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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).

The microstructure (specific surface area and pore size distribution), amount of hydrated water, and proton conductivity were examined for hydrated tin dioxide (SnO2·nH2O) and hydrated zirconia (ZrO2·nH2O), and their relationships were investigated. Both hydrates had many micropores with a pore radius below 1 nm. The average pore size of SnO2·nH2O was smaller than that of ZrO2·nH2O. The amount of hydrated water n was 0.7−1.2 for SnO2·nH2O and 1.0−1.7 for ZrO2·nH2O under a relative humidity (RH) of ∼0 to 95% at 150 °C. Proton conductivity was approximately 10-2 S cm-1 under 95% RH at 150 °C for both hydrates. The conductivity decreased with decreasing RH, but SnO2·nH2O maintained a higher conductivity than that of ZrO2·nH2O under low RH. The high conductivity of SnO2·nH2O at high RH despite low hydrated water content was assumed to be due to the high electronegativity of Sn and resulting high concentration of dissociated protons. The hydrated water in the small micropores of SnO2·nH2O will be maintained even under low humidity (due to the high adsorption energy in the small micropore), and this was considered to result in the small dependence of conductivity on RH for SnO2·nH2O.

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