CaO-TiO2-ZrO2 system at 1200°C and the solubilities of Hf and Gd in zirconolite

Phase equilibria are established in the CaO-TiO₂-ZrO₂ system at 1200°C, using X-ray diffraction and electron probe microanalysis. The existence of two previously reported ternary phases, zirconolite (CaZrTi₂O₇) and calzirtite (Ca₂Zr₅Ti₂O₁₆), is confirmed. Each of these phases exhibits a significant range of homogeneity between TiO₂ and ZrO₂ while maintaining a nearly constant concentration of CaO. The ternary solubilities of the constituent binary phases are found to be negligible, with the exceptions of the perovskites, which display mutual solubility of at least 22 mol.% and may in fact form a series of continuous solid solutions. The solubilities of Hf and Gd in zirconolite are also investigated. While Hf-bearing samples did not reach thermodynamic equilibrium under the experimental conditions employed, the existence of a Hf analog to zirconolite, CaHfTi₂O₇, is conclusively demonstrated. The phase is stable at the stoichiometric composition, and its lattice parameters are very close to those reported in the literature for stoichiometric zirconolite. A Gd-bearing sample of the composition Ca_0.88Zr_0.88Gd_0.24Ti₂O₇ is found to be essentially single phase zirconolite, in agreement with previous investigations at higher temperatures.