A study was made of the synthesis and conductivities of a range of novel mixed Si/Ge-based apatite systems, La8.67BaSi6−yGeyO26 and La9.33Si6−yGeyO26 (0 ≤ y ≤ 6). The results suggested that problems due to Ge volatility increased with increasing Ge content, but were partially reduced by Ba doping, which helped to stabilise the hexagonal apatite lattice. All the phases exhibited high oxide ion conductivities, although the activation energy for oxide ion conduction exhibited a general increase with increasing Ge content. The mixed systems exhibited the highest conductivities (e.g., σ800C = 0.06S/cm for La9.33Si2Ge4O26), and so were promising materials as electrolytes for SOFCs if problems due to Ge loss could be overcome.

Oxide Ion Conductivity in Mixed Si/Ge-Based Apatite-Type Systems. J.E.H.Sansom, A.Najib, P.R.Slater: Solid State Ionics, 2004, 175[1-4], 353-5