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Journal Article

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore


Hu,  Zhiwei
Zhiwei Hu, Physics of Correlated Matter, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Sun, J., Zhou, J., Hu, Z., Chan, T.-S., Liu, R., Yu, H., et al. (2022). Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore. Journal of Synchrotron Radiation, 29, 37-44. doi:10.1107/S1600577521012558.

Cite as: https://hdl.handle.net/21.11116/0000-000A-0F7C-C
As potential nuclear waste host matrices, two series of uranium-doped Nd2Zr2O7 nanoparticles were successfully synthesized using an optimized molten salt method in an air atmosphere. Our combined X-ray diffraction, Raman and X-ray absorption fine-structure (XAFS) spectroscopy studies reveal that uranium ions can precisely substitute the Nd site to form an Nd2-xUxZr2O7+delta (0 <= x <= 0.2) system and the Zr site to form an Nd2Zr2-yUyO7+delta (0 <= y <= 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd (3) over barm) to defect fluorite (Fm3m) structures in both series of U-doped Nd2Zr2O7. The XAFS analysis indicates that uranium exists in the form of high-valent U6+ in all samples. To balance the extra charge for substituting Nd3+ or Zr4+ by U6+, additional oxygen is introduced accompanied by a large structural distortion; however, the Nd2Zr1.6U0.4O7+delta sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd2Zr2O7 host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd2Zr2O7 synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.