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|Title: ||Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9.|
|Authors: ||Ling, CD|
|Issue Date: ||25-Aug-2009|
|Publisher: ||American Chemical Society|
|Citation: ||Ling, C. D., Avdeev, M., Kutteh, R., Kharton, V. V., Yaremchenko, A. A., Fialkova, S., et al. (2009). Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9. Chemistry of Materials, 21(16), 3853-3864.|
|Abstract: ||Ba4Nb2O9 is shown to have two basic polymorphs: a high-temperature γ phase, which represents an entirely new structure typed and a low-temperature (x phase, which has the rare Sr4Ru2O9 structure type. The phases are separated by a reconstructive phase transition at similar to 1370 K, the kinetics of which are sufficiently slow that the γ phase can easily be quenched to room temperature. Below similar to 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-Ba4Nb2O9.1/3H(2)O at room temperature. γ-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase γ-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to γ-I- Ba4Nb2O9. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated (x phases because of the presence in the γ 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 oil reheating a quenched sample occurring at higher temperatures in humid atmospheres. © 2009, American Chemical Society|
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