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Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe2, which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46–0.31 W m−1 K−1) in the 300–673 K. We established an intrinsic rattling nature in TlInTe2 by studying the local structure and phonon vibrations using synchrotron X‐ray pair distribution function (PDF) (100–503 K) and inelastic neutron scattering (INS) (5–450 K), respectively. We showed that while 1D chain of covalently bonded InTe2n-n transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm−1 inside distorted Thompson cage formed by InTe2n-n. This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe2. [ABSTRACT FROM AUTHOR]

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero‐structural nature which has a low lattice thermal conductivity (κlat). BiTe, a unique member of the (Bi2)m(Bi2Te3)n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi2Te3 (where m:n=0:1). Herein, we report intrinsically low κlat of 0.47–0.8 W m−1 K−1 in the 300–650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm2 V−1 s−1 and 707 cm2 V−1 s−1 along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature. [ABSTRACT FROM AUTHOR]

The discovery of topological insulators with unique metallic surface states has added a new dimension in the field of electronics and spintronics. Following the initial breakthrough, the topologically non-trivial materials have been extended to various categories namely weak topological insulator, topological crystalline insulator, Dirac semimetal and Weyl semimetal. Interestingly, most of the topological materials are also good candidates for thermoelectric energy conversion because both demand similar material features such as heavy constituent elements, narrow band gap, and high spin-orbit coupling. Heavy metal chalcogenides satisfy these criteria and present a nice platform for exploring both thermoelectrics and topological insulation. Here, we present a brief overview of both topological materials and thermoelectrics with examples based on heavy metal chalcogenides from the perception of a solid-state chemist. Last part of this review deals with the major challenges at the intersection of these two fields and possible solutions to establish the strong correlation between topological and thermoelectric materials. Image 1 • Topological and thermoelectric materials demand heavy constituent elements, narrow bandgap and strong spin-orbit coupling. • Overview of both topological material and thermoelectrics based on heavy metal chalcogenides is discussed. • Thermoelectric properties of various topological materials such as TI, WTI, TCI and WSM are discussed. [ABSTRACT FROM AUTHOR]

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth's crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi2) sandwiched between two Bi2Se3 quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κlat of ~0.6 W/ mK at 300 K), which is significantly lower than that of Bi2Se3 (κlat of ~1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi2)m(Bi2Se3)n. Phonon dispersion calculated from first-principles and the experimental lowtemperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κlat. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κlat. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ~ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature. [ABSTRACT FROM AUTHOR]

GeTe and its derivatives constituting Pb-free elements have been well known as potential thermoelectric materials for the last five decades, which offer paramount technological importance. The main constraint in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (κlat). Herein, we demonstrate low κlat (~0.7 W/m·K) and a significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in the Sb-doped pseudoternary (GeTe)1-2x(GeSe)x(GeS)x system by two-step strategies. The (GeTe)1-2x(GeSe)x(GeS)x system provides an excellent podium to investigate competition between an entropy-driven solid solution and enthalpy-driven phase separation. In the first step, small concentrations of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the κlat by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly, the system deviates from a solid solution, and phase separation of the GeS1-xSex (5-20 μm) precipitates in the GeTe1-xSex matrix occurs, which does not participate in phonon scattering. In the second stage, κlat of the optimized sample is further reduced to 0.7 W/m·K by Sb alloying and spark plasma sintering (SPS), which introduce additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low κlat in Sb-doped (GeTe)1-2x(GeSe)x(GeS)x is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence, the SPS-processed Ge0.9Sb0.1Te0.9Se0.05S0.05 sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb-doped (GeTe)1-2x(GeSe)x(GeS)x exhibits significantly higher Vickers microhardness (mechanical stability) compared to that of pristine GeTe. [ABSTRACT FROM AUTHOR]

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit ( ZTavg) of materials over the entire working temperature along with a high peak thermoelectric figure of merit ( ZTmax). Herein an ultrahigh ZTavg of 1.4 for (GeTe)80(AgSbSe2)20 [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZTmax of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe- x is attributed to extremely low lattice thermal conductivity ( κlat), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe- x matrix. Addition of AgSbSe2 in GeTe results in κlat of ≈0.4 W mK−1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity ( κmin) of GeTe. Additionally, (GeTe)80(AgSbSe2)20 exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm−2 compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics. [ABSTRACT FROM AUTHOR]

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit ( ZTavg) of materials over the entire working temperature along with a high peak thermoelectric figure of merit ( ZTmax). Herein an ultrahigh ZTavg of 1.4 for (GeTe)80(AgSbSe2)20 [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZTmax of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe- x is attributed to extremely low lattice thermal conductivity ( κlat), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe- x matrix. Addition of AgSbSe2 in GeTe results in κlat of ≈0.4 W mK−1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity ( κmin) of GeTe. Additionally, (GeTe)80(AgSbSe2)20 exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm−2 compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics. [ABSTRACT FROM AUTHOR]

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100-xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100-xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm-1 K-1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses. [ABSTRACT FROM AUTHOR]