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Ionic Size Limits for A Ions in Brannerite (ATi2O6) and Pyrochlore (CaATi2O7) Titanate Structures ( A = tetravalent rare earths and actinides)

Published online by Cambridge University Press:  21 March 2011

E. R. Vance ,
M. L. Carter ,
M. W. A. Stewart ,
R. A. Day ,
B. D. Begg  and
C. J. Ball
E. R. Vance
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
M. L. Carter
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
M. W. A. Stewart
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
R. A. Day
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
B. D. Begg
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
C. J. Ball
Affiliation:
Materials Division, Australian Nuclear Science and Technology Organisation Menai, NSW 2234, Australia
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Abstract

The lower limit of the size of the octahedral A4+ ion in the ATi2O6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi2O6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi2O6 showed that the average B site size could tolerate deviations of < 1%. Ce4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400°C in the A site of brannerite. Lattice parameter data on different endmember ATi2O6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi2O7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO6, whose structure is still uncertain, plus brannerite and rutile.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 713: Symposium JJ – Scientific Basis for Nuclear Waste Management XXV , 2002 , JJ2.6
Copyright
Copyright © Materials Research Society 2002

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References

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