The variable coordination of the dopant ion plays a crucial role in understanding the differing ionic conductivities of these fluorite-type materials. Experimentally, the Bi 0.852 P 0.148 O 1.648 exhibits the lowest conductivity, followed by Bi 0.852 V 0.148 O 1.648 and Bi 0.913 V 0.087 O 1.587 . Although dopants clearly aid in stabilising the cubic δ -Bi 2 O 3 structure, they also tend to disrupt three-dimensional isotropic conduction within the Bi-O sublattice. However, to a limited degree cooperative exchange between the dopant and Bi-O sublattices can offset this effect. The difference in the conductivity of pseudo-cubic Bi 0.913 V 0.087 O 1.587 and monoclinic Bi 0.852 V 0.148 O 1.648 highlights this first point, with the maintenance of the long-range 3D isotropic structure in the former case contributing to its exceptional ionic conduction. In the case of Bi 0.852 P 0.148 O 1.648 , although increasing rates of oxide-ion motion clearly do occur within the Bi-O sublattice as we have shown, the inability of phosphorus to adopt variable O coordination hinders significant sub-lattice exchange and leads to overall lower bulk conductivity. Similar observations have been made