Syntheses and Oxide-Ion Conductivity of Highly c-Axis-Oriented Polycrystals of Apatite-Type Lanthanum Silicates

Abstract

We have successfully prepared the three types of highly c-axis-oriented polycrystalline materials of lanthanum silicate apatite（La_9.33Si₆O₂₆, La_9.50Si₆O_26.25 and La_9.50Si_5.87O₂₆）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 diffration 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, La_9.50Si₆O_26.25,was ca. 2.5 times higher than that of oxygen stoichiometric apatite, La_9.33Si₆O₂₆, at 723-973 K. The polycrystalline material of Si-deficient apatite, La_9.50Si_5.87O₂₆, 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.