Structures, Phase Transitions, Hydration, and Ionic Conductivity of Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub> LingChris D AvdeevMaxim KuttehRamzi KhartonVladislav V YaremchenkoAleksey A FialkovaSvitlana SharmaNeeraj MacquartRené B HoelzelMarkus GutmannMatthias 2009 Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub> 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 Sr<sub>4</sub>Ru<sub>2</sub>O<sub>9</sub> 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-Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub>·1/3H<sub>2</sub>O at room temperature. γ-III-Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub>·1/3H<sub>2</sub>O partially dehydrates at ∼760 K to give another stoichiometric phase γ-II-Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub>·1/6H<sub>2</sub>O, which completely dehydrates at ∼950 K to γ-I-Ba<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub>. 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 Nb<sup>5+</sup> 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.