Crystal Structure and Oxide-Ion Conductivity along c-Axis of Apatite-Type Lanthanum Silicate with Excess Oxide Ions

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• 作者：Koichiro Fukuda ; Toru Asaka ; Masayuki Oyabu ; Daisuke Urushihara ; Abid Berghout ; Emilie B茅chade ; Olivier Masson ; Isabelle Julien ; Philippe Thomas
• 刊名：Chemistry of Materials
• 出版年：2012
• 出版时间：December 11, 2012
• 年：2012
• 卷：24
• 期：23
• 页码：4623-4631
• 全文大小：483K
• 年卷期：v.24,no.23(December 11, 2012)
• ISSN：1520-5002

文摘

We have prepared the highly c-axis-oriented polycrystalline material of apatite-type La_9.50Si₆O_26.25 by isothermal heating of the sandwich-type La₂SiO₅/La₂Si₂O₇/La₂SiO₅ 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 脳 10^鈥? S/cm to 7.9 脳 10^鈥? S/cm with increasing temperature from 723 to 1073 K. The conductivity of this material was, at 723鈥?73 K, about 2.5 times higher than that of the c-axis-oriented apatite polycrystal of La_9.33Si₆O₂₆. 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_9.50Si₆O_26.58 and La_9.33Si₆O₂₆ 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 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.

Keywords:

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