Thermoelectric properties of n-type PbTe driven near ferroelectric phase transition by strain

We recently showed that soft transverse optical (TO) phonons play the key role in high thermoelectric (TE) figure of merit (ZT) of PbTe: they strongly suppress its lattice thermal conductivity[1], but do not degrade its electronic transport properties[2]. In this work, using first principles calculations, we investigate how driving PbTe closer to the soft mode phase transition (PT) via strain affects the TE properties and ZT of n-type PbTe. We find that the lattice thermal conductivity decreases significantly when PbTe approaches the PT, which leads to considerable ZT enhancement. However, if PbTe is driven very close to the PT, the originally negligible electron-TO phonon scattering becomes the strongest scattering channel, due to an increased TO phonon amplitude and the electron-TO phonon scattering phase space. Such increased scattering strength rapidly degrades electrical transport and ZT very near the PT. We show how tuning the proximity to soft mode PT can increase the TE performance of PbTe and other materials with soft phonons that interact weakly with the electronic states relevant for transport.
[1] R. M. Murphy et al. Phys. Rev. B 93, 104304
[2] J. Cao et al. Phys. Rev. B 98, 205202