Phase Evolution in the CaZrTi₂O₇-Dy₂Ti₂O₇ System: A Potential Host Phase for Minor Actinide Immobilization

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy³⁺ incorporation within the self-charge balancing zirconolite Ca_1-Zr_1-Dy₂Ti₂O₇ solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L₃-edge XANES region confirmed that Dy was present uniformly as Dy³⁺, remaining analogous to Am³⁺. Fitting of the EXAFS region was consistent with Dy³⁺ cations distributed across both Ca²⁺ and Zr⁴⁺ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy³⁺ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi₂O₇-REE³⁺Ti₂O₇ binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE³⁺ cations.