Electron-phonon coupling and electronic transport in n-type PbTe: Insights from first principles calculations

Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1]. Soft phonons may lead to low lattice thermal conductivity, while preserving high electronic conductivity. Here I will focus on the unusual electronic transport properties of n-type PbTe, which is a classic thermoelectric material that exists near a soft optical mode phase transition. Our first principles calculations show that longitudinal optical phonon scattering dominates electronic transport, while acoustic phonon scattering is relatively weak [2,3]. We find that scattering due to soft transverse optical phonons is by far the weakest scattering mechanism, due to the symmetry-forbidden scattering between the conduction band minima and the zone center soft modes [3]. Soft phonons thus play the key role in the high thermoelectric figure of merit of n-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity [1].

[1] R. Murphy et al, Phys. Rev. B 93, 104304 (2016)
[2] A. Murphy et al, Phys. Rev. B 98, 085201 (2018)
[3] J. Cao et al, arXiv:1809.03799