Intrinsic ultralow lattice thermal conductivity of the unfilled skutterudite FeSb$_3$

TL;DR

This study reveals that unfilled FeSb$_3$ skutterudite intrinsically exhibits ultralow lattice thermal conductivity due to phonon interactions, challenging the conventional belief that filling is necessary to reduce thermal conductivity.

Contribution

The paper demonstrates that FeSb$_3$ has an ultralow thermal conductivity inherently, without the need for filling, due to softened phonon modes and increased anharmonic scattering, offering a new mechanism for thermal conductivity reduction.

Findings

FeSb$_3$ has lower $$ than CoSb$_3$ by an order of magnitude.

Ultralow $$ results from softened optical phonons and enhanced anharmonic scattering.

Filling skutterudites is not the only way to achieve low thermal conductivity.

Abstract

It has been generally accepted that unfilled skutterudites process high lattice thermal conductivity ($\kappa_{l}$) that can be efficiently reduced upon filling. Here by using first principles Boltzmann-Peierls transport calculations, we find pure skutterudite of FeSb$_3$ with no filler in fact has an intrinsic ultralow $\kappa_{l}$ smaller than that of CoSb$_3$ by one order of magnitude. The value is even smaller than those of most of the fully filled skutterudites. This finding means that with FeSb$_3$ as a reference, filling does not necessarily lower $\kappa_{l}$. The ultralow $\kappa_{l}$ of FeSb$_3$ is a consequence of much softened optical phonon branches associated with the weakly bonded Sb$_4$ rings. They overlap more with heat-carrying acoustic phonons and significantly increase the phase space for three-phonon anharmonic scattering processes. This provides an alternative non-filling related mechanism for lowering the $\kappa_{l}$ of skutterudites.