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 [AnF _x ] ^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.