  The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe
and PbTe are investigated with inelastic neutron scattering (INS) and
first-principles calculations. The experiments show that, surprisingly,
although SnTe is closer to the ferroelectric instability, phonon spectra in
PbTe exhibit a more anharmonic character. This behavior is reproduced in
first-principles calculations of the temperature-dependent phonon self-energy.
Our simulations reveal how the nesting of phonon dispersions induces prominent
features in the self-energy, which account for the measured INS spectra and
their temperature dependence. We establish that the phase-space for
three-phonon scattering processes, rather than just the proximity to the
lattice instability, is the mechanism determining the complex spectrum of the
transverse-optical ferroelectric mode.
