  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.
