Lattice dynamics and reduced thermal conductivity of filled skutterudites

TL;DR

This paper uses first-principles lattice dynamic models to explain how filling skutterudites with rare earth ions reduces their thermal conductivity, highlighting hybridization and scattering mechanisms that improve thermoelectric performance.

Contribution

It provides a microscopic understanding of thermal conductivity reduction in filled skutterudites, challenging previous hypotheses and emphasizing hybridization effects.

Findings

Hybridization between La vibrations and Sb phonons is significant.

Anharmonic RE potential and multiple localized vibrations are refuted.

Harmonic RE motions contribute to phonon scattering and heat conductivity suppression.

Abstract

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.