PROTON AND OXYGEN DIFFUSION IN BACEO3 BASED COMPOUNDS - A COMBINED THERMAL GRAVIMETRIC ANALYSIS AND CONDUCTIVITY STUDY

Ceramics of composition BaO.0.9 CeO2.0.05 Gd2O3 have been prepared with a density of 98-99%. The material consists of two phases, a perovskite of approximate composition (Ba0.965Gd0.035) (Ce0.935Gd0.065)O2.985+delta and an amorphous proton-conducting BaO-rich grain-boundary phase. For the perovskite phase, the proton diffusivity is obtained directly from water-concentration-dependent conductivities and the oxygen diffusion coefficient from O-18-tracer experiments. The results are analyzed consistently in terms of the concentrations and diffusivities of elementary defects. In return, the ionic transference numbers and the chemical diffusion coefficient of water are determined and compared with the experimentally observed kinetics of water uptake. The data suggest structure diffusion as the operating proton conduction mechanism with the proton transfer mode in the configuration OH-/O2- as the rate-limiting step. It is proposed that the mechanism is heavily assisted by extended vibrations of OH-/O2- about their equilibrium position.