Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

Ductile P-Type AgCu(Se,S,Te) Thermoelectric Materials

Ductile inorganic thermoelectric (TE) materials open a new approach to develop high-performance flexible TE devices. N-type Ag2(S,Se,Te) and p-type AgCu(Se,S,Te) pseudoternary solid solutions are two typical categories of ductile inorganic TE materials reported so far. Comparing with the Ag2(S,Se,Te) pseudoternary solid solutions, the phase composition, crystal structure, and physical properties of AgCu(Se,S,Te) pseudoternary solid solutions are more complex, but their relationships are still ambiguous now. In this work, via systematically investigating the phase composition, crystal structure, mechanical, and TE properties of about 60 AgCu(Se,S,Te) pseudoternary solid solutions, the comprehensive composition-structure-property phase diagrams of the AgCuSe-AgCuS-AgCuTe pseudoternary system is constructed. By mapping the complex phases, the "ductile-brittle" and "n-p" transition boundaries are determined and the composition ranges with high TE performance and inherent ductility are illustrated. On this basis, high performance p-type ductile TE materials are obtained, with a maximum zT of 0.81 at 340 K. Finally, flexible in-plane TE devices are prepared by using the AgCu(Se,S,Te)-based ductile TE materials, showing high output performance that is superior to those of organic and inorganic-organic hybrid flexible devices.

The Remarkable Role of Indium in Synergistically Optimizing Carrier Concentration and Phase Distribution of AgCuTe-Based Materials

Carrier regulation has proven to be an effective approach for optimizing the thermoelectric performance of materials. One common method to adjust the carrier concentration is through element doping. In the case of AgCuTe-based materials, it tends to form with cation vacancies, resulting in a high hole concentration and complex phase composition at low temperatures, which also hinders material stability. However, this also offers additional opportunities to manipulate the carrier concentration. In this study, the improved performance of AgCuTe through indium doping is reported, which leads to a reduction in hole concentration. In combination with a significant increase in the effective mass of the carriers, the enhanced Seebeck coefficient is also realized. Particularly, a notable improvement in power factor is observed in the hexagonal phase near room temperature. Furthermore, a lower electron thermal conductivity is achieved, contributing to an average figure of merit value of ≈1.21 (between 523 and 723 K). Additionally, the presence of indium inhibits the formation of the second phase and ensures a homogeneous phase distribution, which reduces the instability arising from phase transition. This work significantly enhances the potential of AgCuTe-based materials for low to medium-temperature applications.

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.

Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2

Noble-metal-based chalcogenide materials recently gained massive attention in the field of thermoelectrics. In most cases, materials are synthesized using (i) high-temperature solid-state reactions or (ii) soft chemical methods where temperature requirements are lower than those of solid-state reactions (generally below 400 °C). Herein, we present a simple, surfactant-free, room-temperature, and energy-efficient synthesis of Ag3CuS2 nanocrystals. The present synthesis technique is scalable and capable of gram-scale production. A spark plasma sintering (SPS) pressed sample exhibits ultralow thermal conductivity (∼0.31 W/mK at room temperature). We found that Ag3CuS2 exhibits low sound velocity, as well as a non-Debye-like behavior based on a low-temperature heat capacity measurement. A high degree of anharmonicity of bonding, soft vibrations modes, and nanoscale grain boundary scattering in Ag3CuS2 lead to ultralow thermal conductivity, which can be important for thermoelectrics, optoelectronics, and thermal barrier coating applications.

Silver Copper Chalcogenide Thermoelectrics: Advance, Controversy, and Perspective

Thermoelectric technology, which enables a direct and pollution-free conversion of heat into electricity, provides a promising path to address the current global energy crisis. Among the broad range of thermoelectric materials, silver copper chalcogenides (AgCuQ, Q = S, Se, Te) have garnered significant attention in thermoelectric community in light of inherently ultralow lattice thermal conductivity, controllable electronic transport properties, excellent thermoelectric performance across various temperature ranges, and a degree of ductility. This review epitomizes the recent progress in AgCuQ-based thermoelectric materials, from the optimization of thermoelectric performance to the rational design of devices, encompassing the fundamental understanding of crystal structures, electronic band structures, mechanical properties, and quasi-liquid behaviors. The correlation between chemical composition, mechanical properties, and thermoelectric performance in this material system is also highlighted. Finally, several key issues and prospects are proposed for further optimizing AgCuQ-based thermoelectric materials.

Incommensurately Modulated Structure in AgCuSe-Based Thermoelectric Materials for Intriguing Electrical, Thermal, and Mechanical Properties

AgCuSe-based materials have attracted great attentions recently in thermoelectric (TE) field due to their extremely high electron mobility, ultralow lattice thermal conductivity, and abnormal "brittle-ductile" transition at room temperature. However, although the investigation on the crystal structure of AgCuSe low-temperature phase (named as β-AgCuSe) was started more than half a century before, it is still in controversy yet, which greatly limits the understanding of its intriguing electrical, thermal, and mechanical performance. In this work, via adopting the advanced three-dimensional electron diffraction technique, this study finds that the AgCuSe-based materials crystalize in an incommensurately modulated structure with an orthorhombic Pmmn(0β1/2)s00 superspace group. The local lattice distortion in the incommensurately modulated structure has weak effects on the conduction band minimum due to the delocalized and isotropic feature of Ag 5s states, leading to high carrier mobility. Likewise, the inhomogeneous, weak, and anisotropic Ag-Se bonds result in the high degree of anharmonicity and ultralow lattice thermal conductivity. Furthermore, alloying S in AgCuSe reinforces the interaction between the adjacent Ag-Se layers, yielding the "brittle-ductile" transition at room temperature. This work well interprets the structure-performance relationship of AgCuSe-based materials and sheds light on the future investigation of this class of promising TE materials.

Ag2 Q-Based (Q = S, Se, Te) Silver Chalcogenide Thermoelectric Materials

Thermoelectric technology provides a promising solution to sustainable energy utilization and scalable power supply. Recently, Ag₂ Q-based (Q = S, Se, Te) silver chalcogenides have come forth as potential thermoelectric materials that are endowed with complex crystal structures, high carrier mobility coupled with low lattice thermal conductivity, and even exceptional plasticity. This review presents the latest advances in this material family, from binary compounds to ternary and quaternary alloys, covering the understanding of multi-scale structures and peculiar properties, the optimization of thermoelectric performance, and the rational design of new materials. The "composition-phase structure-thermoelectric/mechanical properties" correlation is emphasized. Flexible and hetero-shaped thermoelectric prototypes based on Ag₂ Q materials are also demonstrated. Several key problems and challenges are put forward concerning further understanding and optimization of Ag₂ Q-based thermoelectric chalcogenides.

Thermoelectric Silver-Based Chalcogenides

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

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.

Phase Transition Behaviors and Thermoelectric Properties of CuAgTe1-xSex near 400 K

Phase transition is an effective strategy to engineer thermal conductivity and electrical transports. Recently, p-type CuAgTe_1-xSe_x materials were reported to show excellent thermoelectric performance at 300-450 K, but the data are controversial due to the cooccurrence of phase transition in this temperature range. Accurately measuring and analyzing the electrical and thermal transport properties in the narrow phase transition temperature range is a quite challenging task. In this work, we systemically investigate the phase transition behavior, and electrical and thermal transport properties of p-type CuAgTe_1-xSe_x (x = 0.3, 0.4, and 0.5) near 400 K. CuAgTe_1-xSe_x (x = 0.3, 0.4, and 0.5) materials show similar phase transition temperatures but quite different phase transition speeds. The phase transition has a weak influence on the electrical transport properties of CuAgTe_0.7Se_0.3 and CuAgTe_0.6Se_0.4, but a strong influence on those of CuAgTe_0.5Se_0.5. Likewise, an obvious underestimation of thermal diffusivity, with a maximum deviation about 20% off the real value, is observed during the phase transition temperature range for CuAgTe_1-xSe_x. Finally, CuAgTe_0.7Se_0.3 shows a peak zT around 0.9 at 390 K. The present study proves that CuAgTe_1-xSe_x solid solutions are one kind of promising near-room-temperature thermoelectric material.

Surprisingly good thermoelectric performance of monolayer C3N

The rapid emergence of graphene has attracted numerous efforts to explore other two-dimensional materials. Here, we combine first-principles calculations and Boltzmann theory to investigate the structural, electronic, and thermoelectric transport properties of monolayer C₃N, which exhibits a honeycomb structure very similar to graphene. It is found that the system is both dynamically and thermally stable even at high temperature. Unlike graphene, the monolayer has an indirect band gap of 0.38 eV and much lower lattice thermal conductivity. Moreover, the system exhibits obviously larger electrical conductivity and Seebeck coefficients for the hole carriers. Consequently, theZTvalue ofp-type C₃N can reach 1.4 at 1200 K when a constant relaxation time is predicted by the simple deformation potential theory. However, such a largerZTis reduced to 0.6 if we fully consider the electron-phonon coupling. Even so, the thermoelectric performance of monolayer C₃N is still significantly enhanced compared with that of graphene, and is surprisingly good for low-dimensional thermoelectric materials consisting of very light elements.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Enhanced Stability and Thermoelectric Performance in Cu1.85Se-Based Compounds

