High zT and Its Origin in Sb‐doped GeTe Single Crystals

Abstract A record high zT of 2.2 at 740 K is reported in Ge 0.92 Sb 0.08 Te single crystals, with an optimal hole carrier concentration ≈4 × 10 20 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 Ge 0.92 Sb 0.08 Te 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 Ge 0.92 Sb 0.08 Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric ( TE ) performance.