Proton conducting Ba3Ca1.18Nb1.82O8.73H2O:: pressure-compositions isotherms in terms of Fermi-Dirac statistics, concentration and fuel-cell measurements, and impedance spectroscopy

Thin films of Ba3Ca1.18Nb1.82O8.73, prepared in a sol-gel process by multiple dip-coating on silicon wafers, and powder samples, prepared by the conventional carbonate route, were charged with hydrogen by dissociative water absorption at definite values of water vapour pressure and temperature. The hydrogen content was determined ex situ at room temperature using nuclear resonance reaction analysis. From the resulting water vapour pressure-composition isotherms the absorption enthalpies Delta(empty set)H(o), Delta(empty set)H(t) and the absorption entropy Delta(empty set)S were calculated, using a two-site model, based on Fermi-Dirac statistics. On Ba3Ca1.18Nb1.82O8.73 bulk material also impedance spectroscopy, concentration and fuel-cell measurements as well as neutron diffraction were performed. From these data, taken together, the proton diffusion coefficient D-H could be evaluated and compared to quasielastic neutron scattering results. (C) 1999 Elsevier Science B.V. All rights reserved.