Intrinsic ultralow lattice thermal conductivity of the unfilled skutteruditeFeSb3

Abstract

It is generally accepted that unfilled skutterudites process high lattice thermal conductivity ${\ensuremath{\kappa}}_{l}$ that can be efficiently reduced upon filling. Here by using first-principles Boltzmann-Peierls transport calculations, we find pure skutterudite of ${\mathrm{FeSb}}_{3}$ with no filler in fact has an intrinsic ultralow ${\ensuremath{\kappa}}_{l}$ smaller than that of ${\mathrm{CoSb}}_{3}$ by one order of magnitude. The value is even smaller than those of most of the fully filled skutterudites. This finding means that with ${\mathrm{FeSb}}_{3}$ as a reference, filling does not necessarily lower ${\ensuremath{\kappa}}_{l}$. The ultralow ${\ensuremath{\kappa}}_{l}$ of ${\mathrm{FeSb}}_{3}$ is a consequence of the overall softening of phonon spectrum, especially the lowering in frequency of optical phonon branches associated with the weakly bonded ${\mathrm{Sb}}_{4}$ 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 ${\ensuremath{\kappa}}_{l}$ of skutterudites.