Structures, Phase Transitions, Hydration, and Ionic Conductivity of Ba₄Ta₂O₉

Low-temperature α-Ba 4 Ta 2 O 9 is isostructural with α-Ba 4 Nb 2 O 9 (Sr 4 Ru 2 O 9 type), and it undergoes a reconstructive phase transition at approximately the same temperature (1400 K) to a γ form that can easily be quenched to room temperature. However, the γ forms of the two compounds are completely different. Whereas γ-Ba 4 Nb 2 O 9 represents a unique structure type, γ-Ba 4 Ta 2 O 9 adopts a more conventional 6H-perovskite type. The α→γ transition is virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the α form by annealing slightly below the transition temperature. Quenched γ-Ba 4 Ta 2 O 9 is highly strained due to the extreme size mismatch between Ba 2+ (1.35 Å) and Ta 5+ (0.64 Å) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P 6 3 / mmc → P 6 3 / m → P 2 1 / c; the second of these transitions has not previously been observed in a 6H perovskite. Below ∼950 K, both α-Ba 4 Ta 2 O 9 and γ-Ba 4 Ta 2 O 9 hydrate to a greater extent than the corresponding phases of Ba 4 Nb 2 O 9 . Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration.