The calculations on the five XCo8Sb24 models show that the fillers can reduce the 𝜅 latt of pristine CoSb3 by up to 15 % by limiting the thermal transport through the low-lying optic modes from 2.5-6 THz, by reducing the mode group velocities and, for some of the fillers, suppressing the phonon lifetimes. Although this reduction is nowhere near as substantial as those seen in some filled skutterudites, nanostructuring could be used to reduce the heat transport through the acoustic modes. Calculations similar to those in Fig. for the filled models suggest that, for example, nanostructuring to 100 nm could reduce the room-temperature 𝜅 latt of the filled models by ~50 %, in addition to the smaller reductions compared to pristine CoSb3 afforded by filling (Figs. S2.25-2.29).