Phase Evolution in the CaZrTi₂O₇-Dy₂Ti₂O₇ System: A Potential Host Phase for Minor Actinide Immobilization

Area of Science:

• Materials Science
• Nuclear Engineering
• Solid State Chemistry

Background:

• Zirconolite is a promising material for immobilizing minor actinides from nuclear waste.
• Understanding structural behavior under doping is key for effective wasteform design.

Purpose of the Study:

• To investigate the incorporation of dysprosium (Dy3+) into zirconolite (Ca1-xZrxDy2xTi2O7) to simulate trivalent minor actinide immobilization.
• To determine the resulting phase fields and structural transformations.

Main Methods:

• Conventional mixed oxide synthesis and a two-step sintering process at 1400 °C.
• High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) for structural analysis.
• X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) to confirm Dy valence and site occupancy.

Main Results:

• Single-phase zirconolite-2M was observed only at x = 0.10.
• Structural transformation to zirconolite-4M occurred between x = 0.20–0.30.
• Mixed zirconolite-4M and pyrochlore phases formed at x = 0.40–0.50, with single-phase pyrochlore for x ≥ 0.60.
• Dy was confirmed as Dy3+ and distributed across Ca2+ and Zr4+ sites in zirconolite, and 8-fold coordinated in pyrochlore.

Conclusions:

• The study elucidates the phase transitions in Dy-doped zirconolite, driven by ionic radius ratios.
• Findings provide critical insights into the structural stability and suitability of zirconolite-based materials for minor actinide immobilization.
• The observed behavior informs the design of advanced nuclear wasteforms.