Host: The Mineralogical Society of Japan
Perovskite-type oxides such as Ln3+ doped SrCeO3 and BaCeO3 where Ln3+ represents trivalent lanthanide elements, have been found to exhibit appreciable proton conduction in hydrogen-containing atmosphere at high temperature. Among these, the chemical stability of SrZrO3-based oxide is much better, and Yb-doped SrZrO3 oxide shows the highest proton conductivity. The proton conductivity depends on the amount of dopant Yb ions and shows the maximum value when substituted for Yb ions by 10mol%. The oxide ion vacancies and/or electron holes are produced by substitution of tetravalent zirconium ions by trivalent ytterbium ions. In our previous studies, the mass loss and gain of oxygen in the Yb-doped SrZrO3 was observed in heating and cooling process, respectively.In this study, temperature dependence of XAFS spectra of Perovskite-type proton conductor SrZr0.9Yb0.1O3-δ at Sr and Zr K-edges and Yb L3, L2 and L1-edges have been investigated. The chemical shift to lower energy and change in XANES spectra are observed with increasing temperature. No change in XANES spectra and in threshold energies of the Yb L3-, L2- and L1-edge was observed. This indicates that the effective charges (oxidation states) of Sr and Zr ions change with the oxygen loss while the effective charge of Yb ion were maintained. The unit cell volumes increase with increasing temperature with a slight bent between 700 and 950 K. This bent of the unit cell volumes vs. temperature is considered to be due to loss of oxygen, which is consistent with the result of a weight loss in TGA observed in the same temperature range. The mean-square relative displacements (MSRD) of the backscattering atoms with respect to the absorbing atom were also determined by EXAFS. Its temperature dependence increases with a slight bent between 700 and 950 K, which is considered to be due to a local structure change accompanied by the loss of oxygen rather than that by the phase transition at 948 K. It is considered that the changes of local structures and effective charges of Sr and Zr ions must play an important role in the protonic conductivity.
