Ion beam analysis of dependence of the D-H replacement speed on trivalent impurity concentration in D-implanted oxide ceramics exposed to H₂O vapor at room temperature

The speed of D-H replacement in D-implanted BaCe_1-XY _XO_3-δ and SrCe_1-XYb_XO _3-δ, exposed to H₂O vapor at room temperature has been studied as a function of X by use of the ERD technique with 1.7 MeV He ⁺ ion beam. The decay curve of D and growth curve of H in the specimens have been measured as a function of exposure time, and the decay constants of D, namely the D-H replacement speeds, have been obtained by fitting the experimental curves to the theoretical curves of D decay and H growth, which are derived from the mass balance equations described on the one-way diffusion model for the D-H replacement: absorption of H⁺ due to splitting of H₂O physisorbed at the surface, diffusion of H ⁺ and emission of D implanted in the traps as HD molecule due to bulk recombination of H⁺ and subsequent trapping of H⁺ in the vacant trap. It is found for both specimens that the decay constants of D, proportional to the absorption coefficient and the molecular formation coefficient of HD, increase, and after a maximum at X = 0.05-0.1 decrease as X increases, and the absolute experimental values for BaCe_1-XY _XO_3-δ are larger almost by an order of magnitude than those of SrCe_1-XYb_XO_3-δ. These experimental results are discussed in terms of the one-way diffusion model on D-H replacement and water splitting model at the surface.