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 La 2 SiO 5 /La 2 Si 2 O 7 /La 2 SiO 5 diffusion couple at 1873 K for 100 h. The resulting polycrystal of La 9.50 (Si 5.87 □ 0.13 )O 26, 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 La 9.50 (Si 5.87 □ 0.13 )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 Si-deficient apatite was formed by the extraction of the SiO 2 component from the La 2 O 3 -excess apatite according to La 9.33+2 x Si 6 O 26+3 x – 1.5 x SiO 2 → La 9.33+2 x (Si 6–1.5 x □ 1.5 x )O 26 ( x ∼ 0.087).