Synthesis of Ga-doped Ge-based apatites: Effect of dopant and cell symmetry on oxide ion conductivity

Apatite-type rare earth silicates/germanates are attracting significant interest as a new class of oxide ion conductors. For the germanates, La9.33+x(GeO4)6O2+3x/2, it has been shown that, depending on composition, the cell can be either hexagonal or triclinic, with evidence for reduced low temperature conductivities for the latter, attributed to increased defect trapping in this lower symmetry cell. In this paper the effect of Ga doping on the cell symmetry and conductivity is reported, with the observation of triclinic cells across the series La10(GeO4)6−x(GaO4)xO3−x/2. Through co-doping with Y on the La site, i.e. La8Y2(GeO4)6−x(GaO4)xO3−x/2, it is shown that hexagonal cells are obtained, and the dependence of the conductivity on oxygen content/Ga content is discussed. In particular it is shown that the stabilisation of the hexagonal cell through Y doping enhances the low temperature conductivity.