Revisiting PbTe to identify how thermal conductivity is really limited

TL;DR

This study clarifies that low thermal conductivity in PbTe is primarily due to large anharmonic force constants affecting phonons away from the zone center, challenging previous assumptions about zone-center softening.

Contribution

The paper provides an extensive sensitivity analysis showing that anharmonic interactions, not zone-center softening, dominate thermal conductivity reduction in PbTe.

Findings

Large anharmonic force constants are key to low thermal conductivity.

Zone-center phonon softening alone does not reduce thermal conductivity.

Phonons with larger wavevectors in the Brillouin zone are critical in heat transport.

Abstract

Due to the long range interaction in lead telluride (PbTe), the transverse optical (TO) phonon becomes soft around the Brillouin zone center. Previous studies have postulated that this zone-center softening causes the low thermal conductivity of PbTe through either enlarged phonon scattering phase space and/or strengthened lattice anharmonicity. In this work, we reported an extensive sensitivity analysis of the PbTe thermal conductivity to various factors: range and magnitude of harmonic and anharmonic interatomic force constants, and phonon wavevectors in the three-phonon scattering processes. The analysis reveals that the softening by long range harmonic interaction itself does not reduce thermal conductivity and it is the large magnitude of the anharmonic (cubic) force constants that realizes low thermal conductivity, however, not through the TO phonons around the zone center but dominantly through the ones with larger wavevectors in the middle of Brillion zone. The work clarifies that local band softening cannot be a direct finger print for low thermal conductivity and the entire Brillion zone needs to be characterized on exploring low thermal conductivity materials.