where 𝜏 CRTA is an effective phonon lifetime (relaxation time) and the other terms are defined as in Eq. 2. Both 𝜿 latt 𝜏 CRTA ⁄ and 𝜏 CRTA are temperature dependent due to the temperature dependence of the 𝐶 𝜆 and 𝜏 𝜆 . For cubic systems the diagonal elements of 𝜿 latt are equal and the off-diagonal elements are negligible, so it is possible to choose a single 𝜏 CRTA at each temperature such that the separation in Eq. 6 is exact. We discuss the scalar averages 𝜅 latt and 𝜅 latt 𝜏 CRTA ⁄ in the following. and.3. The 𝜅 latt 𝜏 CRTA ⁄ of the filled models are reduced by 4.6 -13 % compared to pristine CoSb3 and fall in a similar order to the rattling frequencies with HeCo8Sb24 > ArCo8Sb24 > NeCo8Sb24 > KrCo8Sb24 ≈ XeCo8Sb24. In the RTA model, these reductions are purely due to changes in the harmonic frequency spectrum. On the other hand, the 𝜏 CRTA in HeCo8Sb24 are reduced by ~5 % compared to pristine CoSb3, whereas filling with Ne, Ar and Kr changes the lifetimes by at most 1 % and in some cases increases them, and filling with Xe decreases the 𝜏 CRTA by a smaller 2.5-3 %.