  Phonon densities of states (DOS) for the high performing thermoelectric
material, AgPbmSbTe2+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.
