Structures, Phase Transitions, Hydration, and Ionic Conductivity of Ba4Nb2O9

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• 作者：Chris D Ling ; Maxim Avdeev ; Ramzi Kutteh ; Vladislav V Kharton ; Aleksey A Yaremchenko ; Svitlana Fialkova ; Neeraj Sharma ; Ren B Macquart ; Markus Hoelzel ; Matthias Gutmann
• 刊名：Chemistry of Materials
• 出版年：2009
• 出版时间：August 25, 2009
• 年：2009
• 卷：21
• 期：16
• 页码：3853-3864
• 全文大小：1145K
• 年卷期：v.21,no.16(August 25, 2009)
• ISSN：1520-5002

文摘

Ba₄Nb₂O₉ is shown to have two basic polymorphs: a high-temperature γ phase, which represents an entirely new structure type; and a low-temperature α phase, which has the rare Sr₄Ru₂O₉ structure type. The phases are separated by a reconstructive phase transition at 1370 K, the kinetics of which are sufficiently slow that the γ phase can easily be quenched to room temperature. Below 950 K, both α and γ phases absorb significant amounts of water. In the case of the γ phase, protons from absorbed water occupy ordered positions in the structure, giving rise to a stoichiometric phase γ-III-Ba₄Nb₂O₉·1/3H₂O at room temperature. γ-III-Ba₄Nb₂O₉·1/3H₂O partially dehydrates at 760 K to give another stoichiometric phase γ-II-Ba₄Nb₂O₉·1/6H₂O, which completely dehydrates at 950 K to γ-I-Ba₄Nb₂O₉. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated α phases because of the presence in the gamma phases of 2D layers containing Nb⁵⁺ cations with unusually low oxygen coordination numbers (4 or 5) separated by discrete OH groups. Hydration appears to play an important role in stabilizing the γ phases at low temperatures, with the γ → α transition on reheating a quenched sample occurring at higher temperatures in humid atmospheres.