Fast oxide ion conductivity and oxygen tracer diffusion in doped La2GeO5−δ

Oxide ion conductivity in La-deficient La2GeO5−δ was investigated with 18O tracer diffusion measurement. It was found that La1.61GeO4.415 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 La1.8GeO4.7 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 La1.5Ca0.2GeO4.45. 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 La1.5Ca0.2GeO4.45 was higher than that of Zr0.84Y0.15O2 at all temperature examined, Ca-doped La2GeO5 based oxide is attractive as a new fast oxide ion conductor.