pubs.acs.org/cm Published on Web 12/18/2009 r 2009 American Chemical Society 532 Chem. Mater. 2010, 22, 532–540 DOI:10.1021/cm903170t Structures, Phase Transitions, Hydration, and Ionic Conductivity of Ba 4 Ta 2 O 9 Chris D. Ling,* ,†,‡ Maxim Avdeev, ‡ Vladislav V. Kharton, § Aleksey A. Yaremchenko, § Ren e B. Macquart, †,‡ and Markus Hoelzel ^ † School of Chemistry, The University of Sydney, Sydney 2006, Australia, ‡ Bragg Institute, ANSTO, PMB 1, Menai 2234, Australia, § Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal, and ^ Darmstadt University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany Received October 15, 2009. Revised Manuscript Received December 2, 2009 Low-temperature R-Ba 4 Ta 2 O 9 is isostructural with R-Ba 4 Nb 2 O 9 (Sr 4 Ru 2 O 9 type), and it under- goes 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 Rfγ transition is virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the R 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 2þ (1.35 A ˚ ) and Ta 5þ (0.64 A ˚ ) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P6 3 /mmcfP6 3 /mfP2 1 /c; the second of these transitions has not previously been observed in a 6H perovskite. Below ∼950 K, both R-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. Introduction Materials that exhibit significant mobility of several different types of charge carriers (e.g., oxide ions, protons, and electrons) have diverse potential applications as fuel-cell membranes, electrodes, batteries, and sensors. New mixed conductors are of perennial interest, especially when they are stable over wide ranges of temperature and redox conditions. Several niobium and tantalum oxides have recently been shown to exhibit promising mixed conducti- vity associated with a surprisingly high uptake of protons at moderate temperatures (below ∼600 °C). These include La(Nb,Ta)O 4 , 1-3 Ba 3 Ca 1þx (Nb,Ta) 2-x O 9-3x/2 , 4-6 HLa- (Nb,Ta) 2 O 7 3 0.5H 2 O, 7 and La 3 (Nb,Ta)O 7 . 8 The niobates and tantalates are isostructural, in most cases (the exception being LaNbO 4 versus LaTaO 4 ), with the niobates generally being the better conductors, because of the fact that Nb is more easily reduced from its standard 5þ oxidation state than is Ta. We recently added Ba 4 Nb 2 O 9 9-13 to the list of proton- conducting niobates. 14 This compound has two basic polymorphs: a high-temperature γ phase, with a unique structure type; and a low-temperature R phase, with the Sr 4 Ru 2 O 9 structure type. The phases are separated by a reconstructive transition at ∼1370 K, the kinetics of which are sufficiently slow, such that the γ phase can easily be quenched to room temperature. Below ∼950 K, both R 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 series of stoichiometric phases: γ-III (Ba 4 Nb 2 O 9 3 1 / 3 H 2 O) at room temperature, γ-II (Ba 4 Nb 2 O 9 3 1 / 6 H 2 O) above ∼760 K, and γ-I (Ba 4 Nb 2 O 9 ) above ∼950 K. 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