Crystal Structure and Oxide-Ion Conductivity along c‑Axis of Si-Deficient Apatite-Type Lanthanum Silicate

We have prepared the highly c-axis-oriented polycrystalline material of Si-deficient apatite-type lanthanum silicate by isothermal heating of the sandwich-type La2SiO5/La2Si2O7/La2SiO5 diffusion couple at 1873 K for 100 h. The resulting polycrystal of La9.50(Si5.87□0.13)­O26, where □ denotes a vacancy in Si site, was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the textured thin-plate electrolyte was obtained. The ionic conductivity (σ) along the c-axis steadily increased from 1.6 × 10–2 S/cm to 1.26 × 10–1 S/cm with increasing temperature from 623 to 1073 K. The Arrhenius plot of σ showed the marked slope change at ca. 800 K; the activation energies of conduction were, above and below 800 K, 0.53 and 0.17 eV, respectively. The crystal structure of La9.50(Si5.87□0.13)­O26 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 Si-deficient apatite was formed by the extraction of the SiO2 component from the La2O3-excess apatite according to La9.33+2xSi6O26+3x – 1.5xSiO2 → La9.33+2x(Si6–1.5x□1.5x)­O26 (x ∼ 0.087).