The κ/τ CRTA decrease in the order Bi 2 SO 2 > Bi 2 SeO 2 > Bi 2 TeO 2 , which reflects the impact of differences in the chalcogen mass and chemical bond strength on the phonon dispersions (c.f. Fig. ). On the other hand, Bi 2 SeO 2 is predicted to have longer τ CRTA than Bi 2 SO 2 , which balances the lower group velocities to yield similar κ latt . This phenomenon has also been observed in calculations on the Sn chalcogenides (c.f. Table ), for which the balance is such that a different ordering of the κ latt can be obtained with different calculation parameters. Taking the averaged lifetimes as indicative of the strength of the intrinsic phonon anharmonicity, this implies that the (heavier) selenide is less anharmonic than the (lighter) sulphide. On the other hand, Bi 2 TeO 2 has both lower group velocities and shorter τ CRTA than Bi 2 SO 2 and Bi 2 SeO 2 and hence the lowest κ latt of the three systems.