Phase diagram, chemical stability and physical properties of the solid-solution Ba{sub 4}Nb{sub 2-x}Ta{sub x}O{sub 9}

Journal Article · Sat Oct 15 00:00:00 EDT 2011 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2011.07.036· OSTI ID:21580017

Southon, Peter D; Kepert, Cameron J ^[1]; Hester, James ^[2]; Kimpton, Justin A ^[3]; Ling, Chris D ^[1]

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1. School of Chemistry, University of Sydney, Sydney, NSW 2006 (Australia)
2. Bragg Institute, ANSTO, PMB 1, Menai 2234 (Australia)
3. Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168 (Australia)

Through the construction of the Ba{sub 4}Nb{sub 2-x}Ta{sub x}O{sub 9} phase diagram, it was discovered that the unique high-temperature {gamma} phase is a thermodynamic intermediate between the low-temperature {alpha} phase (Sr{sub 4}Ru{sub 2}O{sub 9}-type) and a 6H-perovskite. Refined site occupancies for the {gamma} phase across the Ba{sub 4}Nb{sub 2-x}Ta{sub x}O{sub 9} solid-solution indicate that Nb preferentially occupies the tetrahedral sites over the octahedral sites in the structure. When annealed in a CO{sub 2}-rich atmosphere, all of the phases studied absorb large amounts of CO{sub 2} at high temperatures between {approx}700 and 1300 K. In situ controlled-atmosphere diffraction studies show that this behaviour is linked to the formation of BaCO{sub 3} on the surface of the material, accompanied by a Ba{sub 5}(Nb,Ta){sub 4}O{sub 15} impurity phase. In situ diffraction in humid atmospheres also confirms that these materials hydrate below {approx}1273K, and that this plays a critical role in the various reconstructive phase transitions as well as giving rise to proton conduction. - Graphical abstract: Thermodynamic phase diagram of Ba{sub 4}Nb{sub 2-x}Ta{sub x}O{sub 9}. Highlights: > {gamma}-Ba{sub 4}Nb{sub 2}O{sub 9} phase is a structural intermediate between the {alpha} and 6H-perovskite phases. > Ba{sub 4}Nb{sub 2}O{sub 9} and Ba{sub 4}Ta{sub 2}O{sub 9} decompose at high temperatures in the presence of CO{sub 2}. > These materials all absorb between 5% and 6% of CO{sub 2} by mass between {approx}800 and 1200 K.

• OSTI ID: 21580017
• Journal Information: Journal of Solid State Chemistry, Vol. 184, Issue 10; Other Information: DOI: 10.1016/j.jssc.2011.07.036; PII: S0022-4596(11)00415-4; Copyright (c) 2011 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
BARIUM CARBONATES
BARIUM OXIDES
CARBON DIOXIDE
CONTROLLED ATMOSPHERES
DECOMPOSITION
DIFFRACTION
IMPURITIES
MATERIALS
PEROVSKITE
PHASE DIAGRAMS
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
SOLID SOLUTIONS
TEMPERATURE RANGE 0065-0273 K
TEMPERATURE RANGE 0400-1000 K
ALKALINE EARTH METAL COMPOUNDS
ATMOSPHERES
BARIUM COMPOUNDS
CARBON COMPOUNDS
CARBON OXIDES
CARBONATES
CHALCOGENIDES
CHEMICAL REACTIONS
COHERENT SCATTERING
DIAGRAMS
DISPERSIONS
HOMOGENEOUS MIXTURES
INFORMATION
MINERALS
MIXTURES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PEROVSKITES
SCATTERING
SOLUTIONS
TEMPERATURE RANGE