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

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.