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Zirconolite transformation under reducing conditions

Published online by Cambridge University Press:  31 January 2011

B. D. Begg ,
E. R. Vance ,
B. A. Hunter  and
J. V. Hanna
B. D. Begg
Affiliation:
ANSTO, PMB 1, Menai, New South Wales, 2234, Australia
E. R. Vance
Affiliation:
ANSTO, PMB 1, Menai, New South Wales, 2234, Australia
B. A. Hunter
Affiliation:
ANSTO, PMB 1, Menai, New South Wales, 2234, Australia
J. V. Hanna
Affiliation:
CSIRO North Ryde NMR Facility, P.O. Box 52, North Ryde, New South Wales, 2234, Australia
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Abstract

The structural behavior of zirconolite (CaZrTi2O7) under reducing conditions at high temperature has been studied, mainly by scanning electron microscopy (SEM) and x-ray diffraction (XRD), but also with x-ray absorption spectroscopy, thermogravimetry, and electron paramagnetic resonance. The partial reduction of Ti4+ to Ti3+, associated with a reducing atmosphere heat treatment, led to the initial formation of perovskite (CaTiO3) as a second phase. As the concentration of Ti3+ in the zirconolite increased, so did the amount of perovskite until the zirconolite was totally transformed into a fluorite structured phase. Analysis of the reduced zirconolites showed them to be consistently deficient in Ca and enriched in Zr, in proportion to the concentration of Ti3+. To determine how electroneutrality was preserved in these reduced zirconolites, a series of zirconolites were prepared in air using In3+ and Ga3+ as models for Ti3+. These samples were then investigated by neutron and x-ray diffraction, SEM, solid state nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR). 71Ga MAS NMR studies of the Ga substituted zirconolite exhibited a narrow resonance at ˜13 ppm which was attributed to six-coordinate Ga incorporated in a trace perovskite phase. Broadline 71Ga NMR and 69/71Ga NQR were required to characterize the Ga incorporated in the zirconolite. The resultant quadrupolar parameters of CQ = 30.0 ± 0.05 MHz and η = 1.0 ± 0.03 indicate that the Ga site is in a highly distorted environment which would suggest that it is located on the five-coordinate Ti site within the zirconolite lattice. These results were complemented by Rietveld refinement of the neutron diffraction data from the In-doped zirconolite sample, which was optimal when all the In was located on the five-coordinate Ti site with the excess Zr located on the Ca site. It would therefore appear that charge compensation for the presence of Ti3+ in zirconolite is effected via the substitution of an appropriate amount of Zr on the Ca site. The Ti3+-stabilized fluorite structure was readily oxidized back to a single phase zirconolite upon heating in air.

Type
Articles
Information
Journal of Materials Research , Volume 13 , Issue 11 , November 1998 , pp. 3181 - 3190
Copyright
Copyright © Materials Research Society 1998

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References

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