In Table we compare the anharmonicity constants of different materials reported in the literature with that of (PEA) 2 PbI 4 from this work. We tabulate the materials going from covalent to strongly ionic systems, thus moving towards strongly anharmonic lattices. However, it is evident that there is no apparent trend in the value of γ 0 . Given that we are comparing phonons of varying softness, and for our comparison to be independent of the phonon energy, we calculate a dimensionless parameterω/2πγ 0 , which is the ratio of the frequency of the phonon and the anharmonicity constant. We interpret this parameter as the average number of cycles in the phonon oscillation before the phonon encounters a scattering event with another phonon. We clearly observe an increase in the value of this parameter with increasing ionicity of the material. In single-layer MoS 2 , the optical phonon is fairly immune to thermal dephasing, while in GaAs, which is a slightly polar compound, the phonon scatters in about 80 oscilla-tion cycles. Optical phonons in inorganic lead-halide perovskite nanocrystals appear to have comparable anharmonicity to that of GaAs, although the presence of an organic cation is most likely to reduce ω/2πγ 0 . We are not aware of comparable measurements in hybrid perovskites. We do observe much higher anharmonicity in (PEA) 2 PbI 4 with ω/2πγ 0 , comparable to that of strongly ionic solids such as SrTiO 3 .