Electron-phonon coupling and electronic thermoelectric properties of n-type PbTe driven near the soft-mode phase transition via lattice expansion

TL;DR

This study uses first principles calculations to show that soft phonon modes in PbTe near a phase transition do not harm its electronic thermoelectric properties, potentially improving its thermoelectric efficiency.

Contribution

It demonstrates that proximity to a soft-mode phase transition via lattice expansion does not degrade electronic thermoelectric performance in PbTe, unlike its effect on thermal conductivity.

Findings

Soft zone center transverse optical phonons do not impair thermoelectric properties.

Optical deformation potentials change weakly near the phase transition.

Electron-phonon scattering remains relatively weak despite increased soft phonon population.

Abstract

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.