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

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• 作者：Koichiro Fukuda ; Toru Asaka ; Shinji Hara ; Masayuki Oyabu ; Abid Berghout ; Emilie B茅chade ; Olivier Masson ; Isabelle Julien ; Philippe Thomas
• 刊名：Chemistry of Materials
• 出版年：2013
• 出版时间：May 28, 2013
• 年：2013
• 卷：25
• 期：10
• 页码：2154-2162
• 全文大小：557K
• 年卷期：v.25,no.10(May 28, 2013)
• ISSN：1520-5002

文摘

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 La_9.50(Si_5.87鈻?sub>0.13)O₂₆, where 鈻?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 (蟽) along the c-axis steadily increased from 1.6 脳 10^鈥? S/cm to 1.26 脳 10^鈥? S/cm with increasing temperature from 623 to 1073 K. The Arrhenius plot of 蟽 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鈻?sub>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 La_9.33+2xSi₆O_26+3x 鈥?1.5xSiO₂ 鈫?La_9.33+2x(Si_6鈥?.5x鈻?sub>1.5x)O₂₆ (x 0.087).

Keywords:

oxide-ion conductor; Si-deficient lanthanum silicates; apatite; reactive diffusion; crystalline orientation