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The preferential site for oxygen isotope exchange was investigated on layered films of La 2−x Sr x CoO 4 ͑x = 0.5,1.0͒/La 0.6 Sr 0.4 CoO 3 prepared by pulsed laser deposition ͑PLD͒ on a polycrystalline Ce 0.9 Gd 0.1 O 1.95 . An isotope exchange experiment was performed at 773 K in 18 O-enriched oxygen of 0.2 ϫ 10 5 Pa. Three-dimensional imaging of secondary ion mass spectrometry ͑SIMS͒ was carried out to visualize the isotope distribution. The SIMS images suggested that there are fast oxygenincorporation paths along the hetero-phase interface of La 2−x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 . The results have reproduced our former findings on the fast oxygen surface exchange with La 2−x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 composite polycrystals.

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The effects of doping Co for the Ga site on the oxide ion conductivity of La 0.8 Sr 0.2 Ga 0.8 -Mg 0.2 O 3 have been investigated in detail. It was found that doping Co is effective for enhancing the oxide ion conductivity. In particular, a significant increase in conductivity in the low-temperature range was observed. The electrical conductivity was monotonically increased; however, the transport number for the oxide ion decreased with an increasing amount of Co. Considering the transport number and ion transport number, an optimized amount for the Co doping seems to exist at 8.5 mol % for Ga site. The theoretical electromotive forces were exhibited on H 2 -O 2 gas cell utilizing the optimized composition of La 0.8 Sr 0.2 -Ga 0.8 Mg 0.115 Co 0.085 O 3 . The diffusion characteristics of the oxide ion in La 0.8 Sr 0.2 Ga 0.8 Mg 0.115 -Co 0.085 O 3 were also investigated by using the 18 O tracer method. Since the diffusion coefficient measured by the 18 O tracer method was similar to that estimated by the electrical conductivity, the conduction of La 0.8 Sr 0.2 Ga 0.8 Mg 0.115 Co 0.085 O 3 is concluded to be almost ionic. On the other hand, an oxygen permeation measurement suggests that the oxide ion conductivity increased linearly with an increasing amount of Co. Therefore, specimens with Co content higher than 10 mol % can be considered as a superior mixed oxide ion and hole conductor. The UV-vis spectra suggests that the valence number of doped Co was changed from +3 to +2 with decreasing oxygen partial pressure; the origin of hole conduction can thus be assigned to the formation of Co 3+ . Since the amount of dopant in the Ga site was compensated with Mg 2+ , the amount of oxygen deficiency was decreased by doping Co. Therefore, it is likely that the improved oxide ion conductivity observed by doping with Co is brought about by the enhanced mobility of oxide ion.