Effect of Mg substitution on crystal structure and oxide-ion conductivity of apatite-type lanthanum silicates

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• 作者：Tomohiro Kinoshita ; Tomoyuki Iwata ; Emilie Bé ; chade ; Olivier Masson ; Isabelle Julien ; Eric Champion ; Philippe Thomas ; Hideto Yoshida ; Nobuo Ishizawa ; Koichiro Fukuda
• 关键词：Lanthanum silicate ; Oxide-ion conductor ; Apatite-type structure ; Single-crystal X-ray diffraction
• 刊名：Solid State Ionics
• 出版年：2010
• 出版时间：26 July 2010
• 年：2010
• 卷：181
• 期：21-22
• 页码：1024-1032
• 全文大小：1522 K

文摘

Two types of apatite-type lanthanum silicates, La_9.93(Si_5.11 Mg_0.89)O₂₆ (space group P6₃/m) and (La_8.48 Mg_1.28)Si₆O₂₆ (space group P6₃), were successfully synthesized. They were, together with La_9.33Si₆O₂₆ (space group P6₃/m), examined by optical microscopy, impedance spectroscopy and X-ray powder diffraction. The crystal structures of the former two compounds were refined by single-crystal X-ray diffraction. The oxide-ion conductivity at 773 K was the highest for La_9.93(Si_5.11 Mg_0.89)O₂₆, followed by La_9.33Si₆O₂₆ and (La_8.48 Mg_1.28)Si₆O₂₆ in this order. The systematic change in conductivity was well accounted for by the atom arrangements of hexagonal channels, consisting of alternately stacking equilateral triangles of La and O along the c-axis. With La_9.93(Si_5.11 Mg_0.89)O₂₆, the La triangle appreciably expanded as compared with that of La_9.33Si₆O₂₆, while the size of O triangle was kept almost constant. When the La sites were partially occupied by Mg atoms, the space group changed from P6₃/m to P6₃, leading to the transformation of equivalent O triangles into the larger and smaller ones. On the assumption that conduction takes place inside the hexagonal channels through the migration of channel oxide-ions, they necessarily pass through the channel triangles. The smaller O triangle would act as a bottleneck hindering the migration of channel oxide ions. The expansion of La triangle would effectively enhance the conductivity.