice thermal conductivity and TE properties of LaMOCh (M = Cu, Ag; Ch = S, Se). The results indicate that these layered materials exhibit ultralow lattice thermal conductivities of 0.24-0.37 W m^-1 K^-1 along the interlayer direction at room temperature. The low lattice thermal conductivities have been analyzed from some inherent phonon properties, such as low acoustic phonon group velocity, large Grüneisen parameters, and a short phonon relaxation time. Originating from their natural layered crystal structure, the thermal and electronic transports (i.e., thermal conductivity, Seebeck coefficient, and electrical conductivity) are both highly anisotropic between their intralayer and interlayer directions. Finally, we obtained ZT values of 1.17 and 1.26 at 900 K along the interlayer direction for n-type LaCuOSe and LaAgOSe, respectively. Generally, LaMOSe exhibit larger anisotropy than LaMOS, in both n- and p-types of doping. Our findings of low thermal conductivities and large anisotropic TE performances of these layered systems should stimulate much attention in BiCuOSe and alike layered TE families.

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

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

Insights into Low Thermal Conductivity in Inorganic Materials for Thermoelectrics

Efficient manipulation of thermal conductivity and fundamental understanding of the microscopic mechanisms of phonon scattering in crystalline solids are crucial to achieve high thermoelectric performance. Thermoelectric energy conversion directly and reversibly converts between heat and electricity and is a promising renewable technology to generate electricity by recovering waste heat and improve solid-state refrigeration. However, a unique challenge in thermal transport needs to be addressed to achieve high thermoelectric performance: the requirement of crystalline materials with ultralow lattice thermal conductivity (κ_L). A plethora of strategies have been developed to lower κ_L in crystalline solids by means of nanostructural modifications, introduction of intrinsic or extrinsic phonon scattering centers with tailored shape and dimension, and manipulation of defects and disorder. Recently, intrinsic local lattice distortion and lattice anharmonicity originating from various mechanisms such as rattling, bonding heterogeneity, and ferroelectric instability have found popularity. In this Perspective, we outline the role of manipulation of chemical bonding and structural chemistry on thermal transport in various high-performance thermoelectric materials. We first briefly outline the fundamental aspects of κ_L and discuss the current status of the popular phonon scattering mechanisms in brief. Then we discuss emerging new ideas with examples of crystal structure and lattice dynamics in exemplary materials. Finally, we present an outlook for focus areas of experimental and theoretical challenges, possible new directions, and integrations of novel techniques to achieve low κ_L in order to realize high-performance thermoelectric materials.

Ag9GaSe6: high-pressure-induced Ag migration causes thermoelectric performance irreproducibility and elimination of such instability

The argyrodite Ag₉GaSe₆ is a newly recognized high-efficiency thermoelectric material with an ultralow thermal conductivity; however, liquid-like Ag atoms are believed to cause poor stability and performance irreproducibility, which was evidenced even after the 1^st measurement run. Herein, we demonstrate the abovementioned instability and irreproducibility are caused by standard thermoelectric sample hot-pressing procedure, during which high pressure promotes the 3-fold-coordinated Ag atoms migrate to 4-fold-coordinated sites with higher-chemical potentials. Such instability can be eliminated by a simple annealing treatment, driving the metastable Ag atoms back to the original sites with lower-chemical potentials as revealed by the valence band X-ray photoelectron chemical potential spectra and single crystal X-ray diffraction data. Furthermore, the hot-pressed-annealed samples exhibit great stability and TE property repeatability. Such a stability and repeatability has never been reported before. This discovery will give liquid-like materials great application potential.

The Ag₉GaSe₆ is a high-efficient thermoelectric material yet suffers instability. Here, the authors demonstrate the instability is caused by the pressure-induced liquid-like Ag migration, which can be eliminated by a simple annealing treatment.

