Nanoscale Phase Evolution, Substitution Mechanism, and Aqueous Durability of CaZr1−xGdxTi2−xNbxO7 (x = 0.1–1.0) Defect-Fluorite-Derived Ceramics

The safe immobilization of high-level waste (as actinide) remains a critical bottleneck in the disposal of high-level radioactive waste worldwide. Moreover, the higher specific surface area and surface energy of nano-scale powders enable the production of ceramic materials featuring denser crystal structures and superior strength, hardness, and toughness. Therefore, in this study, Gd3+ was used as a surrogate for actinides, and Nb5+ was introduced as a high-valence charge-compensating cation. Nano-scale powders of CaCO3, ZrO2, Gd2O3, TiO2, and Nb2O5 were employed to prepare a series of defect-fluorite-derived ceramics, CaZr1-xGdxTi2-xNbxO7 (x = 0.1–1.0), via a high-temperature solid-state reaction method, aiming to investigate the atomic substitution mechanisms, phase evolution, and chemical stability under high-valence charge compensation. Laboratory X-ray diffraction (XRD), synchrotron X-ray diffraction (SXRD), and backscattered scanning electron microscopy with energy-dispersive X-ray spectroscopy (BSEM-EDX) confirmed a phase evolution sequence from zirconolite-2M to zirconolite-4M and finally to pyrochlore. This behavior is consistent with that reported for other Ln3+-Nb5+ co-doped zirconolite systems. Rietveld refinement of the SXRD data further revealed, for the first time, the site-occupancy mechanism of Gd and Nb in zirconolite-4M. In both zirconolite-2M and zirconolite-4M, Gd preferentially occupies the Ca sites, whereas Nb substitutes at the Ti sites. In the pyrochlore structure, Ca, Zr, and Gd occupy the 16d sites, while Ti and Nb occupy the 16c sites. Static leaching tests following the MCC-1 protocol showed that pyrochlore exhibits the highest leaching resistance, whereas zirconolite-2M shows the lowest. After 28 days, the highest Gd leaching rate was 1.92(1) × 10−5 g m−2 d−1. These results provide new insights into actinide immobilization behavior and compositional design in zirconolite-based waste forms.