The oxide-ion conductivity in La10Ge6O27- and La10Si6O27-based apatites was investigated. It was found that, in spite of the low symmetry of the crystal lattice, both oxides exhibited a high oxide-ion conductivity under a wide range of O partial pressures. The oxide-ion conductivity of La10Si6O27 was increased by inserting Sr into La sites. On the other hand, La10Ge6O27 also exhibited a high oxide-ion conductivity which was comparable to that of the La0.9Sr0.1Ga0.8Mg0.2O3 perovskite oxide. Arrhenius plots of the oxide-ion conductivity of La10Ge6O27 exhibited a knee at about 1000K. However, the knee disappeared after inserting Sr into La sites. The conductivity at low temperature was thereby greatly enhanced. This was attributed to stabilization of the high-temperature phase. The conductivities of Sr-doped La10Si6O27 and La10Ge6O27 were almost independent of O partial pressures ranging from 1 to 10-21atm. This suggested that these materials were ionic over a wide range of O partial pressures. It was concluded that these apatite oxides constituted a new class of fast oxide-ion conductors.

Oxide Ion Conductivity in Sr-Doped La10Ge6O27 Apatite Oxide. H.Arikawa, H.Nishiguchi, T.Ishihara, Y.Takita: Solid State Ionics, 2000, 136-137, 31-7