Charge-Dependent Coordination and Redox Energetics of Actinide Fluoride Complexes in Molten Salts
Redox control in molten fluoride salt reactors determines actinide solubility and corrosion of structural alloys, but the role of local fluoride coordination in this control remains poorly understood. We use density functional theory with explicit LiF microsolvation to study the coordination chemistry, intrinsic redox energetics, and fluoride-exchange kinetics of Th(IV), U(IV), and U(III) fluoride complexes. The coordination response to solvation primarily follows the formal charge of the complex rather than the actinide at its center: we find that cationic and neutral complexes gain fluoride ligands, most anionic complexes keep their coordination number, and $4-$ complexes lose fluoride. Coordination number then strongly controls the intrinsic U(IV)/U(III) reduction energy. From CN 4 to CN 7, this energy rises by up to $11$ eV and changes sign; a continuum-solvation estimate opposes this trend and therefore establishes the intrinsic value as an upper bound. Fluoride exchange barriers are low, so redox partners of different charge do not keep the same coordination numbers in solvated clusters. Finally, above a threshold between CN 5 and CN 6, single-reference DFT converges to a ligand-reduced U(III) state inconsistent with formal $5f^3$ occupation, however the physical nature of this is still unresolved. Therefore, local coordination is a primary variable in the prediction of redox behavior in molten actinide-fluoride salts.