Apatite-type oxides, La10-x(Si/Ge)6O26+z, have been attracting significant interest recently due to their high oxide ion conductivities. Most of the work so far has focused on the Si based systems, since the Ge based systems suffer from problems attributed to Ge loss. In this paper we show that doping divalent cations on the La site or B on the Ge site helps to stabilise the hexagonal apatite lattice for these Ge based systems. These doped phases show high oxide ion conductivities, although results from extended sintering studies suggest that Ge loss is still a problem. In order to limit Ge loss, we have also examined Bi doping to lower the sintering temperature and preliminary results for the novel Bi containing apatite-type phases, La6Bi2M2Ge6O26 (M=Mg, Sr, Ba) and La8-xBi2Ge5GaO26+y, are also reported.
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