For Bi 0.852 P 0.148 O 1.648 , the E A values found for motion within the PO 4 units agree relatively well with the small activation barriers for tetrahedral rotation previously found using O NMR in tetraoxoanion-containing materials, indicating that the relatively straightforward CT linewidth analysis nonetheless leads to reasonable activation energy values. Moreover, the E A values are identical within error to those obtained for motion involving the VO 4 units in the V-substituted materials, suggesting that similar motional processes operate in both types of dopant sublattices.