Structural phase transitions, oxygen vacancy ordering and protonation in doped BaCeO3:: results from time-of-flight neutron powder diffraction investigations

The structural crystallography of the mixed-ion conductor BaCe(1 - x)Ln(x)O(3 - delta) and the parent compound BaCeO3 are reviewed from the perspective of neutron powder diffraction results. High-resolution, time-of-flight, powder diffraction has been used to verify the unit cell metric, and to correct the setting of this metric to the space group for both the undoped phase and for Y- and Nd-doped material. Key superlattice reflections are utilised to disprove the structural phase transitions inferred from Raman spectroscopy in temperature and Nd dopant concentration, with high-resolution powder diffraction showing the correct crystal symmetries in both cases. Crystal structure refinements have shown that negligible perturbation of the parent compound structure occurs on Y- and Nd-doping, and the extrinsic vacancy was found to be ordered on one of the two crystallographically independent oxygen atoms. A possible structural site for the proton has been proposed from measurements made at low temperatures and the potential for more detailed neutron diffraction studies is discussed. (C) 2001 Elsevier Science B.V, All rights reserved.