Apatite germanates doped with tungsten: Synthesis, structure, and conductivity

High oxygen content apatite germanates, La₁₀Ge _6-xW_xO_27+x, have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La₁₀Ge _5.5W_0.5O_27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO₄ tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units. Raman spectroscopy studies suggest that in the case of the WO₅ units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO₅ units, thus not contributing significantly to the conduction process.