Syntheses and oxide-ion conductivity of highly c-axis-oriented polycrystals of apatite-type lanthanum silicates

We have successfully prepared the three types of highly c-axis-oriented polycrystalline materials of lanthanum silicate apatite (La9.33Si6O26, La9.50Si6O26.25 and La9.50Si5.87O26) by isothermal heating of the diffusion couples consisting of La2SiO5 and La2Si2O7 at 1873 K for 50-100 h. The resulting polycrystals were subsequently characterized using polarizing microscopy, X-ray diffraction and impedance spectroscopy. The annealed couples were mechanically processed, and the thin-plate electrolytes consisting of the grain-aligned polycrystals were obtained. The oxide-ion conductivity along the c-axis of oxygen hyper-stoichiometric apatite, La9.50Si6O26.25, was ca. 2.5 times higher than that of oxygen stoichiometric apatite, La9.33Si6O26, at 723-973 K. The polycrystalline material of Si-deficient apatite, La9.33Si6O26, showed, among the three types of apatite polycrystals, the highest conductivity above 850 K; the conductivity at 873 K was 4.2×10-2 S/cm. (author)