In contrast, the τ CRTA are strongly temperature dependent, and the sharp reduction in the phonon lifetimes with temperature, due to larger phonon occupation numbers and stronger scattering, dominate the behaviour of the κ latt once the κ/τ CRTA reach the limiting values. Pnma GeS and GeSe have similar averaged Table Calculated lattice thermal conductivity κ κ κ latt of the Pnma and R3m phases of GeS and GeSe at T = 800 K. The three diagonal components κ xx , κ yy , κ zz , corresponding to transport along the x, y, and z directions, respectively, are shown together with the average κ ave . We also show the averaged harmonic and lifetime components κ/τ CRTA and τ CRTA calculated according to Eq. 3 and the average phonon-phonon interaction strength Pλ for each model. Data for Pnma SnS and SnSe from the similar calculations in Ref. are also given for comparison. lifetimes, so differences in the κ latt are due to the group velocities. R3m GeS and GeSe both have shorter τ CRTA than the corresponding Pnma phases. The averaged lifetimes of GeS are shorter than those of GeSe, which compensates for the higher ν ν ν λ , and, somewhat counter-intuitively, results in a smaller overall predicted κ latt for the sulphide than the selenide. The two Sn chalcogenides both have shorter averaged lifetimes than the equivalent Ge phases, resulting in smaller κ κ κ latt despite the similar κ/τ CRTA . This indicates that, while the tetrel atoms have a minimal effect on the group velocities, they do have an impact on the κ κ κ latt through the phonon lifetimes.