Lattice dynamics and reduced thermal conductivity of filled skutterudites

The great reduction in thermal conductivity of skutterudites upon filling the void sites with rare-earth (RE) ions is key to their favorable thermoelectric properties but remains to be understood. Using lattice dynamic models based on first-principles calculations, we address the most popular microscopic mechanism, reduction via rattling ions. The model withstands inelastic neutron scattering and specific-heat measurements, and refutes hypotheses of an anharmonic RE potential and of two distinct localized RE vibrations of disparate frequencies. It does indicate a strong hybridization between bare La vibrations and certain Sb-like phonon branches, suggesting anharmonic scattering by harmonic RE motions as an important mechanism for suppression of heat conductivity.