Insight into\nγ‑Irradiation-Induced Structure\nEvolution of Uranium(IV) Germanates as Crystalline Nuclear Waste Forms

New kinds of crystalline waste forms with improved structural\nstability\nare desirable for actinide immobilization. In this work, using a molten\nsalt method, two uranium(IV) germanate compounds, namely, K<sub>2</sub>UGe<sub>3</sub>O<sub>9</sub> (<b>1</b>) and K<sub>2</sub>UGe<sub>2</sub>O<sub>7</sub> (<b>2</b>), were synthesized, whose compositions\nconsisted of trimeric and dimeric units of germanate, as well as tetravalent\nuranium, as proved by bond valence calculation and X-ray absorption\nspectra. Radiation stability assessment is further performed by γ-irradiation\nto assess the potential of as-synthesized uranium germanate compounds\nas nuclear waste forms. Powder X-ray diffraction and single-crystal\ndiffraction analyses reveal that <b>1</b> remains stable within\n1 MGy dosage and undergoes a significant structural change with increasing\ndosage at 2 MGy, leading to a transformation of <b>1</b> to <b>1-ir</b> analogous to <b>2</b> in chemical structure. The\nunderlying mechanism was further studied through a combination of\ndifferent characterization techniques, including Raman, UV–vis,\nand electron paramagnetic resonance spectroscopies. Density functional\ntheory calculations of <b>1</b> and <b>2</b> were also\nconducted to probe the coordination interaction of germanium and uranium\nwith oxygen atoms. This work reports new crystalline uranium germanates\nby flux growth and, most importantly, provides insights into the irradiation\nstability of these materials, which will be beneficial to developing\nwaste forms for long-term immobilization of radionuclides.