The calculated phonon dispersion of Y 2 Ti 2 O 5 S 2 (Figure ) contains no imaginary modes, indicating the system is dynamically stable. The same was true for all of the supercell sizes tested (SI Figure ). The dispersion covers a large range of frequencies, with the highest frequency modes reaching 30 THz. The atom-projected (partial) phonon density of states (PDOS) shows that the greatest contribution to modes at frequencies below 8 THz comes from motion of the yttrium atoms, which is expected due to the inverse relationship between frequency and atomic mass. Between 6-10 THz, sulphur motion dominates, while titanium contributions are observed across the whole frequency spectrum. Somewhat surprisingly, oxygen motion also contributes significantly to phonon modes across the spectrum, even at low frequencies. Moreover, both of the symmetry-inequivalent oxygen atoms Phonon dispersion was plotted using ThermoPlotter, and the high-symmetry paths were constructed using the Bradley-Cracknell formalism. contribute to the low-frequency modes. A similar phenomenon, where Ti and O motion contribute to modes over a large range of frequencies, including lower frequencies, has been observed in anatase TiO 2 , SrTiO 3 and BaTiO 3 . All three structures have in common that they contain TiO 6 octahedral structural motifs, so it is likely that the low-frequency oxygen modes in Y 2 Ti 2 O 5 S 2 originate from the TiO 5 S octahedra.