Study on the phase evolution of Nd³⁺-doped zirconolite ceramics （Ca_1-xNd_xZrTi₂O_7+x/2）

This paper used Nd³⁺ as a surrogate element of Am³⁺ to simulate the study of the stable curing effect of zirconolite on the radioactive element Am³⁺. The phase evolution behavior caused by Ca²⁺ sites in the lattice of Ca_1-xNdxZrTi₂O_7+x/2 with Nd³⁺ heterovalent substitution was systematically investigated. The Ca_1-xNdxZrTi₂O_7+x/2 (0≤x≤1.0) ceramic samples were prepared by high temperature solid-state synthesis and characterized by XRD, Raman, BSE, and EDS analysis. The experimental results showed that the solid solution content of Nd³⁺ ion could reach up to 70% (viz. x=0.7) in the zirconolite lattice. This indicated that the Nd³⁺ ion as a substitution element could effectively simulate Am³⁺ solidification in the zirconolite lattice. However, as the doping amount of Nd³⁺ ( x ≤ 0.3) increased, the disordering of the O^2-and Nd³⁺ ions in the zirconolite-2M lattice increased gradually. On the contrary, the (Ca/Nd)-O chemical bond was slightly shortened. Until x=0.4, the O^2- coordination number around some Zr⁴⁺ and Ti⁴⁺ ions changed (ZrO₇→ZrO₈, TiO₅→TiO₆), partly the zirconolite-2M structure rearrangement to the zirconolite-4M structure. When x=0.5, all the zirconolite-2M structure was converted into zirconolite-4M structure, and the ordering of the O^2- and Nd³⁺ cations increased again, with the Ti-O chemical bond slightly elongated. With increasing x ( x=0.6、0.7 ), the sample still retained a single zirconolite-4M structure, but the ordering of the O^2- and Nd³⁺ cations gradually decreased. When x =0.7, the Ca²⁺、Zr⁴⁺、Nd³⁺ions exhibited a subtle amount of pyrochlore structure arrangement in the sublattice. When 0.8≤ x ≤ 0.9, the samples converted to a polyphase coexistence state (Pyrochlore, ZrTiO₄ and Nd₂(TiZr)₄O₁₁). Finally, a single Nd₂(TiZr)₄O₁₁ solid solution phase was observed when x =1.0. Further, the composition and microstructure of coexisting phases were verified by BSE and EDS.