A Correlation Between Relative Cation Radius and the Phase Stability of Zirconolite

Abstract

The maximum solubilities of several different cations in zirconolite (CaZrTi₂O₇) were investigated using X-ray diffraction and electron probe microanalysis. A parameter termed the relative radius ratio, defined as the ratio of the average radius of all atoms substituting for Ca and Zr to the average radius of all atoms substituting for Ti, was calculated for each chemical system, using coordination-dependent ionic radii obtained from the literature. It was found that with the possible exception of systems containing A1³⁺, regardless of the chemical system studied, and for additional chemical systems described in the literature, the relative radius ratio for zirconolite is 1.59, with a less than one percent standard deviation, at its solubility limit. Similar strong correlations were found between relative radius ratio and phase stability for crystallographically related rhombohedral and pyrochlore structures that also appear in these systems. These results suggest that the phase stability of zirconolite is generally governed by geometry and that chemical effects are of secondary importance