where the Γ λ are derived by considering energy-and momentum-conserving three-phonon scattering processes and obtained from the imaginary part of the phonon self-energy. Unlike the group velocities, the lifetimes are scalar quantities. The mode lifetimes are plotted against frequency, with the contributions to κ λ overlaid as a colour scale, in Figure . The lifetimes span a range of 0.1-10 ps, with over 30% of modes possessing lifetimes longer than 1 ps, even at 1000 K. In general, long lifetimes are indicative of weak phonon scattering and thus weak bond anharmonicity, which is expected in Y 2 Ti 2 O 5 S 2 due to the similar ion sizes. This is further evident from the average mode Grüneisen parameter of 0.65, another measure of anharmonicity, which is low compared to PbTe (1.96), SrTiO 3 (1.46) and BiCuOSe (1.5). Bond anharmonicity in Y 2 Ti 2 O 5 S 2 is thus unlikely to be a major contributor to its low lattice thermal conductivity.