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 La 9.50 Si 6 O 26.25 by isothermal heating of the sandwich-type La 2 SiO 5 /La 2 Si 2 O 7 /La 2 SiO 5 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 La 9.33 Si 6 O 26 . 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 9.50 Si 6 O 26.58 and La 9.33 Si 6 O 26 at ambient temperature (space group P 6 3 / m ) showed the appreciable positional disordering of O atoms (12 i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4 f and 6 h sites). The former structure is further characterized by the positional disordering of channel oxide ions (2 a and 4 e sites) as well as the presence of interstitial oxide ions (6 h site), which would contribute to the higher conductivity along the c -axis.