Fig. shows how the rattling frequency and room-temperature 𝜅 latt vary for each filler as the mass 𝑚 X is varied from He to Xe. The frequencies vary inversely with 𝑚 X and fall below 2.5 THz when the mass of the weakly-interacting He filler is increased to 𝑚 Ar , 𝑚 Kr and 𝑚 Xe , or when the mass of Ne is increased to 𝑚 Kr and 𝑚 Xe . The changes in 𝜅 latt in Fig. show two distinct effects. For higher 𝑓 ̃X, the 𝜅 latt follow a general decreasing trend with 𝑚 X to the maximum ~15 % reduction obtained with XeCo8Sb24. Once 𝑓 ̃X falls below a certain threshold, which from these tests appears to be around 1.5 THz, i.e. well within the acoustic bands, there is a large drop in the 𝜅 latt . show that rattling frequencies within the acoustic bands lead to localised modes with narrow dispersion (Figs.