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.     

Oxide Thermoelectric Materials: A Structure–Property Relationship

Recent demand for thermoelectric materials for power harvesting from automobile and industrial waste heat requires oxide materials because of their potential advantages over intermetallic alloys in terms of chemical and thermal stability at high temperatures. Achievement of thermoelectric figure of merit equivalent to unity (ZT ≈ 1) for transition-metal oxides necessitates a second look at the fundamental theory on the basis of the structure–property relationship giving rise to electron correlation accompanied by spin fluctuation. Promising transition-metal oxides based on wide-bandgap semiconductors, perovskite and layered oxides have been studied as potential candidate n- and p-type materials. This paper reviews the correlation between the crystal structure and thermoelectric properties of transition-metal oxides. The crystal-site-dependent electronic configuration and spin degeneracy to control the thermopower and electron–phonon interaction leading to polaron hopping to control electrical conductivity is discussed. Crystal structure tailoring leading to phonon scattering at interfaces and nanograin domains to achieve low thermal conductivity is also highlighted.     

Thermal Conductivity and ZT in Disordered Organic Thermoelectrics

For decades, continuous attempts have been made to improve the figure of merit (ZT) of thermoelectrics. The theory behind the Seebeck effect itself is well researched, but the problem with ZT is related to materials properties that offset one another. This work analyzed the link between the site energy distributions and thermal conductivity of oxidized poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT:Tos), which was reported to be a good organic thermoelectric. To understand how heat flow was affected by “disorder” in PEDOT:Tos and the associated electron–phonon interactions, we computed the values of the thermal conductivity κ and ZT using materials parameters extracted from the open literature. By varying the values of the parameters separately, we were able to identify their individual influence on κ and ZT. Our results suggest that ZT is most sensitive to changes in σ, the bandwidth of the density of states (DOS) of the transport sites, and less so to changes in n _eff, the effective carrier density. Our simulations also suggested that ZT could become exceptionally large (approaching a value of ~20) if σ were lowered to 1 meV to 2 meV. This would be a tremendous approach to increase ZT in oxidized PEDOT:Tos.     

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.     

Extended Antibonding States and Phonon Localization Induce Ultralow Thermal Conductivity in Low Dimensional Metal Halide

Thermal conductivity, which measures the ease at which heat passes through a crystalline solid, is controlled by the nature of the chemical bonding and periodicity in the solid. This necessitates an in-depth understanding of the crystal structure and chemical bonding to tailor materials with notable lattice thermal conductivity (κ_L). Herein, the nature of chemical bonding and its influence on the thermal transport properties (2–523 K) of all-inorganic halide perovskite Cs₃Bi₂I₉ are studied. The κ_L exhibits an ultralow value of ≈0.20  W m⁻¹K⁻¹ in 30–523 K temperature range. The antibonding states just below the Fermi level in the electronic structure arising from the interaction between bismuth 6s and iodine 5p orbitals, weakens the bond and causes soft elasticity in Cs₃Bi₂I₉. First-principles density functional theory (DFT) calculations reveal highly localized soft optical phonon modes originating from Cs-rattling and dynamic double octahedral distortion of 0D [Bi₂I₉]³⁻ in Cs₃Bi₂I₉. These low energy nearly flat optical phonons strongly interact with transverse acoustic modes creating an ultrashort phonon lifetime of ≈1 ps. While the presence of extended antibonding states gives rise to soft anharmonic lattice; Cs rattling provides sharp localized optical phonon modes, which altogether result in strong lattice anharmonicity and ultralow κ_L.     

