Phase Evolution in the CaZrTi₂O₇-Dy₂Ti₂O₇System: A Potential Host Phase for Minor Actinide Immobilization

Zirconolite is considered to be a suitable wasteform material forthe immobilization of Pu and other minor actinide species produced throughadvanced nuclear separations. Here, we present a comprehensive investigation ofDy3+incorporation within the self-charge balancing zirconoliteCa1-xZr1-xDy2xTi2O7solid solution, with the view to simulate trivalent minoractinide immobilization. Compositions in the substitution range 0.10 <= x <= 1.00(Delta x= 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 degrees C in air for 48 h. Three distinct coexisting phasefields were identified, with single-phase zirconolite-2M identified only forx=0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurredin the range 0.20 <= x <= 0.30, while a mixed-phase assemblage of zirconolite-4Mand cubic pyrochlore was evident at Dy concentrations 0.40 <= x <= 0.50.Compositions for whichx >= 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-resolutiontransmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L3-edge XANES regionconfirmed that Dy was present uniformly as Dy3+, remaining analogous to Am3+. Fitting of the EXAFS region was consistent withDy3+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 phasefields werecontextualized within the existing literature, demonstrating that phase transitions in CaZrTi2O7-REE3+Ti2O7binary solid solutionsare fundamentally controlled by the ratio of ionic radius of REE3+cations.