Raman and density functional theory studies of lutecium fluoride and oxyfluoride structures in molten FLiNaK

Combined with Raman spectroscopy and density functional theory (DFT) calculations, the micro-structures in molten FLiNaK-LuF₃ and FLiNaK-LuF₃-Li₂O systems were studied. Both LuF₅^2- (D_3h) and LuF₆^3- (O_h) anions were identified in molten FLiNaK, and their relative content varied with the concentration of LuF₃. For regarding the affection of oxygen anion, Li₂O was added into the molten FLiNaK-LuF₃ (20 mol%) sample. The lutecium oxyfluoride anion Lu₂OF₈^4- was firstly formed which possesses a linear LuOLu geometry with two LuF₄ moieties bridged by one single oxygen atom. Further increasing the Li₂O content to 10 mol% resulted in the formation of two species, which belonged to the Lu₂O₂F₄^2- and Lu₂O₂F₆^4- anions. When increased the concentration of Li₂O to 20 mol%, a new species appeared which was approximate to the oxide structure.

Raman Spectroscopic and Theoretical Study of Scandium Fluoride and Oxyfluoride Anions in Molten FLiNaK

The speciation in the FLiNaK-ScF₃ and FLiNaK-ScF₃-Li₂O melts above 873 K was investigated by Raman spectroscopy and density functional theory (DFT) calculations. Binary scandium fluoride anions in the form of ScF₅^2- and ScF₆^3- were identified in molten FLiNaK containing 20 mol % ScF₃, which were not affected by either temperature or alkali cations in molten fluoride. With the addition of Li₂O into the FLiNaK-ScF₃ (20 mol %) melt, ternary scandium oxyfluoride anion Sc₂OF₆^2- was formed and dominated the spectrum, and it was characterized to possess a linear Sc-O-Sc geometry with two ScF₃ moieties bridged by a single oxygen atom. Further increase in Li₂O content to 40 mol % resulted in the formation of the second oxyfluoride anion Sc₂O₂F₆^4- containing a rhombic Sc₂O₂ ring, while the Raman bands due to Sc₂OF₆^2- disappeared. When the Li₂O concentration went beyond 40 mol %, the sample was no longer homogeneous due to the appearance of insoluble scandium oxide on the surface of the melt, suggesting that scandium oxide is formed via the reactions of ScF₅^2- and ScF₆^3- with O^2- in molten FLiNaK mediated by Sc₂OF₆^2- and Sc₂O₂F₆^4-.

Polarizable force field parameterization and theoretical simulations of ThCl4 -LiCl molten salts

Recycle of thorium is an essential process in the thorium-uranium closed fuel cycle of molten salt reactor (MSR). Pyrochemical treatment of spent nuclear fuel using chloride molten salts as medium has been considered as a promising method. In this article, we performed molecular dynamics simulations on the ThCl₄ LiCl molten salts using a polarizable force field parameterized by us from first-principles calculations. The microscopic structures and macroscopic properties at different compositions were investigated using the developed force field to understand the structure/property relationship in the mixture. The differences between ThCl₄ LiCl and ThF₄ LiF MSs are compared to understand the behaviors of Th⁴⁺ in the fluoride-chloride mixed media. In the molten fluorides, the coordination number of Th⁴⁺ is larger, and the resulting more shared anions lead to lower ThF dissociation barrier and shorter lifetime of the Th⁴⁺ first solvation shell. Our results also indicate the Pauling's structural rules for crystals can be used to rationalize the local structures in molten salts. © 2018 Wiley Periodicals, Inc.

High temperature 35Cl nuclear magnetic resonance study of the LiCl–KCl system and the effect of CeCl3 dissolution

This paper examines the dynamics of the LiCl–KCl system over a range of temperatures in order to understand the local structure surrounding chlorine, which is the common ion in these systems, during molten salt pyro-processing. Chlorine-35 nuclear magnetic resonance (NMR) is sensitive to the local environments of the resonant nuclei and their motion on a diffusive timescale. Thus, it is a good probe of the atomic scale processes controlling the viscosities, diffusivities and conductivities of these molten salts. The average isotropic chemical shifts (^35Clδ) and spin-lattice relaxation times (T₁) of ³⁵Cl in (Li,K)Cl salt mixtures have been obtained over a compositional range of 0–100 mol% KCl with an interval of 10 mol% using high temperature nuclear magnetic resonance (NMR) spectroscopy from room temperature up to 890 °C. The ^35Clδ in the two end member salts are consistent with the cation–anion radius ratio as previously measured on the solid halides and the average radius ratio of cation to anion, can be used to explain the variation of ^35Clδ with composition. The quadrupolar interaction is found to be responsible for the spin-lattice relaxation of the ³⁵Cl, and the activation energies for T₁ relaxation have been obtained for all compositions. The measured T₁ (³⁵Cl) activation energies do not vary linearly with composition and peak at 50% KCl, which also coincides with the Chemla point for this system. They also are in good agreement with the values from equivalent conductivity measurements. To investigate the response of the system to solutes, 8 wt% of CeCl₃ was added to the pure LiCl as a surrogate actinide. The shift induced was 120 ppm and the activation energy for the T₁ (³⁵Cl) increased by a factor of four. This is a promising preliminary result for probing the effect of actinide dissolution on the dynamics of these pyro-processing salts.

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF-LaF3-Li2O systems

The local structure of molten LaF3-LiF-Li2O has been investigated by high temperature NMR spectroscopy. The (139)La and (19)F chemical shifts have been measured as a function of temperature and composition. The NMR spectra show that Li2O reacts completely with LaF3 to form a LaOF compound in the solid state below the melting temperature of the sample. LaOF is not completely dissolved in the fluoride melt and solid LaOF is observed in the (19)F spectra for Li2O concentrations above 10 mol%. We discuss the local environment of lanthanum ions in molten LaF3-LiF-Li2O and compare the results to those with the LaF3-LiF-CaO system. The analysis of the temperature and Li2O concentration dependences of the (139)La and (19)F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaOxFy(3-x-y) units are present in the melt.