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 P6 3 /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.

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 P6 3 /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 SiO 2 component from the La 2 O 3 -excess apatite according to La 9.33+2x Si 6 O 26+3x − 1.5xSiO 2 → La 9.33+2x (Si 6−1.5x □ 1.5x )O 26 (x ∼ 0.087).

We prepared the c-axis-oriented polycrystal of BaO-doped lanthanum silicate oxyapatite (LSO) by a reactive diffusion technique. The sandwich-type ternary diffusion couple, which was made up of La2SiO5/[BaO-doped La2Si2O7 + BaO-doped LSO]/La2SiO5, was heated at 1873 K for 100 h. The thin-plate polycrystalline electrolyte was mechanically extracted from the inner part of the annealed couple and characterized by optical microscopy, transmission electron microscopy, electron probe microanalysis (EPMA), X-ray diffractometry (XRD), Raman spectroscopy, and impedance spectroscopy. On the basis of the numbers of cations determined by EPMA and structural data refined by single-crystal XRD, the chemical formula of the constituent crystallites was determined to be (La9.32Ba0.28)­(Si5.87□0.13)­O26, where □ denotes a vacancy in the Si site. The combined use of BaO-doping and grain-alignment techniques effectively improved the oxide-ion conductivity, which steadily increased from 2.17 × 10–2 to 1.42 × 10–1 S cm–1 as the temperature increased from 723 to 1023 K. The activation energy of conduction was 0.48 eV.