Synthesis of hexagonal lanthanum germanate apatites through site selective isovalent doping with yttrium

Apatite-type lanthanum silicates/germanates have been attracting considerable interest as a new class of oxide ion conductors, whose conductivity is mediated by oxide ion interstitials. For the germanates, 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 we show that site selective doping of Y into the triclinic apatite-type oxide ion conductors, La9.33+z(GeO4)6O2+3z/2 (0.33 ≤ z ≤ 0.67) results in a hexagonal lattice for the complete series with correspondingly enhanced low-temperature conductivity.