High temperature Bi2O3 has the highest oxide ion conductivity known for any material. It has an oxygen-deficient fluorite-based structure, with 25% of oxygen atoms missing from tetrahedral sites in the face centred cubic Bi3+ array. However, delta-Bi2O3 is stable only from 730oC to its melting at 804oC. Its cubic structure can be stabilised by low levels of doping, while higher levels of cation substitution often result in symmetry lowering and formation of complex fluorite-related superstructures.As part of our EPSRC-funded research programme on structure-property relationships in oxide ion conductors, in this proposal we request 1 day of HRPD beam time to perform powder neutron diffraction studies on Bi6WO12 in order to determine its crystal structure and correlate the structure with its oxide ion conducting properties.