Eq. 8 encompasses two types of scattering processes, viz. collisions, where 𝜆 + 𝜆 ′ → 𝜆 ′′ or 𝜆 + 𝜆 ′′ → 𝜆 ′ , and decays, where 𝜆 → 𝜆 ′ + 𝜆 ′′ . These are sometimes referred to in the literature as absorption and emission processes. Broad linewidths Γ 𝜆 , i.e. short 𝜏 𝜆 , arise from strong Φ 𝜆𝜆 ′ 𝜆 ′′ and/or a high density of energyconserving collision and decay pathways. Both factors can lead to short mode lifetimes -for example, the -Page 19 -picosecond lifetimes in the hybrid perovskite (CH3NH3)PbI3 have been attributed to strong Φ 𝜆𝜆 ′ 𝜆 ′′ , whereas the low predicted 𝜅 latt in Bi2Sn2O7 has been ascribed to a high density of scattering channels. Another subtlety of note is that the phonon frequencies 𝜔 𝜆 and phonon-phonon interaction strengths Φ -𝜆𝜆 ′ 𝜆 ′′ are temperature independent in the RTA model, and thus the strong temperature dependence of the 𝜏 𝜆 and 𝜏 CRTA in Figs. and is due entirely to the changes in the 𝑛 𝜆 .