Zirconolite transformation under reducing conditions

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. Broadline⁷¹Ga NMR and^69/71Ga NQR were required to characterize the Ga incorporated in the zirconolite. The resultant quadrupolar parameters ofC_Q= 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.