For all three materials, the O T 1 of the O Bi environment decreases with increased temperature, suggestive of changes in ionic motion within the Bi-O sublattice on the MHz timescale. The motion involving the Bi-O sublattice as probed by the T 1 relaxation measurements, which is on the order of the Larmor frequency, is ascribed to the local movement of oxygen vacancies, which have previously been measured with O NMR relaxometry in other oxide-ion conductors, e.g., Ydoped ceria and Bi 2 WO 6 . The relevant motions in these systems contributing to temperaturedependent changes in the T 1 values are typically considered to comprise short-range displacements such as nearest neighbour and/or nearby vacancy jumps, which may be distinct from motions influencing bulk ionic conductivity.