Nature Chemistry, Published online: 16 June 2026; doi:10.1038/s41557-026-02174-x Phonon engineering of zero thermal expansion materials faces a trade-off between open frameworks, which can enable negative thermal expansion through transverse vibrations but can suffer from thermal instability, and closed frameworks, which are more robust yet exhibit weaker negative thermal expansion. Now, fractional atomic occupancy in sodalite-type closed frameworks is shown to exhibit isotropic zero thermal exp