A detailed investigation was made of the effect of O content on the conductivities of Ba doped lanthanum germanate apatites, La8+xBa2−x(GeO4)6O2+x/2. The results showed that with no nominal O excess (x = 0) the conductivity was low (σ800C = 2 x 10−5S/cm), while the introduction of O vacancies, e.g. La7.8Ba2.2(GeO4)6O1.9, led to a significant further reduction in conductivity (σ800C = 7.2 x 10−7S/cm). In contrast, a systematic increase in conductivity was observed with increasing O excess content reaching a plateau (σ800C  ≈ 0.02S/cm) for x ≥ 1.0. This strong dependence of the conductivity on O excess levels was consistent with the oxide ion conductivity being mediated by oxide ion interstitials, in agreement with previous reports. The effect of cation vacancies on the conductivity has also been investigated, and the results show that the presence of La vacancies also led to an enhancement in conductivity particularly for samples with low O excess. It was suggested that this was due to the presence of cation vacancies increasing the number of O interstitials by enhancing O Frenkel defect formation.

Investigation of the Influence of Oxygen Content on the Conductivities of Ba Doped Lanthanum Germanate Apatites. E.Kendrick, P.R.Slater: Solid State Ionics, 2008, 179[21-26], 981-4