Phonon Dispersions and Relaxation Times in AgSbTe$_2$ and PbTe*

The thermoelectric material AgSbTe$_{2}$ had attracted much interest due to its high thermoelectric figure-of-merit, and its anomalously low thermal conductivity for a nominally simple rock-salt structure, which is glass-like even in bulk single-crystals. We present results of systematic neutron scattering investigations of the phonon density-of-states, dispersions, and relaxation times in AgSbTe$_{2}$, and contrast these with PbTe. A detailed account of the thermal conductivity is obtained in terms of microscopic phonon mean-free-paths, providing good agreement with bulk transport measurements.