  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.