Monolayer SnI2: An Excellent p-Type Thermoelectric Material with Ultralow Lattice Thermal Conductivity

Using density functional theory and semiclassical Boltzmann transport equation, the lattice thermal conductivity and electronic transport performance of monolayer SnI₂ were systematically investigated. The results show that its room temperature lattice thermal conductivities along the zigzag and armchair directions are as low as 0.33 and 0.19 W/mK, respectively. This is attributed to the strong anharmonicity, softened acoustic modes, and weak bonding interactions. Such values of the lattice thermal conductivity are lower than those of other famous two-dimensional thermoelectric materials such as MoO₃, SnSe, and KAgSe. The two quasi-degenerate band valleys for the valence band maximum make it a p-type thermoelectric material. Due to its ultralow lattice thermal conductivities, coupled with an ultrahigh Seebeck coefficient, monolayer SnI₂ possesses an ultrahigh figure of merits at 800 K, approaching 4.01 and 3.34 along the armchair and zigzag directions, respectively. The results indicate that monolayer SnI₂ is a promising low-dimensional thermoelectric system, and would stimulate further theoretical and experimental investigations of metal halides as thermoelectric materials.

Low thermal conductivity and high performance anisotropic thermoelectric properties of XSe (X = Cu, Ag, Au) monolayers

Combining density functional theory (DFT) and semi-classic Boltzmann transport theory, we report the thermoelectric (TE) performance of a family of two-dimensional (2D) group IB-selenides XSe (X = Cu, Ag, Au). The results show that these monolayers exhibit small and anisotropic phonon velocities (0.98-3.84 km s^-1), large Grüneisen parameters (up to 100), and drastic phonon scattering between the optical and acoustic phonons. These intrinsic properties originate from strong phonon anharmonicity and suppress the heat transport capacity, resulting in low lattice thermal conductivities (12.54 and 1.22 W m^-1 K^-1) along the x- and y-directions for a CuSe monolayer. Among our studied monolayers, the 2D CuSe monolayer possesses the most remarkable TE performance with ultrahigh ZT (3.26) for n-type doping along the y-direction at 300 K. CuSe monolayer can achieve higher thermoelectric conversion efficiency at a lower synthetic preparation cost than the expensive AgSe and AuSe monolayers, and our work provides a theoretical basis for paving the way for further experimental studies.

High-Throughput Screening of Rattling-Induced Ultralow Lattice Thermal Conductivity in Semiconductors

Thermoelectric (TE) materials with rattling model show ultralow lattice thermal conductivity for high-efficient energy conversion between heat and electricity. In this work, by analysis of the key spirit of the rattling model, we propose an efficient empirical descriptor to realize the high-throughput screening of ultralow thermal conductivity in a series of semiconductors. This descriptor extracts the structural information of rattling atoms whose bond lengths with all the nearest neighboring atoms are larger than the sum of corresponding covalent radiuses. We obtain 1171 candidates from the Materials Project (MP) Database that contains more than 100 000 materials. Combining the empirical equation of high-throughput computation with a machine learning algorithm, we compute the approximate lattice thermal conductivities (κ_L) and find the κ_L values of 532 materials are less than 2.0 W m^-1 K^-1 at 300 K, which can be regarded as the criteria of ultralow κ_L in general. In particular, we demonstrate that halide double perovskites structures show ultralow κ_L, which provides valuable references for promising low κ_L materials in future experiments. In order to further verify our computational results, we calculate accurate κ_L for Rb₂SnBr₆ and CsCu₃O₂ as candidates with the low lattice thermal conductivity by solving the phonon Boltzmann transport equation. In particular, we demonstrate that Rb₂SnBr₆ has the lowest κ_L value of 0.1 W m^-1 K^-1 at 300 K of all known thermal conductivity materials with the rattling model so far.

Syntheses and characterization of two new layered ternary chalcogenides NaScQ2 (Q = Se and Te)

Two new metal ternary chalcogenides, NaScSe2 and NaScTe2, have been synthesized via high-temperature reaction.

Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted by firstprinciples calculations

Tailoring novel thermoelectric materials (TMs) with a high efficiency is challenging due to a difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose...

Mixed-Valence CsCu4Se3: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu+)4(Se2-)2](Se-) Double Anti-CaF2 Layer and the Soft Cs+ Sublattice

