The two mechanisms by which filler atoms in cage compounds suppress the heat transport are through resonant scattering, which reduces the lifetimes of framework modes close in frequency to localised "rattling" modes, and by reducing the group velocities. Calculations suggest that these are not mutually exclusive, as the changes to the phonon spectrum that reduce the 𝒗 𝜆 can introduce new phonon-scattering channels that reduce the 𝜏 𝜆 of modes over a broader spectrum of frequencies. The small differences in the 𝜅 latt of the filled models makes it difficult to draw quantitative conclusions by inspecting the spectra of modal properties (Figs.