Location of Deuterium Positions in the Proton-Conducting Perovskite BaCe0.4Zr0.4Sc0.2O2.90•xD2O by Neutron Powder Diffraction

Structural studies from X-ray and high-resolution neutron powder diffraction data collected on deuterated and nondeuterated BaCe0.4Zr0.4Sc0.2O2.9 center dot x(D2O) have been carried out. Although X-ray diffraction suggested that a sample of the nominal composition was single phase cubic, neutron diffraction showed the structure as single phase orthorhombic in the space group Pbnm. The structural distortion can be related to the determination of accurate oxygen positions in the structure by neutron diffraction. Rietveld refinement of the neutron diffraction data and subsequent Fourier nuclear density maps have proved to be successful in locating the deuterium (D) positions despite the low occupancies present, with a final composition of BaCe0.4Zr0.4Sc0.2O2.90 center dot 0.10(D2O) and the D position associated with the O1 position in the Ba-containing plane. TGA analysis in dry air on the deuterated sample shows the 0.70% weight loss for heating from room temperature to 900 C which corresponds to the D2O loss of 0.10/formula unit. The unit-cell volume for the deuterated phase was found to be higher than the nondeuterated phase (i.e., dry) at 295 K, i.e., V = 311.59(5) angstrom(3) compared with 310.91(5) angstrom(3). The O1-D distance was found to be 1.06(1) angstrom. The presence of anisotropic displacements for oxygen was observed during the refinement.