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

ABSTRACT

Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF-Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (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 C2221 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of ß-U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ 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.