As noted earlier, the temperature dependence of the phonon dephasing dynamics is interpreted as the consequence of their elastic scattering by defects and phonon-phonon interactions that are mediated by the anharmonicity of lattice. The latter was theoretically modelled as a third-order anharmonic Hamiltonian considering, three-phonon interactions, which reads as : serving both energy and momentum act as plausible pathways for the phonon decoherence. In the case of (PEA) 2 PbI 4 , the higherenergy phonons correspond to localized, dispersionless vibrations of the organic moiety and it is unlikely that such an upconversion process will conserve momentum. Thus, we only consider the downconversion processes as the primary dephasing mechanism. It is worth noting that four-phonon interactions, where the phonon decays into three or more phonons, are also plausible decay pathways; however, they are not significant at low temperatures . We thus consider the simplest case of the downconversion of the optical phonon into two acoustic phonons of equal energy, referred to as the cubic overtone model. We disregard any constraints from the wavevector mismatch, but we do note that that this mechanism can be expected given the measured phonon dispersion for similar metal-halide derivatives . In such a scenario, the anharmonic potential results in a dephasing rate Γ which can be written as: