ZIRCONOLITE TRANSFORMATION UNDER REDUCING CONDITIONS

The structural behavior of zirconolite (CaZrTi2O7) under reducing cond itions at high temperature has been studied, mainly by scanning electr on microscopy (SEM) and x-ray diffraction (XRD), but also with x-ray a bsorption spectroscopy, thermogravimetry, and electron paramagnetic re sonance. The partial reduction of Ti4+ to Ti3+, associated with a redu cing atmosphere heat treatment, led to the initial formation of perovs kite (CaTiO3) as a second phase. As the concentration of Ti3+ in the z irconolite increased, so did the amount of perovskite until the zircon olite was totally transformed into a fluorite structured phase. Analys is of the reduced zirconolites showed them to be consistently deficien t in Ca and enriched in Zr, in proportion to the concentration of Ti3. To determine how electroneutrality was preserved in these reduced zi rconolites, a series of zirconolites were prepared in air using In3+ a nd Ga3+ as models for Ti3+. These samples were then investigated by ne utron and x-ray diffraction, SEM, solid state nuclear magnetic resonan ce (NMR), and nuclear quadrupole resonance (NQR). Ga-71 MAS NMR studie s of the Ga substituted zirconolite exhibited a narrow resonance at si milar to 13 ppm which was attributed to six-coordinate Ga incorporated in a trace perovskite phase. Broadline 71Ga NMR and Ga-69/71 NQR were required to characterize the Ga incorporated in the zirconolite, The resultant quadrupolar parameters of C-Q = 30.0 +/- 0.05 MHz and eta = 1.0 +/- 0.03 indicate that the Ga site is in a highly distorted enviro nment 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-dop ed zirconolite sample, which was optimal when all the In was located o n the five-coordinate Ti site with the excess Zr located on the Ca sit e. It would therefore appear that charge compensation for the presence of Ti3+ in zirconolite is effected via the substitution of an appropr iate amount of Zr on the Ca site. The Ti3+-stabilized fluorite structu re was readily oxidized back to a single phase zirconolite upon heatin g in air.