Influence of Nd³⁺ Doping Content on the Phase Transition Behavior of Ca_1-<italic>_x</italic>Zr_1-<italic>_x</italic>Nd₂<italic>_x</italic>Ti₂O₇ Ceramics

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 Ca1-xZr1-xNd2xTi2O7(0.0≤x≤1.0) via the high-temperature solid-state method, focusing on the equivalent substitution of Nd3+ at both Ca2+ and Zr4+ lattice sites. The Ca1-xZr1-xNd2xTi2O7(0.0≤x≤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, x=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≤≤x≤0.8. When x=0.2 and 0.3, the degree of disorder involving O2- and Nd3+ in the ceramic lattice increases significantly, resulting in changes to the coordination numbers of O2- 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%(x=0.5), a partial lattice transformation from zirconolite-4M to the pyrochlore structure occurs in the ceramic matrix. At x=0.6, zirconolite-4M undergoes complete transformation into the pyrochlore structure, accompanied by an extremely high degree of disorder in O2- and Nd3+ ions. As the doping level increases further (x=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 x=0.9, the ceramic matrix transforms into a single-phase Nd2Ti2O7 solid solution. When Nd3+ completely substitutes for the Ca2+ and Zr4+ sites(x=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.