As we have already discussed, in Fig. the mode M1 exhibits weak temperature dependence and thus we deduce a very low degree of anharmonicity. One could rationalize this with the cubic overtone model, wherein M1 does not have access to two acoustic modes of exactly half its energy. It can be qualitatively substantiated through the density of states for acoustic phonons of low frequency and in isotropic media, which can be shown to scale as ω 2 , where hω is the energy of the acoustic phonon . This suggests that M1 will have access to a lower density of acoustic phonons and thus will be less likely to undergo downconversion. Similar scenarios have been proposed for CsPbBr 3 , where a higher density of states, between 1-3 THz, increase the dephasing rate in comparison to lower energy modes . Similarly, for (n-butylammonium) 2 PbI 4 in which the phonon lifetime was calculated to be higher at lower frequencies following an ω -2 trend . This interpretation relies on the assumption of decay of the M1 into two acoustic phonons of similar energy. However, different pathways including phonons of higher energy with larger density of states might also be plausible. It is imperative here to have detailed knowledge of the entire dispersion of the low-energy acounstic phonons to obtain greater insight into the different scattering pathways that are energetically allowed and that obey momentum conservation. Unfortunately, such data is not yet available for Ruddlesden-Popper metal halides.