Intrinsic ultralow lattice thermal conductivity of the unfilled skutterudite FeSb_3

摘要

It is generally accepted that unfilled skutterudites process high lattice thermal conductivity κ_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 κ_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 κ_l. The ultralow κ_l of 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 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 κ_l of skutterudites.