Thermoelectric properties of n-type PbTe driven near soft-mode phase transition via lattice expansion*

We have recently shown that soft transverse optical (TO) phonons play the key role in the high thermoelectric figure of merit (ZT) of PbTe: they strongly suppress the lattice thermal conductivity [1], but do not degrade the electronic transport properties [2]. Using first principles calculations, here we investigate how driving PbTe closer to the soft-mode phase transition via lattice expansion affects the thermoelectric properties and ZT of n-type PbTe. We find that the very soft TO phonons do not deteriorate the electronic thermoelectric properties of PbTe. Scattering between electrons and soft phonons remains weak even very near the phase transition as a result of symmetry restrictions. In contrast, the phonon softening considerably reduces the lattice thermal conductivity of expanded PbTe and doubles the ZT with respect to the equilibrium structure. Our results indicate that the ZT of materials with soft phonon modes that interact strongly with other phonons but weakly with conducting electronic states could be significantly increased by bringing these materials closer to the soft-mode phase transition.

[1] R. M. Murphy et al, Phys. Rev. B 93, 104304 (2016)

[2] J. Cao et al, Phys. Rev. B 98, 205202 (2018)