Phase evolution and polytype analysis in Ce‒Fe‒Cr doped zirconolite glass‐ceramics for nuclear waste immobilization

Abstract Zirconolite‐based glass‐ceramics offered promising potential for immobilizing actinides due to their chemical durability and structural adaptability. In this study, Ca 1‒ x Ce x ZrTi 2‒2 x Fe x Cr x O 7 ‒Na 2 Al 2 Si 6 O 16 ( x = 0‒0.2) glass‐ceramics were synthesized at 1250°C under air atmosphere to investigate the effects of dopant concentration on zirconolite polytypism. Phase identification confirmed the direct formation of the predominant zirconolite phase in all samples, with the observed transformation from the monoclinic 2M to the trigonal 3T polytype as Ce‒Fe‒Cr content increased to x = 0.2. Detailed selected area electron diffraction and high‐resolution transmission electron microscopy analysis validated the presence of zirconolite‐3T polytype through characteristic d ‐spacings and interplanar angles, alongside the presence of stacking faults. Microstructural observations revealed homogeneous distribution of zirconolite grains within dense glass matrix, and energy dispersive X‐ray spectrometry and X‐ray photoelectron spectroscopy proved successful dopant incorporation and the near‐equimolar Ce 3+ /Ce 4+ ratio. These results indicated that the glass matrix enhanced densification at moderate temperature and facilitated polytype transformation of zirconolite, where the 2M‐to‐3T transition occurred at lower doping‐level and temperature than in ceramic‐only systems, indicative of the catalytic influence of the glass phase on zirconolite polytypism.