To investigate which types of vibration contribute most to the κ l , the cumulative in-plane and out-of-plane κ l as a function of frequency was calculated at 1000 K (Figure ) and compared to the atom-projected density of states. It is clear from this analysis that the majority of the heat transport occurs through the acoustic ThermoPlotter. To decrease the κ l , one would need to reduce the group velocities and/or lifetimes of the low-frequency modes. To achieve this heavier and larger substitutional point defects should be introduced into the structure to create greater variation in the ion sizes in the crystal lattice, which would act as scattering centres and promote "electron-crystal, phonon-glass" behaviour. As shown in Figures and, modes involving the Y atoms predominantly occur below 4 THz, so substituting Y with a heavier atom of similar electronegativity and bonding, for example La, would likely decrease the κ l . Alternatively, substituting S for Se in BiCuOS and BiCuOSe substantially improves the ZT, so Y 2 Ti 2 O 5 Se 2 could potentially be a better thermoelectric than Y 2 Ti 2 O 5 S 2 . The cumulative κ l can also be examined as a function of the phonon mean free path (MFP), Λ λ = υ λ τ λ . This is shown for the in-plane and out-of-plane directions at 300 K and 1000 K in Figures and, respectively.