High-temperature anion and proton conduction in RE₃NbO₇ (RE=La, Gd, Y, Yb, Lu) compounds

The oxide-ion and proton conduction properties of RE₃NbO₇ (RE=La, Gd, Y, Yb, Lu) compounds were investigated. For the bigger rare-earth cation, i.e. La³⁺, the compound crystallises in a weberite-type structure and the oxide-ion conductivity is low owing to the lack of intrinsic oxygen vacancies. Consequently, the resultant proton incorporation and conductivity in La₃NbO₇ are also low. For small rare-earth cations, i.e. from Gd³⁺ to Lu³⁺ and for RE=Y, materials adopt a fluorite-like structure confirmed from X-ray powder diffraction. In this latter case, materials include intrinsic oxygen vacancies leading to a higher oxygen conductivity. For these compounds, a proton incorporation takes place at low temperature under wet conditions giving rise to proton conductivity. Nevertheless, both oxygen and proton conductivities are low in these materials, which can be explained by the ordering of oxygen vacancies observed by Transmission Electron Microscopy.