High and Its Origin in Sb-doped GeTe Single Crystals.

A record high of 2.2 at 740 K is reported in GeSbTe single crystals, with an optimal hole carrier concentration ≈4 × 10 cm that simultaneously maximizes the power factor () ≈56 µW cmK and minimizes the thermal conductivity ≈1.9 Wm K. In addition to the presence of herringbone domains and stacking faults, the GeSbTe exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5-6 meV at 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 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 GeSbTe, in contrast to a dominant four-phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric () performance.