V is the unit-cell volume, C λ are the modal heat capacities, υ λ are the modal group velocities and τ λ are the phonon lifetimes. The contributions of individual modes to κ l can be examined from scatter plots of the three modal quantities in Eq 3 (i.e. C λ , υ λ , τ λ ) against the phonon frequency, coloured according to the modal contributions to κ l (κ λ ; the summand in Eq. 3). The effect of the C λ on the κ l is not considered here, as this quantity is a shallow function of frequency and the variation between phonon modes is therefore minimal.