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Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p‐type polycrystalline rhombohedral (GeSe)0.9(AgBiTe2)0.1 , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8–0.29 in (GeSe)1−x(AgBiTe2)x. The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)1−x(AgBiTe2)x. (GeSe)0.9(AgBiTe2)0.1 has closely lying primary and secondary valence bands (within 0.25–0.30 eV), which results in high power factor 12.8 μW cm−1 K−2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm−1 K−1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)0.9(AgBiTe2)0.1. We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3–30 K) and switching spectroscopy in piezoresponse force microscopy, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Crystalline solids with intrinsically low lattice thermal conductivity (κL) are crucial to realizing high‐performance thermoelectric (TE) materials. Herein, we show an ultralow κL of 0.35 Wm−1 K−1 in AgCuTe, which has a remarkable TE figure‐of‐merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First‐principles DFT calculation reveals several soft phonon modes in its room‐temperature hexagonal phase, which are also evident from low‐temperature heat‐capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κL in the room‐temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high‐temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor. [ABSTRACT FROM AUTHOR]