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Title: Incorporation of uranium in pyrochlore oxides and pressure-induced phase transitions

Uranium-doped gadolinium zirconates with pyrochlore structure were studied at ambient and high-pressure conditions up to 40 GPa. The bonding environment of uranium in the structure was determined by x-ray photoelectron and Raman spectroscopies and x-ray diffraction. The uranium valence for samples prepared in air is mainly U{sup 6+}, but U{sup 4+} is present in pyrochlores fabricated in an argon atmosphere. Rietveld refinement of the XRD pattern suggests that uranium ions in pyrochlores are on the 16d site in 6-fold coordination with oxygen. At pressures greater than 22 GPa, the pyrochlore structure transformed to a cotunnite-type phase. The cotunnite high-pressure phase transformed to a defect fluorite structure on the release of pressure. - Graphical abstract: In U-bearing pyrochlore, U ions mainly occupy the 16d site and replace the smaller Zr{sup 4+}, part of the oxygen will occupy the 8b site, which is empty to most pyrochlores. At pressure of 22 GPa, the pyrochlore lattice is not stable and transforms to a cotunnite-type structure. The high-pressure structure is not stable and transform to a fluorite or back to the pyrochlore structure when pressure is released. - Highlights: • We found that U ions mainly occupy the smaller cation site in U-bearing pyrochlore.more » • Pyrochlore structure is not stable at pressure of more than 20 GPa. • The quenched sample has a pyrochlore or a disordered fluorite structure.« less
Authors:
 [1] ; ; ;  [1] ; ;  [2]
  1. Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109 (United States)
  2. Institute of Materials Engineering, ANSTO, Locked Bag 2001, Kirrawee DC 2232, NSW (Australia)
Publication Date:
OSTI Identifier:
22443440
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 219; Other Information: Copyright (c) 2014 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CATIONS; DOPED MATERIALS; FLUORITE; GADOLINIUM; OXIDES; OXYGEN; PHASE TRANSFORMATIONS; PRESSURE RANGE MEGA PA 10-100; PYROCHLORE; RAMAN SPECTROSCOPY; URANIUM; URANIUM IONS; X RADIATION; X-RAY DIFFRACTION; ZIRCONATES; ZIRCONIUM IONS