The κ κ κ/τ CRTA iso as a function of temperature are shown for the ten compounds in Fig. , and the values at 300 K are provided in Table . As found in our previous work on the silicon clathrates, we see a strong dependence on the number of atoms n a in the primitive cell and the crystal symmetry. The four Pnma phases, which have the largest n a = 8, have the lowest values, the two R3m phases have intermediate values (n a = 2, lower symmetry), and the three Fm 3m phases have the largest values (n a = 2, higher symmetry). The large unit cell of the Pnma structure therefore lends itself to lower phonon group velocities, as does the symmetry breaking in the R3m structure compared to the Fm 3m phase. Cmcm SnSe is an outlier here, as despite its relatively large primitive cell (n a = 4) and low orthorhombic symmetry it has a larger κ κ κ/τ CRTA iso than R3m GeTe and SnTe, and, indeed, than Fm 3m SnTe. We attribute this to the imaginary phonon mode leading to a sharp dispersion and large ν ν ν λ in some of the phonon branches (c.f. Fig. and Eq. 5). In reality, above the Pnma → Cmcm transition temperature we would expect the imaginary modes to be "renormalised" to real frequencies, although in our previous calculations we found the impact of this on the dispersion and thermal conductivity to be minimal. However, the experiments in Ref. noted a substantial phonon softening with temperature, leading to a reduction in the ν ν ν λ on the order of 20%, which would not be captured by the fixed-volume structures used in our calculations. It is therefore unclear whether this phenomenon is a feature of materials with soft-mode phase transitions. Finally, for a given crystal structure we observe a clear dependence of the κ κ κ/τ CRTA iso on the average mass, such that the values fall in the order of SnTe < SnSe ≃ GeTe < GeSe, SnTe < GeTe, and SnTe << GeTe < GeSe for the Pnma, R3m and Fm 3m phases.