Lattice dynamical origin of peak thermoelectric performance in AgPb _mSbTe_2+m observed by inelastic neutron scattering

Phonon densities of states (DOS) for the high performing thermoelectric material, AgPb_mSbTe_2+m (LAST-m, m = 16, 18, and 20), were extracted from time-of-flight inelastic neutron scattering measurements. The phonon DOS of LAST-18 differs remarkably from LAST-16 and LAST-20 by exhibiting a dramatic broadening of its acoustic modes that increases on heating. This broadening coincides with a minimum in the thermal conductivity, a maximum in the electrical conductivity and Seebeck coefficient, and a related peak in thermoelectric performance. We argue that the anomalous broadening originates with scattering enhanced by modifications to Te-Ag(Sb) bonds caused by their resonant electronic states falling near the Fermi energy for m = 18.