Improved oxide ion conductivity in La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ by doping Co

The effects of doping Co for the Ga site on the oxide ion conductivity of La_0.8Sr_0.2Ga_0.8-Mg_0.2O₃ 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 H₂-O₂ gas cell utilizing the optimized composition of La_0.8Sr_0.2-Ga_0.8Mg_0.115Co _0.085O₃. The diffusion characteristics of the oxide ion in La_0.8Sr_0.2Ga_0.8Mg_0.115-Co _0.085O₃ were also investigated by using the ¹⁸O tracer method. Since the diffusion coefficient measured by the ¹⁸O tracer method was similar to that estimated by the electrical conductivity, the conduction of La_0.8Sr_0.2Ga_0.8Mg_0.115Co _0.085O₃ 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⁺, 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.