Our XRD data, above, indicate that Ba and Sn are randomly distributed across the fluorite 4a cation sites, and our AIMD simulations predict a complex fluorine substructure. Both results imply that c-BaSnF 4 contains a rich variety of fluoride-ion environments, which might be expected to be observable in the experimental 19 F MAS NMR spectrum, as in other mixed-cation fluorites ;and yet we observe only two peaks. This apparent contradiction can be reconciled with our expectation of a complex fluorine substructure if we consider fluorine exchange between different sites within the host framework . Fluorine exchange between Ba-rich sites can cause individual peaks associated with different Ba-rich environments to coalesce, giving a single observed resonance. The same reasoning applies to Sn-rich environments, suggesting that they too exhibit fluorine exchange on the NMR timescale.