Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe show 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 energy spectra and their temperature dependence. The contributions to the complex features of the transverse-optic ferroelectric mode from phase-space for three-phonon scattering processes and the lattice instability are compared.