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

authors	Ling, CD; Avdeev, M; Kutteh, R; Kharton, VV; Yaremchenko, AA; Fialkova, S; Sharma, N; Macquart, RB; Hoelzel, M; Gutmann, M
nationality	International
journal	CHEMISTRY OF MATERIALS
keywords	TEMPERATURE PROTON CONDUCTIVITY; BARIUM NIOBATE; CERAMICS; PEROVSKITES; ELECTROLYTE; CONDUCTORS; TRANSPORT; NUMBERS; OXIDES; CELLS
abstract	Ba4Nb2O9 is shown to have two basic polymorphs: a high-temperature gamma 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 gamma phase can easily be quenched to room temperature. Below similar to 950 K, both (alpha and gamma phases absorb significant amounts of water. In the case of the gamma phase, protons from absorbed water occupy ordered positions in the Structure, giving rise to a stoichiometric phase gamma-III-Ba4Nb2O9.1/3H(2)O at room temperature. gamma-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase gamma-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to gamma-I- Ba4Nb2O9. The hydrated gamma phases exhibit faster protonic and oxide ionic transport than the hydrated (x 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 gamma phases at low temperatures, with the gamma -> alpha transition oil reheating a quenched sample occurring at higher temperatures in humid atmospheres.
publisher	AMER CHEMICAL SOC
issn	0897-4756
year published	2009
volume	21
issue	16
beginning page	3853
ending page	3864
digital object identifier (doi)	10.1021/cm901644e
web of science category	Chemistry, Physical; Materials Science, Multidisciplinary
subject category	Chemistry; Materials Science
unique article identifier	WOS:000269043900016