Polymorphic Transitions in Cerium-Substituted Zirconolite (CaZrTi₂O₇).

Area of Science:

• Materials Science
• Solid-State Chemistry
• Ceramic Engineering

Background:

• Zirconolite is a critical material for nuclear waste immobilization.
• Actinide elements, relevant to nuclear waste, are challenging to study directly.
• Cerium (Ce) serves as a viable surrogate for actinium elements in material studies.

Purpose of the Study:

• To investigate the structural evolution of zirconolite (CaZr_1-xCe_xTi₂O₇) with varying cerium doping levels.
• To understand the influence of synthesis conditions on zirconolite phase transitions.
• To explore the potential of cerium-doped zirconolite as an actinide immobilization matrix.

Main Methods:

• Solid-state reaction synthesis of CaZr_1-xCe_xTi₂O₇ compounds.
• Spark plasma sintering (SPS) for material consolidation.
• X-ray absorption near edge spectroscopy (XANES) to determine cerium valence states.
• Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) for microstructural analysis.
• High-temperature X-ray diffraction (HT-XRD) to study phase transition kinetics.

Main Results:

• A structural transition from the 2M to the 4M zirconolite polymorph was observed with increasing cerium content.
• Both trivalent and tetravalent cerium were detected, indicating substitution on both Ca and Zr sites.
• Spark plasma sintering stabilized a perovskite phase (Ca_0.4Ce_0.4TiO₃) under reducing conditions.
• The 2M zirconolite phase showed lower cerium concentration compared to the 4M phase.
• CaCeTi₂O₇ (cubic pyrochlore) was identified as an intermediate phase during the perovskite-to-zirconolite transition.
• The transition from 2M to 4M zirconolite is controllable via oxygen partial pressure (P_O2) and heat treatment temperature.

Conclusions:

• Increasing cerium content promotes the transformation from 2M to 4M zirconolite.
• The valence state and site occupancy of cerium are crucial factors in zirconolite structural stability.
• Controlling synthesis parameters like oxygen partial pressure and temperature allows for tuning the zirconolite structure and phase.
• These findings contribute to the development of advanced ceramic materials for nuclear waste management.