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, Pr1.5(H2O)6[(UO2)10UO13(OH)4] (UOHF-Pr) or Nd1.5(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.