Phase Evolution in the CaZrTiO-DyTiO 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 Dy incorporation within the self-charge balancing zirconolite CaZrDyTiO solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ ≤ 1.00 (Δ = 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 = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ ≤ 0.50. Compositions for which ≥ 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 L-edge XANES region confirmed that Dy was present uniformly as Dy, remaining analogous to Am. Fitting of the EXAFS region was consistent with Dy cations distributed across both Ca and Zr sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTiO-REETiO binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE cations.