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 [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 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.
Highlights
Molten salts are a class of ionic liquids which have in recent years been the focus of extensive fundamental research
Bessada et al.[10] pointed out that the viscosity of LiF–AnF4 (A = Th, U) mixtures should be greater than that of LiF–ZrF4 at a the Structural studies of alkali fluoride/actinide fluoride salt mixtures in the molten state have been presented, using a combined Extended X-ray Absorption Fine Structure (EXAFS)-Molecular Dynamics (MD) approach which has proven to be a valuable tool to explore the rich chemistry of molten salt systems
Fitting the standard EXAFS equation was done in parallel, and was shown to be an adequate means to extract information about the first coordination shell of thorium and uranium, provided there is another structural model, e.g. obtained from MD, to constrain the fit to physically realistic values
Summary
Molten salts are a class of ionic liquids which have in recent years been the focus of extensive fundamental research. This can be explained by the fact that they are a versatile class of reaction media, e.g. carbonates, nitrates, fluorides, chlorides, with a variety of melting points, heat capacities, vapor pressures, densities, and other thermophysical properties suited for a variety of industrial applications. Molten salts may be used for processes as important as the production of hydrogen[1] and ammonia,[2] carbon capture and storage,[3] nuclear fuel pyroprocessing,[4] and as nuclear fuels.[5,6]. EXAFS data for pure molten ThF4 and UF4 were collected for the first time
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