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

We have prepared the highly c-axis-oriented polycrystalline material of apatite-type La9.50Si6O26.25 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 c-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 c-axis-oriented apatite polycrystal of La9.33Si6O26. 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 La9.50Si6O26.58 and La9.33Si6O26 at ambient temperature (space group P63/m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The former structure is further characterized by the positional disordering of channel oxide ions (2a and 4e sites) as well as the presence of interstitial oxide ions (6h site), which would contribute to the higher conductivity along the c-axis.