Crystalline solids that exhibit inherently low lattice thermal conductivity (κ_lat) have attracted a great deal of attention because they offer the only independent control for pursuing a high thermoelectric figure of merit (ZT). Herein, we report the successful preparation of CsCu₄Q₃ (Q = S (compound 1), Se (compound 2)) with the aid of a safe and facile boron-chalcogen method. The single-crystal diffraction data confirm the P4/mmm hierarchical structures built up by the mixed-valence [(Cu⁺)₄(Q^2-)₂](Q^-) double anti-CaF₂ layer and the NaCl-type Cs⁺ sublattice involving multiple bonding interactions. The electron-poor compound CsCu₄Q₃ features Cu-Q antibonding states around E_F that facilitates a high σ value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow κ_lat value of 2, 0.20 W/m/K at 650 K (70% lower than that of Cu₂Se), is mainly driven by the vibrational coupling of the rigid double anti-CaF₂ layer and the soft NaCl-type sublattice. The hierarchical structure increases the bond multiplicity, which eventually leads to a large phonon anharmonicity, as evidenced by the effective scattering of the low-lying optical phonons to the heat-carrying acoustic phonons. Consequently, the acoustic phonon frequency in 2 drops sharply from 118 cm^-1 (of Cu₂Se) to 48 cm^-1. In addition, the elastic properties indicate that the hierarchical structure largely inhibits the transverse phonon modes, leading to a sound velocity (1571 m/s) and a Debye temperature (189 K) lower than those of Cu₂Se (2320 m/s; 292 K).

Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal

Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2-2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m^-1 K^-1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material.

High thermoelectric performance enabled by convergence of nested conduction bands in Pb7Bi4Se13 with low thermal conductivity

Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450-800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

Rb2CuSb7S12: Quaternary Antimony-Rich Semiconductor Featuring a Three-Dimensional Open Framework and Exhibiting an Intriguing Photocurrent Response

Metal-rich chalcogenides with unique network architectures are rare but are of considerable interest because of their intriguing physical properties. In this work, a novel quaternary thioantimonate, Rb₂CuSb₇S₁₂, has been discovered by a facile surfactant-thermal reaction. It crystallizes monoclinic space group P1̅ (No. 2) and exhibits a unique Sb-rich three-dimensional (3D) [CuSb₇S₁₂]^2- framework surrounded by charge-compensating Rb⁺ cations. It is interesting to note that the Cu/Sb ratio of Rb₂CuSb₇S₁₂ represents the lowest limit in the quaternary A/Cu/Sb/Q (A = alkali metals; Q = chalcogen) system. Moreover, Rb₂CuSb₇S₁₂ shows rapid response and good reproducibility based on the photoelectrochemical tests. This study opens up opportunities for discovering the desirable physical properties in metal-rich chalcogenides.

Intrinsically ultralow thermal conductive inorganic solids for high thermoelectric performance

Thermoelectric materials which can convert heat energy to electricity rely on crystalline inorganic solid state compounds exhibiting low phonon transport (i.e. low thermal conductivity) without much inhibiting the electrical transport. Suppression of phonons traditionally has been carried out via extrinsic pathways, involving formation of point defects, foreign nanostructures, and meso-scale grains, but the incorporation of extrinsic substituents also influences the electrical properties. Crystalline materials with intrinsically low lattice thermal conductivity (κlat) provide an attractive paradigm as it helps in simplifying the complex interrelated thermoelectric parameters and allows us to focus largely on improving the electronic properties. In this feature article, we have discussed the chemical bonding and structural aspects in determining phonon transport through a crystalline material. We have outlined how the inherent material properties like lone pair, bonding anharmonicity, presence of intrinsic rattlers, ferroelectric instability, weak and rigid substructures, etc. influence in effectively suppressing the heat transport. The strategies summarized in this feature article should serve as a general guide to rationally design and predict materials with low κlat for potential thermoelectric applications.

Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4 TiSe4

Ultralow thermal conductivity draws great attention in a variety of fields of applications such as thermoelectrics and thermal barrier coatings. Herein, the crystal structure and transport properties of Cu₄ TiSe₄ are reported. Cu₄ TiSe₄ is a unique example of a non-toxic and low-cost material that exhibits a lattice ultra-low thermal conductivity of 0.19 Wm^-1  K^-1 at room temperature. The main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics. This ultralow value of lattice thermal conductivity (k_L ) can be attributed to the presence of the localized modes of Cu, which partially hybridize with the Se atoms, which in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with a reduced frequency. Like a phonon glass electron crystal, Cu₄ TiSe₄ could also open a route to efficient thermoelectric materials, even, with chalcogenides of relatively high electrical resistivity and a large band gap, provided that their structures offer a sublattice with lightly bound cations.

Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe1-xSex through Isoelectronic Substitution

In this paper, we report a series of x polycrystalline AgCuTe_1-xSe samples with high thermoelectric performance. X-ray photoelectron spectroscopy data suggest the observation of Ag⁺, Cu⁺, Te^2-, and Se^2- states of Ag, Cu, Te, and Se. Meanwhile, the carrier concentration of the obtained p-type samples changes from 9.12 × 10¹⁸ to 0.86 × 10¹⁸ cm^-3 as their carrier mobility varies from 698.55 to 410.12 cm²·V^-1·s^-1 at 300 K. Compared with undoped AgCuTe, an ultralow thermal conductivity is realized in AgCuTe_1-xSe_x due to the enhanced phonon scattering. Ultimately, a maximum figure of merit (ZT) of ∼1.45 at 573 K and a high average ZT above 1.0 at temperatures ranging from room temperature to 773 K can be achieved in AgCuTe_0.9Se_0.1, which increases by 186% compared to that of the undoped AgCuTe (0.82 at 573 K). This work provides a viable insight toward understanding the effect of the Se atom on the lattice structure and thermoelectric properties of AgCuTe and other transition-metal dichalcogenides.

Syntheses of five new layered quaternary chalcogenides SrScCuSe3, SrScCuTe3, BaScCuSe3, BaScCuTe3, and BaScAgTe3: crystal structures, thermoelectric properties, and electronic structures

Five new layered transition metal-based chalcogenides (SrScCuSe3, SrScCuTe3, BaScCuSe3, BaScCuTe3, and BaScAgTe3) were discovered by the exploratory solid-state method.

Origin of the difference in thermal conductivity and anharmonic phonon scattering between LiNbO3 and LiTaO3

The origin of the different thermal conductivities between LiNbO3 and LiTaO3 stems from the different scattering channels throughout the whole Brillouin zone, as well as the different emission channels of the LA branch along the Γ–F direction.

A new type of novel salt-inclusion chalcogenide with ultralow thermal conductivity

The design and development of novel chalcogenides with ultralow thermal conductivity is extremely important but very challenging for promoting the efficiencies of thermoelectric (TE) materials. Herein, a new type of salt-inclusion chalcogenide (SIC), [Rb6Cl][RE23Mn7Se44] (RE = Ho-Yb), was discovered via a modified flux method. They possessed [RESe6] and [MSe6] (M = RE/Mn) octahedra as basic building units, which interlinked to form a three-dimensional quasi-NaCl-type [RE23Mn7Se44]5- host framework, where the [Rb6Cl]5+ guest ions resided. Interestingly, these isomorphic compounds showed ultralow thermal conductivities (0.28-0.37 W m-1 K-1) at 673 K, which are reported for the first time in SICs. This work not only enriches SIC chemistry but also broadens the application of SICs in the TE field.

Dimer rattling mode induced low thermal conductivity in an excellent acoustic conductor

A solid with larger sound speeds usually exhibits higher lattice thermal conductivity. Here, we report an exception that CuP₂ has a quite large mean sound speed of 4155 m s⁻¹, comparable to GaAs, but single crystals show very low lattice thermal conductivity of about 4 W m⁻¹ K⁻¹ at room temperature, one order of magnitude smaller than GaAs. To understand such a puzzling thermal transport behavior, we have thoroughly investigated the atomic structures and lattice dynamics by combining neutron scattering techniques with first-principles simulations. This compound crystallizes in a layered structure where Cu atoms forming dimers are sandwiched in between P atomic networks. In this work, we reveal that Cu atomic dimers vibrate as a rattling mode with frequency around 11 meV, which is manifested to be remarkably anharmonic and strongly scatters acoustic phonons to achieve the low lattice thermal conductivity.

CuP2 has a puzzling thermal transport behavior, with low thermal conductivity but quite large mean sound speeds. Here, the authors conduct a systematical study of the atomic structure and lattice dynamics of CuP2 to reveal the origin, finding a dimer rattling behavior.

Achieving High Thermoelectric Performance in Rare-Earth Element-Free CaMg2Bi2 with High Carrier Mobility and Ultralow Lattice Thermal Conductivity

