Examination of the short-range structure of molten salts: ThF4, UF4, and related alkali actinide fluoride systems

Abstract

The short-range structures of LiF–ThF₄, NaF–AnF₄, KF–AnF₄, and Cs–AnF₄ (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 ThF₄ and UF₄ 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 AnF₄ 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.