The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: Pnma), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: R3m), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferro­electric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe2. We show ultralow lattice thermal conductivity (κL) of 0.74–0.47 W/mK in the 300–723 K range and high zT ≈ 1.25 at 723 K in the p-type rhombohedral (GeSe)0.9­(AgBiSe2)0.1 crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferro­electric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferro­electric instability in (GeSe)0.9­(AgBiSe2)0.1 are directly evident in the low temperature heat capacity (C p) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferro­electric instability induced soft transverse optical phonons significantly reduces the κL and enhances the thermoelectric performance in rhombohedral (GeSe)0.9­(AgBiSe2)0.1 crystals.

A two‐step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self‐compensated SnTe (that is, Sn1.03Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn1.03−xAgxTe resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔEv decreases to 0.10 eV in Sn0.83Ag0.03Mn0.17Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 μW cm−1 K−2 at 816 K in Sn0.83Ag0.03Mn0.17Te was attained. The lattice thermal conductivity of Sn0.83Ag0.03Mn0.17Te reached to an ultralow value (ca. 0.3 W m−1 K−1) at 865 K, owing to the formation of Ag7Te4 nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn0.83Ag0.03Mn0.17Te.

Orthorhombic GeSe is ap romising thermoelectric material. However,l arge band gap and strong covalent bonding result in al ow thermoelectric figure of merit, zT % 0.2. Here,w ed emonstrate am aximum zT % 1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe) 0.9 (AgBiTe 2 ) 0.1 , which is the highest value reported among GeSe based materials.T he rhombohedral phase is stable in ambient conditions for x = 0.8-0.29 in (GeSe) 1Àx (AgBiTe 2 ) x .T he structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe) 1Àx (AgBiTe 2 ) x .(GeSe) 0.9 (AgBiTe 2 ) 0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 mWcm À1 K À2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm À1 K À1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of aferroelectric instability,with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number,l ow band gap and moderate electrical conductivity,corroborate metavalent bonding in (GeSe) 0.9 (AgBiTe 2 ) 0.1 .W econfirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopyinpiezoresponse force microscopy, respectively.

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.

Thermoelectric materials which can convert heat energy to electricity relies on crystalline inorganic solid state compounds exhibiting low phonon transport (i.e. low thermal conductivity) without much inhibiting the electrical transport....