The range of the frequency spectra closely track the average masses. The phonon frequencies in Fm 3m SnTe, which has the largest average mass of the four systems for which phonon spectra are shown in Fig. , range up to ∼4 THz, whereas the frequencies in Pnma GeSe, which has the smallest average mass, range up to ∼7 THz. The frequencies in Cmcm SnSe and R3m GeTe, which have similar average masses, both range up to around 6 THz, i.e. intermediate between the limits set by SnTe and GeSe. This pattern is observed across all ten systems, regardless of structure type. The "compression" of the phonon spectrum with increasing average mass would be expected to increase the density of allowed phonon scattering pathways at a given frequency, i.e. the scattering phase space, which we again investigate further below.