On the basis of X-ray diffraction the oxygen deficient La2-xNdxCe2O7 series displays a transition from disordered fluorite to the C-type superstructure in which the cations remain statistically distributed around two distinct oxygen sites. The transition is accompanied by a decrease in the oxide ion conductivity. Work by the applicants has established that La2Ce2O7 also displays significant proton conductivity and preliminary neutron data revealed a strongly modulated background indicating that the local structure is more complex than a randomly disordered fluorite. We here request 4 days on POLARIS to study the structural transition in La2-xNdxCe2O7 using the total scattering-RMC methodology to resolve local cation and/or oxygen vacancy ordering. We also aim to determine proton/deuteron sites in selected samples by collecting low T data to help identify proton migration pathways.