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 both the protonic conductor BaCe0.9,Y-0.1,O-2.95 and the undoped parent compound BaCeO3 at a temperature of 4.2 K. Analysis of difference nuclear density maps from the doped material showed the presence of two weak negative peaks which were tentatively assigned to hydrogen. On refinement only one of these sites behaved in a stable manner resulting in a chemically sensible bond length to a neighbouring oxygen atom. As the number density of incorporated protons is low in the doped compound, this structural site is only proposed and additional work will be required for confirmation. However, the site 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 relaxation 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 rights reserved.