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Synthesis and Examination of New Actinide Pyrochlores

Published online by Cambridge University Press:  21 March 2011

N.P. Laverov ,
S.V. Yudintsev ,
S.V. Stefanovsky ,
Y.N. Jang  and
R.C. Ewing
N.P. Laverov
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 109017, Russia
S.V. Yudintsev
Affiliation:
IGEM RAS, Staromonetny 35, Moscow 109017, Russia
S.V. Stefanovsky
Affiliation:
SIA Radon, 7th Rostovskii per., 2/14, Moscow 119121, Russia
Y.N. Jang
Affiliation:
KIGAM, 30 Kajung-Dong, Yusung-Ku, Taejon, Korea
R.C. Ewing
Affiliation:
University of Michigan, Ann Arbor, MI, 48109-2104, U.S.A.
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Abstract

Pyrochlore is a complex oxide with general formula VIIIA2VIB2O6X (X – additional anion in specific position, usually O2-). It has a fluorite-derived structure (space group Fd3m) where one-eighth of oxygens are missing. Two different structural sites for cations exist, resulting in the cell parameter of pyrochlore is twice that of the fluorite lattice. Some pyrochlore-based actinide waste forms have been produced in the systems: CaO-CeO2-TiO2, CaO-UO2(ThO2)-ZrO2, CaO-UO2(ThO2)-Gd2O3-TiO2-ZrO2, and CaO-ThO2-SnO2. The oxide precursors were cold pressed at 200 – 400 MPa and sintered at 1500 – 1550 °C for 6-10 hours in air or at 1300 – 1350 °C for 5-50 hours in pure oxygen (Ce-doped batches only). The use of pure oxygen was used to try to put Ce in the tetravalent state required to form a pyrochlore.

The products were examined with XRD, SEM/EDS, and TEM. In the ceramics with bulk compositions: CaCeTi2O7, CaThZr2O7, (Ca0.5GdTh0.5)(TiZr)O7, (Ca0.5GdU0.5)(TiZr)O7, and CaThSn2O7, pyrochlore was the major phase, but fluorite-structured oxide was also present. In the samples with target compositions CaUZr2O7 and (Ca0.5GdU0.5)Zr2O7 pyrochlore was not identified, rather a fluorite-structured oxide appeared. Different occurrences of pyrochlore and defect fluorite in the ceramics produced are due to crystal chemical constraints.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 713: Symposium JJ – Scientific Basis for Nuclear Waste Management XXV , 2002 , JJ11.13
Copyright
Copyright © Materials Research Society 2002

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