Structures, phase transitions, hydration, and ionic conductivity of Ba4Ta2O9
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Date
2010-01-26
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Publisher
American Chemical Society
Abstract
Low-temperature α-Ba4Ta2O9 is isostructural with α-Ba4Nb2O9 (Sr4Ru2O9 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-Ba4Nb2O9 represents a unique structure type, γ-Ba4Ta2O9 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-Ba4Ta2O9 is highly strained due to the extreme size mismatch between Ba2+ (1.35 Å) and Ta5+ (0.64 Å) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P63/mmc→P63/m→P21/c; the second of these transitions has not previously been observed in a 6H perovskite. Below 950 K, both α-Ba4Ta2O9 and γ-Ba4Ta2O9 hydrate to a greater extent than the corresponding phases of Ba4Nb2O9. Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration. © 2010, American Chemical Society
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Keywords
Phase transformations, Hydration, Ionic conductivity, Perovskite, Tantalates, Annealing
Citation
Ling, C. D., Avdeev, M., Kharton, V. V., Yaremchenko, A. A., Macquart, R. B., & Hoelzel, M. (2010). Structures, phase transitions, hydration, and ionic conductivity of Ba4Ta2O9. Chemistry of Materials, 22(2), 532-540. doi:10.1021/cm903170t