At high temperature, the phonon MFPs in Y 2 Ti 2 O 5 S 2 are short, with only 6% of the phonon modes possessing MFPs longer than 20 nm in either direction. A slightly larger proportion of modes have MFPs longer than 10 nm -17% in the xy plane and 14% along the z direction. At 300 K, longer phonon lifetimes are observed, such that 25% of the κ l arises from modes with MFPs above 20 nm and 40% from modes with MFPs longer than 10 nm. This suggests that nanostructuring to limit the phonon mean-free paths could in principle reduce the κ l , in particular at lower temperature, which we explore in the following subsection.