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

Abstract

It has been generally accepted that unfilled skutterudites process high lattice thermal conductivity ($\hat{I}{º}_l$) 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 $\hat{I}{º}_l$ 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 $\hat{I}{º}_l$. The ultralow $\hat{I}{º}_l$ 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 $\hat{I}{º}_l$ of skutterudites.

6 pages, 5 figures + Supplementary information