Towards uranium oxide hydrate framework with holmium(III) ions: High or low symmetry structure?

Uranium oxide hydrate frameworks (UOHFs) with lanthanide ions are fundamentally important to uranium crystal chemistry. Earlier works revealed a possible phase transition from high symmetry space groups (monoclinic or orthorhombic) for UOHF-Ln (Ln = Pr → Dy) to a low symmetry space group (triclinic) for UOHF-Ln (Ln = Er → Lu). However, given holmium lies close to this apparent transition boundary, whether these frameworks with holmium adopt a high or low symmetry structure is unknown. Herein, we report the synthesis of Ho 2 (H 2 O) 4 (OH) 2 [(UO 2 ) 10 UO 13 (OH) 4 ]·H 2 O ( UOHF-Ho ) and subsequent structural and spectroscopic investigations. Synchrotron single crystal X-ray diffraction confirmed that UOHF-Ho crystallises in the orthorhombic C 222 1 space group, further validated by electron diffraction with transmission electron microscopy. The framework structure is constructed with β-U 3 O 8 type layers linked by double uranyl units in pentagonal bipyramids, with disordered Ho 3+ ions lying inside the channels. While Raman spectroscopy revealed the U 6+ dominant vibrational modes, diffuse reflectance spectroscopy unearthed characteristic absorption bands for both U 6+ and Ho 3+ ions. This work uncovers the exact phase transition for UOHF-Ln series and has implications to the uranium structural chemistry and possible spent nuclear fuel alterations. The previously absent uranium oxide hydrate framework with holmium(III) ions has been synthesized and characterized showing a high symmetry structure on the phase transition boundary along the UOHF-Ln series. • A uranium oxide hydrate framework with Ho 3+ ions was synthesized and characterized. • Synchrotron single-crystal XRD and electron diffraction with TEM revealed a high symmetry structure. • A phase transition for uranium oxide hydrate frameworks with lanthanide ions was further validated. • Vibrational modes and electronic structures were also investigated.