CaMg₂Bi₂-based compounds, a kind of the representative compounds of Zintl phases, have uniquely inherent layered structure and hence are considered to be potential thermoelectric materials. Generally, alloying is a traditional and effective way to reduce the lattice thermal conductivity through the mass and strain field fluctuation between host and guest atoms. The cation sites have very few contributions to the band structure around the fermi level; thus, cation substitution may have negligible influence on the electric transport properties. What is more, widespread application of thermoelectric materials not only desires high ZT value but also calls for low-cost and environmentally benign constituent elements. Here, Ba substitution on cation site achieves a sharp reduction in lattice thermal conductivity through enhanced point defects scattering without the obvious sacrifice of high carrier mobility, and thus improves thermoelectric properties. Then, by combining further enhanced phonon scattering caused by isoelectronic substitution of Zn on the Mg site, an extraordinarily low lattice thermal conductivity of 0.51 W m⁻¹ K⁻¹ at 873 K is achieved in (Ca_0.75Ba_0.25)_0.995Na_0.005Mg_1.95Zn_0.05Bi_1.98 alloy, approaching the amorphous limit. Such maintenance of high mobility and realization of ultralow lattice thermal conductivity synergistically result in broadly improvement of the quality factor β. Finally, a maximum ZT of 1.25 at 873 K and the corresponding ZT_ave up to 0.85 from 300 K to 873 K have been obtained for the same composition, meanwhile possessing temperature independent compatibility factor. To our knowledge, the current ZT_ave exceeds all the reported values in AMg₂Bi₂-based compounds so far. Furthermore, the low-cost and environment-friendly characteristic plus excellent thermoelectric performance also make the present Zintl phase CaMg₂Bi₂ more competitive in practical application.

Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs_{2}AgBiBr_{6}, the prototypical compound in this class and show that the lattice dynamics of Cs_{2}AgBiBr_{6} is highly anharmonic, largely in regards to tilting of AgBr_{6} and BiBr_{6} octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs_{2}AgBiBr_{6} yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.

Origin of High Thermoelectric Performance in Earth-Abundant Phosphide-Tetrahedrite

Phosphide-based thermoelectrics are a relatively less studied class of compounds, primarily due to the presence of light elements, which result in high thermal conductivity and inherent stability problems. In this work, we present a stable phosphide-tetrahedrite, Ag₆Ge₁₀P₁₂, which possesses the highest zT (∼0.7) among all known phosphides at intermediate temperatures (750 K). We examine the intrinsic electronic and thermal transport properties of this compound by expressing the transport properties in terms of weighted mobility (μ_W), transport coefficient (σ_E0), and material quality factor (B), from which we are able to elucidate that the origin of its high zT can be attributed to the platelike Fermi surface and high level of band multiplicity related to its complex band structure. Finally, we discuss the origin of the low lattice thermal conductivity observed in this compound using experimental sound velocity, elastic properties, and Debye-Callaway model, thus laying the foundation for similar stable phosphides as potentially earth-abundant and nontoxic intermediate-temperature thermoelectric materials.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

Enhanced Thermoelectric Properties of p-Type CaMg2Bi2 via a Synergistic Effect Originated from Zn and Alkali-Metal Co-doping

Bi-based Zintl phase CaMg₂Bi₂ is a promising thermoelectric material. Here, we report that the high-concentration point defects induced by equivalent Zn doping on the Mg site significantly enhance phonon scattering and then suppress lattice thermal conductivity by 50% at room temperature. Subsequently, partial substitution of divalent calcium ions with alkali-ion doping (Li, Na, K) not only optimizes the electrical transport properties by increasing the carrier concentration but also further reduces the lattice thermal conductivity through crystal disorder. Finally, the synergistic effect of Zn and Li co-doping leads to a high ZT of ∼1.0 at 873 K and an average ZT of 0.6 between 300 and 873 K for Ca_0.995Li_0.005Mg_1.9Zn_0.1Bi_1.98. This work demonstrates an instructive method to reduce the lattice thermal conductivity via doping at the Mg site, which has never been reported in the CaMg₂Bi₂ system. Moreover, high-performance Ca_0.995Li_0.005Mg_1.9Zn_0.1Bi_1.98 alloy does not contain any toxic elements and expensive rare earth elements, which is of great significance for the development of environment-friendly thermoelectric materials.

