Fast oxide ion conductivity and oxygen tracer diffusion in doped La₂GeO_5-δ

Oxide ion conductivity in La-deficient La₂GeO_5-δ was investigated with ¹⁸O tracer diffusion measurement. It was found that La_1.61GeO_4.415 exhibits a large diffusion coefficient, which is in good agreement with that estimated from electrical conductivity. Therefore, it was confirmed that La-deficient La₂GeO₅ is a pure oxide ionic conductor. The effects of doping alkaline earth cations to the La site of La_1.8GeO_4.7 were also investigated. Although the crystal structure was slightly changed, alkaline earth cations seem to substitute for the La site in La₂GeO₅. The electrical conductivity at low temperature is increased by doping alkaline earth cations, in particular, it was found that doping with Ca is the most effective for increasing the electrical conductivity. The electrical conductivity increased with increasing Ca and attained the maximum at a composition of La_1.5Ca_0.2GeO_4.45. The transport number of the oxide ion, estimated by H₂-O₂ cell, was almost 1.0. Therefore, Ca-doped La₂GeO₅ oxide is also a pure oxide ionic conductor. Since the electrical conductivity of La_1.5Ca_0.2GeO_4.45 was higher than that of Zr_0.84Y_0.15O₂ at all temperature examined, Ca-doped La₂GeO₅ based oxide is attractive as a new fast oxide ion conductor.