The structural evolution and chemical durability of the CaZr1-xSmxTi2-xNbxO7 defect fluorite-derived ceramics

Zirconolite and pyrochlore are recognized as the promising candidates for HLW immobilization because of their excellent chemical durability, radiation resistance and so on. In this study, the nominal CaZr_1-xSm_xTi_2-xNb_xO₇ (x = 0.1–1.0) compounds were synthesized to explore their microstructure, phase evolution and chemical durability. The X-ray diffraction (XRD), back-scattered scanning electron microscopy with energy dispersive X-Ray spectrometry (BSEM-EDS) and Raman analysis results displayed that the phase transformation was two-layer to four-layer monoclinic zirconolite (2M to 4M) and then to cubic pyrochlore with concentration of Sm³⁺ and Nb⁵⁺ raising. The crystalline structure evolution of 2M to 4M zirconolite and zirconolite-4M to cubic pyrochlore were determined firstly at x = 0.2 and x = 0.3 sample, respectively. Zirconolite-4M with trace perovskite was prepared at x = 0.2 sample. Near single-phase pyrochlore would be obtained when x ≥ 0.4. Moreover, the chemical durability of zirconolite-4M was stronger than zirconolite-2M and the composite of zirconolite-4M and pyrochlore. The findings could be helpful for the design of zirconolite-based waste forms for actinides such as Np³⁺, Am³⁺, and others.