Rich Chemical Behavior and Topological Variation in Uranium Phosphates via Flux Growth under an Inert Atmosphere

Flux synthesis in molten salts, benefiting from high ionic strength and strong solvation ability, is an effective route to crystalline actinide materials. Here, we report three new uranium phosphates with three-dimensional (3D) anionic frameworks: Cs₇U^IV₅Al(PO₄)₁₀ (1), Cs₄(U^VIO₂)Al₂(PO₄)₄ (2), and Cs_1.45K_14.55(U^VIO₂)₁₀(PO₄)₁₂ (3), using different metal halide salts and varying U/P ratios under an inert atmosphere. Single-crystal X-ray diffraction analyses reveal that the utilization of different flux conditions leads to rich chemical behavior, including varying oxidation states of uranium and diverse coordination modes of phosphate groups as well as the formation of new polymeric species, and finally contributes to topological variation of resultant 3D frameworks for these three uranium phosphate compounds. Characterization of the physicochemical properties of these compounds was further conducted, confirming their chemical components and key spectroscopic information. Moreover, density functional theory calculations of the formation enthalpies and lattice energies were used to understand their structural stability, which demonstrates the superior stability of compound 1 among these uranium phosphate compounds. This work not only expands the family of uranium phosphates, but also proves the feasibility via the flux method to explore novel actinide materials with high structure stability, thus providing significant implications for nuclear waste form development.