Liquid-like copper selenium compounds have attracted considerable interest in recent years for their excellent thermoelectric performance, abundant element reserves, and low toxicity. However, the related applications are still limited due to the phase transition and precipitation of Cu under an external field. Here, the cubic Cu_1.85Se-based compounds with suppressed phase transition and improved critical voltage (V_c) are first studied. In particular, Li/Bi co-doping effectively optimizes hole concentration and the ZTs are substantially improved from 0.2 in Cu_1.85Se to 0.7 in Li_0.03Cu_1.81Bi_0.04Se at 760 K. Meanwhile, the latter shows an outstanding V_c above 0.22 V at 750 K, which is the highest value in Cu_2-xSe thermoelectric compounds to date. Moreover, S is alloyed in Li_0.03Cu_1.81Bi_0.04Se to greatly reduce the thermal conductivity and the ZT is further enhanced to 0.9 for Li_0.03Cu_1.81Bi_0.04Se_0.9S_0.1 at 760 K. Our work sheds light on a new strategy to realize good stability and enhanced thermoelectric performance, which provides a new direction for further research.

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.

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.

Hierarchical Structures Advance Thermoelectric Properties of Porous n-type β-Ag2Se

Owing to the intrinsically good near-room-temperature thermoelectric performance, β-Ag₂Se has been considered as a promising alternative to n-type Bi₂Te₃ thermoelectric materials. Herein, we develop an energy- and time-efficient wet mechanical alloying and spark plasma sintering method to prepare porous β-Ag₂Se with hierarchical structures including high-density pores, a metastable phase, nanosized grains, semi-coherent grain boundaries, high-density dislocations, and localized strains, leading to an ultralow lattice thermal conductivity of ∼0.35 W m^-1 K^-1 at 300 K. A relatively high carrier mobility is obtained by adjusting the sintering temperature to obtain pores with an average size of ∼260 nm, therefore resulting in a figure of merit, zT, of ∼0.7 at 300 K and ∼0.9 at 390 K. The single parabolic band model predicts that zT of such porous β-Ag₂Se can reach ∼1.1 at 300 K if the carrier concentration can be tuned to ∼1 × 10¹⁸ cm^-3, suggesting that β-Ag₂Se can be a competitive candidate for room-temperature thermoelectric applications.

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.

Ultralow Thermal Conductivity in Chain-like TlSe Due to Inherent Tl+ Rattling

Understanding the mechanism that correlates phonon transport with chemical bonding and solid-state structure is the key to envisage and develop materials with ultralow thermal conductivity, which are essential for efficient thermoelectrics and thermal barrier coatings. We synthesized thallium selenide (TlSe), which is comprised of intertwined stiff and weakly bonded substructures and exhibits intrinsically ultralow lattice thermal conductivity (κ_L) of 0.62-0.4 W/mK in the range 295-525 K. Ultralow κ_L of TlSe is a result of its low energy optical phonon modes which strongly interact with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic rattler-like vibration of Tl⁺ cations in the cage constructed by the chains of (TlSe₂)_n^n-, as evident in low temperature heat capacity, terahertz time-domain spectroscopy, and temperature dependent Raman spectroscopy. Density functional theoretical analysis reveals the bonding hierarchy in TlSe which involves ionic interaction in Tl⁺-Se while Tl³⁺-Se bonds are covalent, which causes significant lattice anharmonicity and intrinsic rattler-like low energy vibrations of Tl⁺, resulting in ultralow κ_L.

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.

Bonding heterogeneity and lone pair induced anharmonicity resulted in ultralow thermal conductivity and promising thermoelectric properties in n-type AgPbBiSe3

Synergistic effect of bonding inhomogeneity and local off-centering within global cubic structure results in ultralow thermal conductivity of n-type AgPbBiSe₃.

Efficiency in generation and utilization of energy is highly dependent on materials that have the ability to amplify or hinder thermal conduction processes. A comprehensive understanding of the relationship between chemical bonding and structure impacting lattice waves (phonons) is essential to furnish compounds with ultralow lattice thermal conductivity (κ_lat) for important applications such as thermoelectrics. Here, we demonstrate that the n-type rock-salt AgPbBiSe₃ exhibits an ultra-low κ_lat of 0.5–0.4 W m^–1 K^–1 in the 290–820 K temperature range. We present detailed analysis to uncover the fundamental origin of such a low κ_lat. First-principles calculations augmented with low temperature heat capacity measurements and the experimentally determined synchrotron X-ray pair distribution function (PDF) reveal bonding heterogeneity within the lattice and lone pair induced lattice anharmonicity. Both of these factors enhance the phonon–phonon scattering, and are thereby responsible for the suppressed κ_lat. Further optimization of the thermoelectric properties was performed by aliovalent halide doping, and a thermoelectric figure of merit (zT) of 0.8 at 814 K was achieved for AgPbBiSe_2.97I_0.03 which is remarkable among n-type Te free thermoelectrics.

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.

The journey of tin chalcogenides towards high-performance thermoelectrics and topological materials

Sn-Chalcogenides are recognized as high performance thermoelectrics and topological insulators due to their unique crystal and electronic structures and lattice dynamics.