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

Low-temperature alpha-Ba4Ta2O9 is isostructured With alpha-Ba4Nb2O9 (Sr4Ru2O9 type), and it undergoes a reconstructive phase transition at approximately the same temperature (1400 K) to a gamma form that can easily be quenched to room temperature. However, the gamma forms of the two compounds are completely different. Whereas gamma-Ba4Nb2O9 represents a unique Structure type, gamma-Ba4TaO9 adopts a more conventional 6H-perovskite type. The alpha ->gamma transition is Virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the a form by annealing slightly below the transition temperature. quenched gamma-Ba4Nb2O highly strained due to tile extreme size mismatch between Ba2+ (1.35 angstrom) and Ta5+, (0.64 angstrom) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P6(3)/nunc -> P6(3)/m -> P2(1)/c; the second of these transitions has not previously been observed in a 6H perovskite. Below similar to 950 K. both alpha-Ba4Ta4O9 and gamma-Ba4Ta2O9 hydrate to a greater extent than the corresponding phases of Ba4Ta4O9 Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration,