THz are raised, with the largest increases towards the 𝐻 wavevector, while the frequencies of the higher-energy optic modes are reduced. The reduction in bandwidth ("depression") of the phonon spectrum would lead to a reduction in the mode group velocities, one of the two main mechanisms by which fillers can suppress the thermal conductivity of cage compounds. This mechanism is most usually observed when the rattling frequency falls among the acoustic modes and introduces avoided crossings into the phonon dispersion. In these systems, however, the noble gas fillers have higher rattling frequencies and would therefore mainly affect the optic modes, and the density of phonon branches makes it difficult to clearly identify any avoided crossings.