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 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.