A record high zT of 2.2 at 740 K is reported in Ge0.92Sb0.08Te single crystals, with an optimal hole carrier concentration ≈4 × 1020 cm−3 that simultaneously maximizes the power factor (PF) ≈56 µW cm−1 K−2 and minimizes the thermal conductivity ≈1.9 Wm−1 K−1. In addition to the presence of herringbone domains and stacking faults, the Ge0.92Sb0.08Te exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5–6 meV at Γ point of the Brillouin zone as confirmed through inelastic neutron scattering (INS) measurements. Density functional theory (DFT) confirmed this phonon excitation, and predicted another higher energy phonon excitation ≈12–13 meV at W point. These phonon excitations collectively increase the number of phonon decay channels leading to softening of phonon frequencies such that a three‐phonon process is dominant in Ge0.92Sb0.08Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.
Recently, the anisotropic single crystalline SnSe has gained tremendous interest as a promising thermoelectric material. The elastic constants of such anisotropic crystals are notoriously difficult to measure yet play a crucial role in many thermodynamic properties. We report for the first time the nine independent elastic constants of its stiffness tensor as measured by resonant ultrasound spectroscopy. Our experimental values of the elastic constants are in good agreement with those reported by the density functional theory of SnSe, except for C 12 . The Voigt−Reuss− Hill method was used to determine the isotropic polycrystalline elastic moduli from the measured elastic constants of SnSe, which were found to be in agreement with theoretical values. Notably, the heat capacity of single crystalline SnSe deduced from our measured elastic moduli is in excellent agreement with the room temperature value of heat capacity determined from thermal transport measurements.
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