Powder neutron diffraction studies of BaCe0.9Y0.1O2.95 and BaCeO3 at 4.2 K: a possible structural site for the proton

High-resolution, powder, neutron diffraction data have been collected on bo th the protonic conductor BaCe0.9,Y-0.1,O-2.95 and the undoped parent compo und BaCeO3 at a temperature of 4.2 K. Analysis of difference nuclear densit y maps from the doped material showed the presence of two weak negative pea ks which were tentatively assigned to hydrogen. On refinement only one of t hese sites behaved in a stable manner resulting in a chemically sensible bo nd length to a neighbouring oxygen atom. As the number density of incorpora ted protons is low in the doped compound, this structural site is only prop osed and additional work will be required for confirmation. However, the si te does give Ba-H and Ce-H distances close to those calculated in a quantum molecular dynamics simulation of the aristotype phase at high temperatures and is in good agreement with that inferred from a recent muon spin relaxa tion study of positive muons implanted in Sc-doped SrZrO3. The orientation of the O-H bond is in agreement with atomistic lattice simulations carried out on the aristotype phase of LaMnO3. (C) 2000 Elsevier Science B.V. All r ights reserved.