Oxide ion conductivity in Sr-doped La₁₀Ge₆O₂₇ apatite oxide

Oxide ion conductivity in La₁₀Si₆O₂₇ and La₁₀Ge₆O₂₇-based apatite oxides was investigated in this study. In spite of the low symmetry of the crystal lattice, both oxides exhibited high oxide ion conductivity over a wide range of oxygen partial pressures. Oxide ion conductivity of La₁₀Si₆O₂₇ was increased by doping Sr for La sites. On the other hand, it was found that La₁₀Ge₆O₂₇ also exhibited high oxide ion conductivity which is comparable with that of La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ perovskite oxide. Arrhenius plots of the oxide ion conductivity in La₁₀Ge₆O₂₇ show a knee around 1000 K. However, the knee in the Arrhenius plot of conductivity disappeared by doping Sr for La sites. Thus, the conductivity at low temperature was greatly enhanced by doping Sr for La sites in La₁₀Ge₆O₂₆. This may be due to stabilization of the high temperature crystal phase. The electrical conductivities of Sr-doped La₁₀Si₆O₂₇ and La₁₀Ge₆O₂₇ were almost independent of the oxygen partial pressure from P_O(2) = 1 to 10^-21 atm. This suggests that these materials are ionic over a wide P_O(2) range. This study revealed that apatite oxide of La₁₀M₆O₂₇ (M = Si, Ge) is a new class of the fast oxide ion conductors.