Crystal Structure and Oxide-Ion Conductivity along <i>c</i>‑Axis of Apatite-Type Lanthanum Silicate with Excess Oxide Ions

We have prepared the highly <i>c</i>-axis-oriented polycrystalline material of apatite-type La_9.50Si₆O_26.25 by isothermal heating of the sandwich-type La₂SiO₅/La₂Si₂O₇/La₂SiO₅ diffusion couple at 1873 K for 50 h. The resulting polycrystal was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the <i>c</i>-axis steadily increased from 2.0 × 10^–2 S/cm to 7.9 × 10^–2 S/cm with increasing temperature from 723 to 1073 K. The conductivity of this material was, at 723–973 K, about 2.5 times higher than that of the <i>c</i>-axis-oriented apatite polycrystal of La_9.33Si₆O₂₆. These two materials have the identical activation energy of conduction (0.35 eV), and hence the conduction mechanism must be the same. Both crystal structures of La_9.50Si₆O_26.58 and La_9.33Si₆O₂₆ at ambient temperature (space group <i>P</i>6₃/<i>m</i>) showed the appreciable positional disordering of O atoms (12<i>i</i> site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4<i>f</i> and 6<i>h</i> sites). The former structure is further characterized by the positional disordering of channel oxide ions (2<i>a</i> and 4<i>e</i> sites) as well as the presence of interstitial oxide ions (6<i>h</i> site), which would contribute to the higher conductivity along the <i>c</i>-axis