i.e., on the NMR timescale, no O sites are isolated from vacancies. Over the same temperature range, both the 17 O and 31 P NMR spectra reveal an increasing number of different O P environments that can be resolved due to motional narrowing; however, this motion must be restricted to localised tetrahedral rotations because coalescence between the Bi-O and PO 4 sublattices is not observed. In contrast, the same behaviour cannot be observed in O NMR spectra of the Vsubstituted samples due to the coalescence of the O Bi and O V resonances; however, the 51 V NMR spectra of Bi 0.852 V 0.148 O 1.648 still indicate the presence of V sites with differing coordination numbers even at high temperature.