Influence of Nd<sup>3+</sup> Doping Content on the Phase Transition Behavior of Ca<sub>1-</sub><sub>x</sub>Zr<sub>1-</sub><sub>x</sub>Nd<sub>2</sub><sub>x</sub>Ti<

To investigate the stable immobilization of the radioactive element Am³⁺ in perovskite zirconolite, Nd³⁺ was employed as a chemical analogue. A systematic study was conducted on the phase evolution behavior during the synthesis of Ca_1-<italic>_x</italic>Zr_1-<italic>_x</italic>Nd₂<italic>_x</italic>Ti₂O₇(0.0≤<italic>x</italic>≤1.0) via the high-temperature solid-state method, focusing on the equivalent substitution of Nd³⁺ at both Ca²⁺ and Zr⁴⁺ lattice sites. The Ca_1-<italic>_x</italic>Zr_1-<italic>_x</italic>Nd₂<italic>_x</italic>Ti₂O₇(0.0≤<italic>x</italic>≤1.0) ceramic samples were prepared by solid state synthesis and characterized by XRD, Raman, BSE and EDS analyses. When the solid-solution capacity of Nd³⁺ ions in the zirconolite lattice reaches 10% (Atomic fraction, <italic>x</italic>=0.1), the primary crystalline phase in the ceramic matrix is zirconolite-2M. However, a minor amount of calcium titanate crystalline phase is present and persists over the composition range of 0.1≤≤<italic>x</italic>≤0.8. When <italic>x</italic>=0.2 and 0.3, the degree of disorder involving O^2- and Nd³⁺ in the ceramic lattice increases significantly, resulting in changes to the coordination numbers of O^2- around Zr⁴⁺ and Ti⁴⁺ ions in certain regions of the zirconolite-2M structure(ZrO₇→ZrO₈ and TiO₅→TiO₆), indicating a partial transformation toward the zirconolite-4M lattice. When x reaches 0. 4, zirconolite-2M undergoes complete transformation into the zirconolite-4M structure, accompanied by a decrease in the (Ca/ Nd)—O bond length and an increase in the Ti—O bond length. When the Nd³⁺ doping level reaches 50%(<italic>x</italic>=0.5), a partial lattice transformation from zirconolite-4M to the pyrochlore structure occurs in the ceramic matrix. At <italic>x</italic>=0.6, zirconolite-4M undergoes complete transformation into the pyrochlore structure, accompanied by an extremely high degree of disorder in O^2- and Nd³⁺ ions. As the doping level increases further (<italic>x</italic>=0.7, 0.8), the solubility of Nd³⁺ in the pyrochlore structure reaches saturation, and the Nd₂Ti₂O₇ solid solution begins to precipitate in the ceramic matrix. By <italic>x</italic>=0.9, the ceramic matrix transforms into a single-phase Nd₂Ti₂O₇ solid solution. When Nd³⁺ completely substitutes for the Ca²⁺ and Zr⁴⁺ sites(<italic>x</italic>=1.0), the ceramic matrix exists in a two-phase coexisting state, namely Nd₂Ti₂O₇ and TiO₂. BSE and EDS analyses of the ceramic grains confirm this phase transformation behavior.