Li‐Doped Cr2MnO4: A New p‐Type Transparent Conducting Oxide by Computational Materials Design

To accelerate the design and discovery of novel functional materials, here, p‐type transparent conducting oxides, an inverse design approach is formulated, integrating three steps: i) articulating the target properties and selecting an initial pool of candidates based on “design principles”, ii) screening this initial pool by calculating the “selection metrics” for each member, and iii) laboratory realization and more‐detailed theoretical validation of the remaining “best‐of‐class” materials. Following a design principle that suggests using d5⁵ cations for good p‐type conductivity in oxides, the Inverse Design approach is applied to the class of ternary Mn(II) oxides, which are usually considered to be insulating materials. As a result, Cr₂MnO₄ is identified as an oxide closely following “selection metrics” of thermodynamic stability, wide‐gap, p‐type dopability, and band‐conduction mechanism for holes (no hole self‐trapping). Lacking an intrinsic hole‐producing acceptor defect, Li is further identified as a suitable dopant. Bulk synthesis of Li‐doped Cr₂MnO₄ exhibits at least five orders of magnitude enhancement of the hole conductivity compared to undoped samples. This novel approach of stating functionality first, then theoretically searching for candidates that merits synthesis and characterization, promises to replace the more traditional non‐systematic approach for the discovery of advanced functional materials.     

Low-temperature soft-chemical synthesis and thermoelectric properties of barium-filled p-type skutterudite nanocrystals

Bulk skutterudites, such as cobalt triantimonide (CoSb₃) are promising inorganic materials for thermoelectric power generation at high temperatures. Generally, bulk CoSb₃ is synthesized by high temperature solid state reactions. Herein, we demonstrate the low temperature solution phase synthesis of p-type nanocrystalline CoSb₃ and Ba-filled CoSb₃. Increase in the temperature dependent Seebeck coefficient with simultaneous increase in temperature dependent electrical conductivity has been observed in the present nanocrystalline samples, which is unusual in the case of bulk CoSb₃. Efficient phonon scattering by nanoscale grain boundaries and the additional phonon damping due to the rattling of Ba in the void of nanocrystalline CoSb₃ give rise to low thermal conductivity, which results in improved thermoelectric performance in nanocrystalline p-type Ba_0.048CoSb₃.     

Anomalous Low Thermal Conductivity of Atomically Thin InSe Probed by Scanning Thermal Microscopy

The ability of a material to conduct heat influences many physical phenomena, ranging from thermal management in nanoscale devices to thermoelectrics. Van der Waals 2D materials offer a versatile platform to tailor heat transfer due to their high surface-to-volume ratio and mechanical flexibility. Here, the nanoscale thermal properties of 2D indium selenide (InSe) are studied by scanning thermal microscopy. The high electrical conductivity, broad-band optical absorption, and mechanical flexibility of 2D InSe are accompanied by an anomalous low thermal conductivity (κ). This can be smaller than that of low-κ dielectrics, such as silicon oxide, and it decreases with reducing the lateral size and/or thickness of InSe. The thermal response is probed in free-standing InSe layers as well as layers supported by a substrate, revealing the role of interfacial thermal resistance, phonon scattering, and strain. These thermal properties are critical for future emerging technologies, such as field-effect transistors that require efficient heat dissipation or thermoelectric energy conversion with low-κ, high electron mobility 2D materials, such as InSe.     

Ultralow Thermal Conductivity of Single‐Crystalline Porous Silicon Nanowires

Porous materials provide a large surface‐to‐volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure–property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single‐crystalline silicon nanowires using a novel electron‐beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m^?1 K^?1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems.     

Thermoelectric Properties of Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript> and Ni<Subscript>0.06</Subscript>Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript>

Mo₃Sb₇, crystallizing in the Ir₃Ge₇ type structure, has poor thermoelectric (TE) properties due to its metallic behavior. However, by a partial Sb-Te exchange, it becomes semiconducting without noticeable structure changes and so achieves a significant enhancement in the thermopower with the composition of Mo₃Sb₅Te₂. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the thermal conductivity by the so-called rattling effect. As part of the effort to verify this idea, we report herein the growth as well as measurements of the thermal and electrical transport properties of Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.     

