where the κ κ κ λ are the contributions to the thermal conductivity from individual phonon modes λ with heat capacities C λ , group velocities ν ν ν λ and lifetimes τ λ , and the sum is normalised by the volume V and the number of phonon wavevectors N included in the summation. (Full details of how the macroscopic κ κ κ latt and the constituent modal terms are calculated can be found in Ref. .) Fig. (b)/(c) compares the harmonic components and averaged lifetimes of the four Ge chalcogendies to SnS and SnSe. The κ/τ CRTA rapidly approaches a constant value as the C λ saturate to the Dulong-Petit limit, such that above ∼400 K this function is indicative of differences in the group velocities ν ν ν λ . Comparison of the κ/τ CRTA reveal three trends, viz. that the ν ν ν λ are smaller in the Pnma phases than in the R3m phases, are around 50% smaller for the selenides than the sulphides, and are similar for GeS/SnS and GeSe/SnSe. The latter implies that the tetrel atom has little impact on the group velocities.