Improved oxide ion conductivity in La0.8Sr0.2Ga0.8Mg0.2O3 by doping Co

The effects of doping Co for the Ga site on the oxide ion conductivity of La0.8Sr0.2Ga0.8Mg0.2O3 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 La0.8Sr0.2Ga0.8Mg0.115Co0.085O3. The diffusion characteristics of the oxide ion in La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 were also investigated by using the O-18 tracer method. Since the diffusion coefficient measured by the 180 tracer method was similar to that estimated by the electrical conductivity, the conduction of La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 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 Co3+. Since the amount of dopant in the Ga site was compensated with Mg2+, th, 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.