Examination of the short-range structure of molten salts: ThF$_{4}$, UF$_{4}$, and related alkali actinide fluoride systems

The short-range structures of LiF–ThF$_{4}$, NaF–AnF$_{4}$, KF–AnF$_{4}$, and Cs–AnF$_{4}$ (An = Th, U), were probed using in situ high temperature Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Signally, the EXAFS spectra of pure molten ThF$_{4}$ and UF$_{4}$ were measured for the first time. The data were interpreted with the aid of Molecular Dynamics (MD) and standard fitting of the EXAFS equation. As in related studies, a speciation distribution dominated by [AnFx]$^{4-x}$ (x = 7, 8, 9) coordination complexes was observed. The average coordination number was found to decrease with the increasing size of the alkali cation, and increase with AnF$_{4}$ content. An average coordination number close to 6, which had not been detected before in melts of alkali actinide fluorides, was seen when CsF was used as solvent.