Effects of In Substitution for Ga on the Thermoelectric Properties of Type-VIII Clathrate Ba8Ga16Sn30 Single Crystals

We have prepared single crystals of type-VIII clathrate Ba₈Ga_15.9?x In _x Sn_30.1 for x ≤ 0.60 by the Sn-flux method. As x is increased from 0 to 0.60, the lattice parameter increases by 0.2%, which is consistent with the larger covalent diameter for In than for Ga. The Seebeck coefficient α, electrical resistivity ρ, and thermal conductivity κ were measured in the temperature range from 300 K to 600 K. For all samples, α is negative, indicating the dominant charge carriers are electrons. With increasing x from 0 to 0.20, ρ and \(\left| \alpha \right|\) decrease by 50% and 30%, respectively. As a result, the lattice thermal conductivity at 300 K decreases from 0.58 W/Km to 0.41 W/Km, which is ascribed to enhancement of rattling of the guest atoms. It is found that the maximum of the dimensionless figure of merit ZT reaches 1.05 at 540 K for x = 0.20.     

Neodymium-Strontium Titanate: A New Ceramic for an Old Problem

High density ceramics based on neodymium-calcium titanate (RE_0.6II_0.1TiO₃) where RE = Nd, Pr, Sm and II = Ca, Sr, were prepared by the mixed oxide route. All products exhibited low thermal conductivity due to the presence of A-site vacancies assisting strong phonon scattering. The moderate electrical conductivity and high Seebeck coefficient resulted in the highest thermoelectric figure of merit (ZT) in neodymium-strontium titanate (NT-ST) ceramic with a value of 0.03 at 900 K. The temperature stable ZT behavior of NT-ST is promising for device applications.     

Improved Thermoelectric Properties in Ga2Te3-GaSb Vacancy Compounds

Thermoelectric materials with high figure of merit, which requires large Seebeck coefficient, large electrical conductivity, and low thermal conductivity, are of great importance in solid-state cooling and power generation. Solid-solution formation is one effective method to achieve low thermal conductivity by phonon scattering due to mass and strain field fluctuations. This type of scattering is maximized in structures containing vacancies. The thermoelectric properties of Ga₂Te₃-GaSb vacancy compounds were studied in this work. We find that the lattice thermal conductivity is reduced by over an order of magnitude with the addition of only very moderate amounts of Ga₂Te₃. Additionally, both the carrier type and concentration can be modified. While the vacancy structure induced by Ga₂Te₃ addition to GaSb can effectively reduce phonon conductivity, carrier mobility is also degraded, and optimized thermoelectric properties require careful control of the vacancy content in these solid solutions.     

Effects of Defects on the Temperature‐Dependent Thermal Conductivity of Suspended Monolayer Molybdenum Disulfide Grown by Chemical Vapor Deposition

It is understood that defects of the atomic arrangement of the lattice in 2D molybdenum disulfide (MoS₂) grown by chemical vapor deposition (CVD) can have a profound effect on the electronic and optical properties. Beyond these it is a major prerequisite to also understand the fundamental effect of such defects on phonon transport, to guarantee the successful integration of MoS₂ into the solid‐state devices. A comprehensive joint experiment‐theory investigation to explore the effect of lattice defects on the thermal transport of the suspended MoS₂ monolayer grown by CVD is presented. The measured room temperature thermal conductivity values are 30 ± 3.3 and 35.5 ± 3 W m^?1 K^?1 for two samples, which are more than two times smaller than that of their exfoliated counterpart. High‐resolution transmission electron microscopy shows that these CVD‐grown samples are polycrystalline in nature with low angle grain boundaries, which is primarily responsible for their reduced thermal conductivity. Higher degree of polycrystallinity and aging effects also result in smoother temperature dependency of thermal conductivity (κ) at temperatures below 100 K. First‐principles lattice dynamics simulations are carried out to understand the role of defects such as isotopes, vacancies, and grain boundaries on the phonon scattering rates of our CVD‐grown samples.     

