  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. It has been shown that the lattice thermal
conductivity of PbTe can be considerably reduced by bringing PbTe closer to the
phase transition e.g. via lattice expansion. However, the effect of soft phonon
modes on the electronic thermoelectric properties of such system remains
unknown. 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, and thus enhance the thermoelectric figure of merit. We
find that the optical deformation potentials change very weakly as the
proximity to the phase transition increases, but the population and scattering
phase space of soft phonon modes increase. Nevertheless, scattering between
electronic states near the band edge and soft optical phonons remains
relatively weak even very near the phase transition.
