NEUTRON-DIFFRACTION INVESTIGATION OF THE ATOMIC DEFECT STRUCTURE OF Y-DOPED SRCEO3, A HIGH-TEMPERATURE PROTONIC CONDUCTOR

The structures of SrCeO3 and its Y-doped equivalent Sr[Ce0.85Y0.15]O-2.925 have been examined by neutron powder diffraction at room temperature. Both compounds crystallize in a distorted perovskite-like structure and were refined in space group Pnma (no. 62) by full-profile Rietveld methods. The structure refinements show that substitution of yttrium into SrCeO, creates an increased lattice distortion relative to the cubic perovskite structure. Changes in neutron scattering densities, Delta rho, due to this substitution are illustrated by difference scattering density maps around the atoms constructed from F-obs, phased by the F-calc, obtained from the Rietveld refinement. Delta rho is computed as rho[Sn(Ce0.85Y0.15)O-2.995]-rho(SrCeO3) with the coordinates of the atoms in question translated to the origin (0,0,0). These maps provide a direct picture of the average in space and time of thermal vibrations and occupancies on atomic sites less biased by least-squares methods than parameters obtained from refinement. Oxygen vacancies induced in order to maintain electroneutrality upon substitution of Ce4+ by Y3+ are confined to one of two non-equivalent oxygen sites. This structural feature is consistent with experiments which show a very low contribution to the total conductivity from oxygen ion conductivity relative to that of protonic conductivity.