From Bonding Asymmetry to Anharmonic Rattling in Cu12Sb4S13 Tetrahedrites: When Lone‐Pair Electrons Are Not So Lonely

Some of the best thermoelectrics are complex materials with rattling guests inside oversized atomic cages. Understanding the chemical and structural origins of the rattling behavior is essential to the design of thermoelectric materials. In this work, a clear connection is established between the local bonding asymmetry and anharmonic rattling modes in tetrahedrite thermoelectrics, enabled by the chemically active electron lone pairs. The studies reveal a five‐atom atomic cage Sb[CuS₃]Sb in Cu₁₂Sb₄S₁₃ tetrahedrites that exhibits strong local bonding asymmetry: covalent bonding inside the CuS₃ trigonal plane and weak out‐of‐plane bonding induced by the lone‐pair electrons of Sb. This bonding asymmetry leads to out‐of‐plane rattling modes that are quasilocalized and anharmonic with low frequency and large amplitude, and are likely the origin of low thermal conductivity in tetrahedrites. Such knowledge highlights the importance of local structure asymmetry and lone‐pair atoms in driving anharmonic rattling, providing a stepping stone to the discovery and design of next‐generation thermoelectrics.     

Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Multiple-Filled Skutterudites

Filled skutterudites are prospective intermediate temperature materials for␣thermoelectric power generation. CoSb₃-based n-type filled skutterudites have good electrical transport properties with power factor values over 40 μW/cm K² at elevated temperatures. Filling multiple fillers into the crystallographic voids of skutterudites would help scatter a broad range of lattice phonons, thus resulting in lower lattice thermal conductivity values. We report the thermoelectric properties of n-type multiple-filled skutterudites between 5 K and 800 K. The combination of different fillers inside the voids of the skutterudite structure shows enhanced phonon scattering, and consequently a strong suppression of the lattice thermal conductivity. Very good power factor values are achieved in multiple-filled skutterudite compared with single-element-filled materials. The dimensionless thermoelectric figure of merit for n-type filled skutterudites is improved through multiple-filling in a wide temperature range.     

Thermal Conductivity of Diameter-Modulated Silicon Nanowires Within a Frequency-Dependent Model for Phonon Boundary Scattering

Modulated nanowires have been proposed as candidates for efficient thermoelectric applications. It has been previously shown within the low-temperature ballistic regime of phonon transport that the thermal conductivity can be significantly reduced when the width of the nanowire is modulated. Here, we report on the thermal conductivity of modulated Si nanowires calculated within a kinetic theory model. The size dependence is taken into account through the sampling of k-points in the first Brillouin zone and a frequency-dependent calculation of the boundary scattering length. It has been found that the thermal conductivity of modulated nanowires can be drastically reduced compared with that of nanowires with constant width. Interestingly, the thermal conductivity is even smaller than that of corresponding straight wires with width equal to the smallest width in the modulated nanowires. The dramatic decrease of the thermal conductivity of modulated nanowires is attributed to their small transmissivity.     

Vacancy Manipulation Induced Optimal Carrier Concentration,Band Convergence and Low Lattice Thermal Conductivity in Nano-Crystalline SnTe Yielding Superior Thermoelectric Performance

Synergetic optimization of electrical and thermal transport properties is achieved for SnTe-based nano-crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m⁻¹ K⁻² for Sn_0.96Gd_0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd-doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano-pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe-based nanoscale polycrystalline materials.     

Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

Development of a Thermal Conductivity Measurement System Using the 3ω Method and Application to Thermoelectric Particles

For this study, we developed a thermal conductivity, κ, measurement system using 3ω method. We checked the system accuracy by measuring κ for a glass substrate (1737; Corning). Conventional evaporated aluminum wire and ink-jet printed silver wire were used as sensor wires. The system realized a κ measurement of glass within 10 % error. We estimated κ of aggregated p-type (Bi_1?x Sb _x )₂Te₃ particles using a two heat flow model. The estimated thermal conductivity of the sample κ _sample are 0.06–0.27 WK^?1 m^?1, which is smaller than the bulk value.