Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9

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

Keywords

Hydration, Ionic conductivity, Phase transformations, Niobates, Barium, Molecular structure

Citation

Ling, C. D., Avdeev, M., Kutteh, R., Kharton, V. V., Yaremchenko, A. A., Fialkova, S., Sharma, N., Macquart, R. B., Hoelzel, M., & Gutmann, M. J.(2009). Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9. Chemistry of Materials, 21(16), 3853-3864. doi:10.1021/cm901644e