Impact of Rattlers on Thermal Conductivity of a Thermoelectric Clathrate: A First-Principles Study

TL;DR

This study uses first-principles calculations to show that rattler atoms in clathrates significantly reduce thermal conductivity by increasing phonon scattering across a broad frequency range, not just through resonant effects.

Contribution

It provides a detailed first-principles analysis revealing that rattlers cause widespread phonon relaxation time reduction, challenging the resonant scattering explanation.

Findings

Rattlers cause a 10-fold reduction in phonon relaxation time.

The reduction occurs over a wide frequency range, not just at resonant frequencies.

Impact on phonon group velocity is minimal due to limited dispersion flattening.

Abstract

We investigate the role of rattling guest atoms on the lattice thermal-conductivity of a type-I clathrate Ba$_{8}$Ga$_{16}$Ge$_{30}$ by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause 10-fold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining unusually low thermal-conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.