The structures, phase transitions and dynamics behind mixed ionic and electronic conduction in hydrated Ba4Nb2O9
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Date
2010-02-03
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Australian Institute of Physics
Abstract
Although Ba4Nb2O9 was first synthesised in 1965, [1] its structure remained unsolved until a recent study [2] in which we showed that Ba4Nb2O9 has 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 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 absorb significant amounts of water. In the γ phase, protons occupy ordered positions, 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 γ-II-Ba4Nb2O9.1/6H2O, then completely dehydrates at ~950 K to γ-I-Ba4Nb2O9. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated " phases, due to 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. In this paper, we will discuss the structures and mechanisms of hydration – and,
therefore, of ionic conduction – in the various phases of Ba4Nb2O9 on the basis of neutron diffraction experiments and ab initio (density functional theory) dynamics simulations.
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Keywords
Phase transformations, Hydration, Ionic conductivity, Perovskite, Tantalates, Oxygen, Neutron diffraction
Citation
Ling, C. D., Avdeev, M., & Johnson, M. R. (2010). The structures, phase transitions and dynamics behind mixed ionic and electronic conduction in hydrated Ba4Nb2O9. Paper presented to the 34th Annual Condensed Matter and Materials Meeting 2010, Waiheke Island Resort, Waiheke, Auckland, New Zealand 2 - 5 February 2010. Retrieved from: https://physics.org.au/wp-content/uploads/cmm/2010/