Possible deuterium positions in the high-temperature deuterated proton conductor Ba3Ca1+yNb2-yO9-? studied by neutron and X-ray powder diffraction

High-temperature proton conductors with perovskite structures are a class of well-known systems with high protonic conductivity, which is of high technological interest in view of the possible applications in solid oxide fuel cells. We present neutron and X-ray diffraction studies of the crystal structure of the Ba3Ca1.18Nb1.82O9??+zD2O (BCN18) system that exhibits high proton conductivity. It is assumed that the mechanism of proton conductivity in BCN18 is connected with trapping of protons at some interstitial positions in the crystal lattice as deduced from earlier quasielastic neutron scattering and muon spin rotation studies published in the literature. In order to get more information on the location of deuterium, systematic high-resolution neutron diffraction studies of the stoichiometric Ba3Ca1Nb2O9 and non-stoichiometric BCN18 compounds with and without D2O were performed. The ROTAX time-of-flight (TOF) diffractometer at the ISIS neutron spallation source and the SV7 double-axis diffractometer at the DIDO reactor at Jülich were used. Refinements of the crystal structures were performed by using the FULLPROF and GSAS programs. The possible deuterium positions in the crystallographic unit cell are discussed.