  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.
