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 temperat ure. Both compounds crystallize in a distorted perovskite-like structu re and were refined in space group Pnma (no. 62) by full-profile Rietv eld methods. The structure refinements show that substitution of yttri um 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 scat tering density maps around the atoms constructed from F-obs, phased by the F-calc, obtained from the Rietveld refinement. Delta rho is compu ted 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 pr ovide a direct picture of the average in space and time of thermal vib rations and occupancies on atomic sites less biased by least-squares m ethods than parameters obtained from refinement. Oxygen vacancies indu ced in order to maintain electroneutrality upon substitution of Ce4+ b y Y3+ are confined to one of two non-equivalent oxygen sites. This str uctural feature is consistent with experiments which show a very low c ontribution to the total conductivity from oxygen ion conductivity rel ative to that of protonic conductivity.