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_0.92Sb_0.08Te 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 Ge_0.92Sb_0.08Te 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 Ge_0.92Sb_0.08Te, in contrast to a dominant four-phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.