Low charge compensator (Mg2+) causing a new REE-end 3O structure (REE=Rare Earth Element) and a different phase transformation in Nd3+ Co-doped zirconolite: Investigation by X-ray structural analysis

Zirconolite-derived structures have shown promising applications for the immobilization of high-level radioactive wastes, especially for minor actinides. This study evaluated the effect of magnesium (Mg, one of the main impurities in natural zirconolites) incorporation into zirconolite on structural evolution and neodymium (Nd, surrogates to minor actinides) solubility in the designed zirconolite matrix. X-ray diffraction results showed the phase transformation from zirconolite-2M to zirconolite-3O with increasing Mg²⁺ incorporated into the target structure. The lattice parameters of zirconolites, Ca_(0.99-2x)Nd_2xZrTi_(2-x)Mg_xO₇, also showed a linear relationship with the amount of Mg²⁺ being substituted. The Rietveld refinement results showed that Nd³⁺ preferred occupying the Ca sites, while Mg²⁺ substituted the Ti sites in 5-fold coordination (TiO₅). X-ray adsorption near edge spectroscopy further revealed that the ratio of TiO₅ and TiO₆ in the zirconolites decreased and less distortions of TiO₆ polyhedra were induced with increasing Mg²⁺ concentration in the zirconolites. Moreover, a new mineral phase (REE-end zirconolite-3O) with the chemical formula of NdZrTi_1.5Mg_0.5O₇ was reported in this study, and the single target phase was synthesized without the coexistence of perovskite. The combination of selected area electronic diffraction and Rietveld refinement revealed that the structure of NdZrTi_1.5Mg_0.5O₇ was similar with zirconolite-3O - Nd dominated Ca in the 8-fold coordinated site, and Mg occupied the Ti sites in both 4-fold and 5-fold coordination. This study demonstrates that the new crystalline structure explored from the process of magnesium incorporation into zirconolites can provide insights about the design and optimization of reliable waste forms for the immobilization of nuclear wastes.