Formation mechanism of oxygen deficient region in electrochemically reduced Y-PSZ crystal

Formation mechanism of the oxygen deficient regions in electrochemically reduced 4.5 mol%Y₂O₃-ZrO₂ crystals was examined by electric measurement and X-ray photoelectron spectroscopy (XPS) technique. Reduced samples were composed of two regions: severely reduced region near the cathode and weakly reduced region near the anode. During passage of d.c. current with a constant current density, the voltage across the sample decreased linearly with time. The severely reduced region progressed through the sample toward the anode linearly with time. These results showed that the resistivity of the severely reduced region was lower than that of the weakly reduced region, and the total resistivity of the sample decreased with increasing volume fraction of the severely reduced region and that the rate limiting process of formation of the severely reduced region was considered to be mainly surface reaction at the boundary of severely and weakly reduced regions. The measurement of oxidation state of zirconium by XPS showed that both severely and weakly reduced regions were uniformly reduced and that degree of reduction was independent of the reduction time. This was a direct evidence that during electrochemical reduction, the degree of reduction of both severely and weakly reduced regions was constant and that total reduction progressed with increasing volume fraction of the severely reduced region. On the other hand, there was no clear difference of oxidation states of yttrium between as-grown and reduced samples.