Prediction of large thermoelectric figure of merit in n-type PbTe driven near the soft-mode phase transition via lattice expansion

IV-VI materials are some of the most efficient bulk thermoelectric materials due to their proximity to soft-mode phase transitions, which leads to low lattice thermal conductivity. However, the effect of soft phonon modes on their electronic thermoelectric properties remains poorly understood. Using first principles calculations, we show that the soft zone center transverse optical phonons do not deteriorate the electronic thermoelectric properties of PbTe driven closer to the phase transition via lattice expansion due to external stress. Scattering between electrons and soft phonons remains weak even very near the phase transition as a result of symmetry restrictions. In contrast, phonon softening considerably reduces the lattice thermal conductivity of PbTe with the lattice constants $\sim 1.5$\% larger than that at equilibrium and doubles the thermoelectric figure of merit. Our results indicate that the thermoelectric figure of merit of materials with soft phonon modes that interact strongly with other phonons, but weakly with electronic states relevant for transport, could be significantly increased by bringing these materials closer to the soft-mode phase transition.