Avoided crossings in the phonon dispersion tend to "depress" the phonon frequencies and lower the band curvature and group velocities, thereby also lowering the lattice thermal conductivity. Such features typically indicate the presence of "rattling" species, such as heavy filler atoms in the cavities in, for example, skutterudites and clathrates. With a quasi-layered structure and fairly light constituent atoms, rattling behaviour would not be expected in Y 2 Ti 2 O 5 S 2 . However, several avoided crossings can be seen along the Γ-X, P-N and Γ-N paths in the phonon dispersion, both between the optic and acoustic modes from 0-4 THz and also between different optic modes in the 6-16 THz range (Figure ).