Expanding Uranium Oxide Hydrate Frameworks toward Early Lanthanides: Cases for Pr(III) and Nd(III) Ions

We report the hydrothermal syntheses and structural and spectroscopic characterization of two new uranium oxide hydrate frameworks (UOHFs) with either Pr³⁺ or Nd³⁺ ions, Pr_1.5(H₂O)₆[(UO₂)₁₀UO₁₃(OH)₄] (UOHF-Pr) or Nd_1.5(H₂O)₆[(UO₂)₁₀UO₁₃(OH)₄] (UOHF-Nd). Both UOHFs crystallize in the orthorhombic C222₁ space group and display needle crystal morphologies. Their crystal structures are composed of β-U₃O₈-type layers connected by double uranium polyhedra to form the frameworks, with disordered Pr³⁺/Nd³⁺ ions within the framework channels, as revealed by synchrotron single-crystal XRD. The presence of pentavalent uranium in both UOHFs was confirmed by a combination of diffuse reflectance and synchrotron X-ray absorption spectroscopies. The formation and stabilization of UOHF-Ln with earlier lanthanide ions in high symmetry structures are confirmed, and the strong correlation of the structural trends with the ionic radius of the lanthanide cation has been further validated. This work further emphasizes the complex uranium chemistry in the presence of lanthanide ions and has implications for uranium geochemistry and possible alterations of spent nuclear fuel under geological disposal.