Intrinsic localized mode and low thermal conductivity of PbSe

TL;DR

This paper uses advanced ab-initio calculations to reveal that the low thermal conductivity of PbSe is due to an unusually strong anharmonic interaction, leading to an intrinsic localized mode in the acoustic phonons.

Contribution

It demonstrates the importance of strong anharmonicity in PbSe's thermal properties, revealing a localized phonon mode not captured by previous models.

Findings

PbSe exhibits a lower thermal conductivity than PbTe despite being stiffer.

Anomalously large anharmonic interaction between phonon branches in PbSe.

Identification of an intrinsic localized mode in the acoustic phonons of PbSe.

Abstract

Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity despite being stiffer, a trend that prior first-principles calculations have not reproduced. Here, we use ab-initio calculations that explicitly account for strong anharmonicity to identify the origin of this low thermal conductivity as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches. The strong anharmonicity is reflected in the striking observation of an intrinsic localized mode that forms in the acoustic frequencies. Our work shows the deep insights into thermal phonons that can be obtained from ab-initio calculations that are not confined to the weak limit of anharmonicity.