Controllable sites and high-capacity immobilization of uranium in Nd(2)Zr(2)O(7) pyrochlore

As potential nuclear waste host matrices, two series of uranium-doped Nd(2)Zr(2)O(7) 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 Nd(2–x )U( x )Zr(2)O(7+δ) (0 ≤ x ≤ 0.2) system and the Zr site to form an Nd(2)Zr(2–y )U( y )O(7+δ) (0 ≤ y ≤ 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd 3 m) to defect fluorite (Fm 3 m) structures in both series of U-doped Nd(2)Zr(2)O(7). The XAFS analysis indicates that uranium exists in the form of high-valent U(6+) in all samples. To balance the extra charge for substituting Nd(3+) or Zr(4+) by U(6+), additional oxygen is introduced accompanied by a large structural distortion; however, the Nd(2)Zr(1.6)U(0.4)O(7+δ) sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd(2)Zr(2)O(7) host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd(2)Zr(2)O(7) synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.