Comparison of the unfolded phonon dispersions to the dispersion of CoSb3 (c.f. Figs. and; see also Figs. S2.2-S2.6) shows the heavier filler atoms have a progressively larger effect on the vibrations of the host framework. In HeCo8Sb24, the spectrum is largely unchanged save for the localised branch in the phonon bandgap. From NeCo8Sb24 to XeCo8Sb24 the frequencies of the acoustic and low-lying optic modes up to ~2.5