Cu4Bi4Se9: A Thermoelectric Symphony of Rattling, Anharmonic Lone-pair, and Structural Complexity

Thermoelectric technology, enabling an environmentally friendly direct heat to electricity conversion, provides a possible alternative energy solution. To obtain a higher thermoelectric conversion efficiency, a larger dimensionless figure of merit ZT is required, which is, however, very difficult owing to the mutually restricted and even reversely correlated key property parameters. Herein, we report for the first time the thermoelectric properties of novel Cu₄Bi₄S₉ and Cu₄Bi₄Se₉ materials with complicated orthorhombic Pnma structures. Cu₄Bi₄Se₉ exhibits an extremely low lattice thermal conductivity of about 0.29-0.35 W m^-1 K^-1 that is mainly ascribed to the high lattice anharmonicity coming from the synergistic effect of the crystal structure complexity, soft Cu-Se bonds with lower bonding energy, rattling of the Cu atoms, and the high anharmonicity of Bi atoms carrying stereochemically active lone-pair electrons. In spite of its poor electrical conductivity of 7.33 S cm^-1, Cu₄Bi₄Se₉ realizes a power factor of about 1.37 μW cm^-1 K^-2 at ∼530 K, and a figure of merit, ZT ∼0.24 at ∼530 K. Such a value is comparable with those of the Cu/Ag-Bi/Sb-S/Se-based ternary compounds, particularly, the 19 times higher ZT improvement with respect to the isostructural Cu₄Bi₄S₉ suggests that the enhancing factors mentioned above play significant roles.

Manipulating Localized Vibrations of Interstitial Te for Ultra-High Thermoelectric Efficiency in p-Type Cu-In-Te Systems

Thermoelectric materials are of imperative need on account of the worldwide energy crisis. However, their efficiency is limited by the interplay of high electrical and lower thermal conductivities, that is, the figure of merit (ZT). Owing to their unique crystal structures, Cu-In-Te-based chalcogenides are suitable for both and thus have attracted much attention recently as potential thermoelectrics. Here we explore a newly developed Cu-In-Te derivative compound Cu_3.52In_4.16Te₈. With a proper adjustment of Cu₂Te doping, this material shows an ultralow lattice thermal conductivity (κ_L) (0.3 WK^-1m^-1) and, consequently, a figure of merit (ZT) as high as 1.65(±0.15) at 815 K: the highest value reported for p-type Cu-In-Te to date. The reduction in κ_L is directly related to the alteration of local symmetry around the interstitial Te, resulting in an effectively optimized phonon transport through localized "rattling" of the same. Although the Hall carrier concentration reduces upon Cu₂Te addition due to the unpinning of the Fermi level (E_Fermi) toward the conduction band minimum, the power factor remains stable. The knowledge depicted here not only demonstrates the potential of Cu_3.52In_4.16Te₈-based alloys as a promising TE, but also provides guidelines for developing further high-performance thermoelectric materials by enhancing the electronic conductivity.

Revelation of Inherently High Mobility Enables Mg3Sb2 as a Sustainable Alternative to n-Bi2Te3 Thermoelectrics

Over the past years, thermoelectric Mg3Sb2 alloys particularly in n-type conduction, have attracted increasing attentions for thermoelectric applications, due to the multivalley conduction band, abundance of constituents, and less toxicity. However, the high vapor pressure, causticity of Mg, and the high melting point of Mg3Sb2 tend to cause the inclusion in the materials of boundary phases and defects that affect the transport properties. In this work, a utilization of tantalum-sealing for melting enables n-type Mg3Sb2 alloys to show a substantially higher mobility than ever reported, which can be attributed to the purification of phases and to the coarse grains. Importantly, the inherently high mobility successfully enables the thermoelectric figure of merit in optimal compositions to be highly competitive to that of commercially available n-type Bi2Te3 alloys and to be higher than that of other known n-type thermoelectrics at 300-500 K. This work reveals Mg3Sb2 alloys as a top candidate for near-room-temperature thermoelectric applications.

Thermoelectric Material SnPb2Bi2S6: The 4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity

Although the binary sulfides Bi₂S₃, PbS, and SnS have attracted extensive interest as thermoelectric materials, no quaternary sulfides containing Sn/Pb/Bi/S elements have been reported. Herein, we report the synthesis of a new quaternary sulfide, SnPb₂Bi₂S₆, which crystallizes in the orthorhombic space group Pnma with unit cell parameters of a = 20.5458(12) Å, b = 4.0925(4) Å, c = 13.3219(10) Å. SnPb₂Bi₂S₆ has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS₇ monocapped trigonal prisms. In the lillianite homologous series, SnPb₂Bi₂S₆ can be classified as ^4,4L, where the superscripted numbers indicate the maximum numbers of edge-sharing octahedra in the two adjacent NaCl-shaped slabs along the diagonal direction. The obtained SnPb₂Bi₂S₆ phase exhibited good thermal stability up to 1000 K and n-type degenerate semiconducting behavior, with a power factor of 3.7 μW cm^-1 K^-2 at 773 K. Notably, this compound exhibited a very low lattice thermal conductivity of 0.69-0.92 W m^-1 K^-1 at 300-1000 K. Theoretical calculations revealed that the low thermal conductivity is caused by the complex crystal structure and the related elastic properties of a low Debye temperature, low phonon velocity, and large Grüneisen parameters. A reasonable figure of merit (ZT) of ∼0.3 was obtained at 770 K.

