Uranium Oxide Hydrate Frameworks with Dy(III) or Lu(III) Ions: Insights Into the Framework Structures With Lanthanide Ions.

Two uranium oxide hydrate frameworks (UOHFs) with either Dy or Lu ions, Dy(HO)[(UO)UO(OH)] (UOHF-Dy) or Lu(HO)[(UO)UO(OH)] (UOHF-Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM-EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF-Dy in the orthorhombic C222 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of β-UO type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy/Lu ions inside the channels. However, the arrangements of Dy/Lu ions are different, with disordered Dy ions well aligned at the centers of the channels and single Lu ions well-separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF-Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal.