Structural and computational studies of Bi2WO6 based oxygen ion conductors

A combination of neutron powder diffraction and computer simulation techniques was performed on undoped and doped Bi2WO6 Aurivillius type compounds to clarify some of the factors controlling oxygen transport in these materials. Oxygen vacancies in doped compounds are randomly distributed within the perovskite-like slab. The most favourable dopants are predicted to be NbVand TaV on WVI and LaIII on BiIII in accordance with the experimental results. The calculated migration energy of 0.63 eV is in agreement with the values deduced from impedance spectroscopy data for Ta and Nb doped Bi2WO6 at T>550 °C. At lower temperatures, pair clusters are predicted to form with a 0.25 eV mean binding energy, leading to a 0.88 eV activation energy for oxygen vacancy migration, in good correlation with experimental values. Finally, consideration of possible oxygen ion migration pathways in the structure showed that energy barriers to migration are lowest between adjacent apical and equatorial sites of WO6 oxygen octahedra.