CsCu5S3: Promising Thermoelectric Material with Enhanced Phase Transition Temperature

Cu₂S, featuring low cost, nontoxicity, and earth abundance, has been recently recognized as a high efficiency thermoelectric (TE) material. However, before reaching the maximum of the figure of merit ( ZT), Cu₂S undergoes three phase transformations starting at 370 K, which give rise to severe problems, such as possible decomposition and low reliability. Herein, we discover CsCu₅S₃ with phase transformation at 823 K, which is significantly higher than the 370 K value of Cu₂S. Single crystal diffraction data reveal that its two phases are constructed by the same Cu₄S₄ columnar building unit via propagating either at the opposite sides into a layered o-CsCu₅S₃, or at the four apexes into a 3D t-CsCu₅S₃, respectively. Interestingly, the o-to- t transformation is quick, but the reverse one is relatively slow. Theoretical studies reveal that the Cu₄S₄ column exhibits not only the most condensed atomic aggregation ( D_column) but also the lightest effective mass ( m*), along which higher σ is realized. More interestingly, both phases exhibit remarkable ZT enhancements, 0.46 at 800 K for o-CsCu₅S₃, and 0.56 at 875 K for t-CsCu₅S₃, which are 170% and 175% that of Cu₂S at the same temperature.

High thermoelectric efficiency in monolayer PbI2 from 300 K to 900 K

By using a first-principles approach, monolayer PbI₂ is found to have great potential in thermoelectric applications.

Bi2+2 nO2+2 nCu2-δSe2+ n-δXδ (X = Cl, Br): A Three-Anion Homologous Series

Both layered multiple-anion compounds and homologous series are of interest for their electronic properties, including the ability to tune the properties by changing the nature or number of the layers. Here we expand, using both computational and experimental techniques, a recently reported three-anion material, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, to the homologous series Bi_{2+2 n}O_{2+2 n}Cu_2-δSe_{2+ n-δ}X_δ (X = Cl, Br), composed of parent blocks that are well-studied thermoelectric materials. All of the materials show exceptionally low thermal conductivity (0.2 W/mK and lower) parallel to the axis of pressing of the pellets, as well as narrow band gaps (as low as 0.28 eV). Changing the number of layers affects the band gap, thermal conductivity, carrier type, and presence of a phase transition. Furthermore, the way in which the different numbers of layers are accessed, by tuning the compensating Cu vacancy concentration and halide substitution, represents a novel route to homologous series. This homologous series shows tunable properties, and the route explored here could be used to build new homologous series out of known structural blocks.

Cs2Ge3In6Se14: A Structure Transformation Driven by the Size Preference and Its Properties

The new selenide Cs₂Ge₃In₆Se₁₄, featuring its own structure type with germanium in mixed-valence states, is discovered via a solid-state reaction at 1173 K. The compound crystallizes in the R3̅ m space group with a = 7.9951(6) Å and c = 41.726(4) Å. Two adjacent condensed layers of InSe₄ tetrahedra are linked by a [Ge²⁺Se₆] octahedron into a double slice that is further stacked along the c direction with a packing sequence of ··· abca··· through the [Ge³⁺₂Se₆] dimer via its Ge-Ge metallic bond. The coexistence of Ge²⁺/Ge³⁺ and Ge-Ge metallic bonding has been confirmed by XPS and ELF analyses, respectively. More interestingly, although sharing many structure similarities, Cs₂Ge₃In₆Se₁₄ and our previously reported Cs₂Ge₃In₆Te₁₄ reveal a R3̅ m to P3̅ m1 structure transformation with a tripled c parameter. Single-crystal diffraction data and a thorough structure survey of related compounds point out that such a transformation is driven by the size preference of the [Ge₂Q₆] dimer. The title compound possesses a band gap of 2.08 eV and shows photodegradation of RhB under visible light that is more efficient than that for the commercial P25.