Ceramics with the composition, BaO-0.9CeO2-0.05Gd2O3, were prepared with densities of 98 to 99%. The material consisted of 2 phases: a perovskite with the approximate composition, (Ba0.965Gd0.035)(Ce0.935Gd0.065)O2.985+, and an amorphous proton-conducting BaO-rich grain boundary phase. In the case of the perovskite phase, the proton diffusivity was deduced directly from water concentration-dependent conductivities and the O diffusion coefficient was obtained from 18O tracer experiments. The results were analyzed in terms of the concentrations and diffusivities of elementary defects. The ionic transference numbers and the chemical diffusion coefficient of water were then determined and compared with experimentally observed water uptake kinetics. The data suggested that structure diffusion was the operative proton conduction mechanism, with the proton transfer mode in the configuration, OH-/O2-, being the rate-limiting step. It was proposed that the mechanism was heavily assisted by extended vibrations of OH-/O2- about their equilibrium position.

K.D.Kreuer, E.Schönherr, J.Maier: Solid State Ionics, 1994, 70-71, 278-84