Uranium-Incorporated Pyrochlore La₂(UMgZr_1-2)₂O₇ Nuclear Waste Form: Structure and Phase Stability

As efficient and stable nuclear waste forms, single-phase uranium (U⁶⁺)-incorporated La₂Zr₂O₇ nanoparticles were designed and synthesized in an air atmosphere. To obtain a high U loading, divalent magnesium (Mg²⁺) was introduced to balance the extra charge from the substitution of tetravalent zirconium (Zr⁴⁺) by U⁶⁺ with a minimized impact to the lattice. There is a composition-driven phase transition from order pyrochlore to defect fluorite as the U concentration increases from 10 to 30 mol %, demonstrating both good solubility and stability of the La₂Zr₂O₇ host for U and potentially for other actinides. La₂(UMgZr_1-2)₂O₇ (x = 0-0.3) nanoparticles showed good dispersity and crystallinity with an average particle size of ∼48 nm. Furthermore, X-ray photoelectron spectroscopy, Raman spectroscopy, and emission spectroscopy revealed that U was stabilized in the hexavalent state in the form of a UO₂²⁺ ion. Spectroscopic methods also demonstrated that our samples caused a scintillating response with an orange emission (597 nm) by 230 nm excitation. In addition, density functional theory simulations were employed to investigate the atomic structures and electronic properties of the U-incorporated pyrochlores. The calculated bond lengths, atomic charges, and charge density confirm the existence of UO₂²⁺ ions. Supported by both experimental and computational results, a novel geometrical structure was proposed to explain the Mg²⁺-U⁶⁺ substitution. This work demonstrated the successful development of U-incorporated La₂Zr₂O₇ nanoparticles and provided an efficient way to immobilize U in these ceramic waste matrixes.