Zirconolite transformation under reducing conditions
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Zirconolite transformation under reducing conditions B. D. Begg, E. R. Vance, and B. A. Hunter ANSTO, PMB 1, Menai, New South Wales, 2234, Australia
J. V. Hanna CSIRO North Ryde NMR Facility, P.O. Box 52, North Ryde, New South Wales, 2234, Australia (Received 27 May 1997; accepted 26 January 1998)
The structural behavior of zirconolite (CaZrTi2 O7 ) 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 Ti41 to Ti31 , associated with a reducing atmosphere heat treatment, led to the initial formation of perovskite (CaTiO3 ) as a second phase. As the concentration of Ti31 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 Ti31 . To determine how electroneutrality was preserved in these reduced zirconolites, a series of zirconolites were prepared in air using In31 and Ga31 as models for Ti31 . These samples were then investigated by neutron and x-ray diffraction, SEM, solid state nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR). 71 Ga 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 71 Ga NMR and 69/71 Ga NQR were required to characterize the Ga incorporated in the zirconolite. The resultant quadrupolar parameters of CQ 30.0 6 0.05 MHz and h 1.0 6 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 Ti31 in zirconolite is effected via the substitution of an appropriate amount of Zr on the Ca site. The Ti31 -stabilized fluorite structure was readily oxidized back to a single phase zirconolite upon heating in air.
I. INTRODUCTION
Zirconolite (CaZrTi2 O7 ) is the primary actinidebearing phase in Synroc, a multiphase mineral-analogue based ceramic designed to immobilize high-level nuclear waste from the reprocessing of spent nuclear fuel.1 Zirconolite is derived from an anion-deficient fluorite structure and has the generic form CaZrx Ti32x O7 with 0.8 , x , 1.37.2 It consists of layers of cornerlinked TiO6 octahedra separated by planes of Ca and Zr atoms. Titanium occupies three distinct lattice sites in zirconolite, two of which are octahedrally coordinat
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