ZT enhanced by electron-phonon interactions on lattice thermal conductivity in Sb-Bi GeTe crystal

GeTe-based system has attracted intensive attention due to its ZT can be largely enhanced by reducing the thermal conductivity with element doping. Here we report a record high ZT =2.7 at 700 K was found on an Sb and Bi co-doped (Ge_0.86Sb_0.08Bi_0.06)Te crystal. In addition to the structural lattice anomaly such as Umklapp processes, point/planar defects, lattice dislocations, and herringbone domains appearing in the crystal, a strong electron-phonon (EP) interaction was also discovered in our inelastic neutron scattering (INS) measurements with a Kohn anomaly in the phonon dispersion. This phenomenon has never been reported in the system. Furthermore, the Density Functional Theory (DFT) calculation on phonon dispersion showed some of the optical modes are reduced to the energy around the acoustic modes, causing a dramatic reduction in the lattice thermal conductivity. Even though the single Sb and Sb/Bi doping have a similar total density of modes (DOM), the Bi atoms contribute more to the soft modes than the Sb atoms. This explains why Sb/Bi co-doping has a larger lattice thermal conductivity reduction in comparison with Sb single doping in the experimental measurements. From high-resolution transmission electron microscope HRTEM images and mass density calculation, we found that the Sb and Bi doping didn’t decrease the Ge vacancies but created ~15% more than the pristine for Ge_0.86Sb_0.08B_0.06 crystals. The consequence did increase the structural lattice anomaly and further reduce thermal conductivity in Sb/Bi-doped crystal. In conclusion, this significant finding sheds light on the origin of the ultralow lattice phonon thermal conductivity in the doping GeTe crystal and provides an enlightening strategy to further improve the thermoelectric performance in thermoelectric materials.