Synthesis and characterisation of lanthanum germanate-based apatite phases

La germanate apatite-based materials, La9.33 + x(GeO4)6O2 + 1.5x, are good oxide ion conductors but there are uncertainties over the solid solution limits, the defect crystal structure and conductivity variations with composition and temperature. Volatilisation of GeO2 during synthesis is increasingly important for temperatures above ∼1300 °C. The solid solution range at 1100 °C is ∼0.17 ≤ x ≤ ∼0.50 at which temperature GeO2 loss appears not to be significant. With increasing x, the symmetry changes from hexagonal to triclinic; for high x compositions, high temperature X-ray diffraction shows a gradual triclinic to hexagonal transition over the range 600–800 °C. Conductivity data of the high x solid solutions also show evidence of the phase transition region, with lower activation energy and higher conductivity in the high temperature hexagonal structure. The extent of, and structural changes within, the apatite domain in the LaO1.5–GeO2–SrO ternary system at 1100 °C have been studied and single-phase samples are obtained for La9.33 + x − 2y / 3Sry(GeO4)6O2 + 1.5x y = 1.0 with x = 0.17 and 0.34. The hexagonal to triclinic transition is clearly associated with increasing oxygen content rather than filling the La sites by addition/substitution of Sr into the structure.