A comparison of the normalised N2 of the ten systems (Fig. ) reveals some notable trends. After normalisation the scattering phase space depends strongly on n a , such that at most frequencies the N2 are highest for the Pnma phases (n a = 8), intermediate for Cmcm SnSe (n a = 4), and lowest and comparable for the R3m and Fm 3m phases (n a = 2). This can be attributed to the more complex phonon spectra enabling more scattering pathways (c.f. Fig. ). Also, the fine structure in the phonon DoS curves is partially reflected in the N2 , with those of the Pnma phases showing a reduction at mid frequencies reminiscent of the phonon bandgaps in the DoS. However, whereas the phonon bandgap in Pnma GeSe occurs at ∼3.5-3 THz, the minimum in the N2 occurs around 6 THz, and the absence of modes in the bandgap therefore affects the scattering (largely decay processes) of the high-frequency op-Fig. Analysis of the phonon lifetimes of the ten systems examined in this work. a) Averaged two-phonon weighted joint density of states (w-JDoS) functions N2 ( f ), defined in Eq. 16, for the ten materials examined in this study. The functions have been normalised by 1/(3n a ) 2 to be comparable between the phases with different n a . b) Comparison of the averaged lifetimes τ CRTA and inverse three-phonon interaction strengths P for the ten materials examined in this work at T = 300 K. The trendline shows the correlation for the three Fm 3m systems (R 2 ≈ 1). tic modes. However, the reduction in the DoS of R3m GeTe at intermediate frequencies in Fig. does not appear to be reflected in the N2 , at least not as clearly as in the corresponding Pnma phase. Finally, the spread of the N2 depends on the spread of the frequency spectra. Among the four Pnma phases, the N2 of SnTe is notably "compressed" compared to that of GeTe, with larger features at lower frequencies, while SnSe and GeTe have similar N2 . A crude comparison of the size of the phase space obtained by integrating the N2 from zero to the maximum frequency f max in the phonon spectra confirms that a narrower frequency spectrum generally results in a larger scattering phase space (Table ).