High-energy EXAFS study of molten GdCl3 systems

Structure and Dynamics of NaCl/KCl/CaCl2-EuCln (n = 2, 3) Molten Salts

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu²⁺ and Eu³⁺ ions in CaCl₂, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K⁺ to Na⁺ to Ca²⁺, the coordination number (CN) of Cl^- in the first solvation shell increases from 5.6 (Eu²⁺) and 5.9 (Eu³⁺) in KCl to 6.9 (Eu²⁺) and 7.0 (Eu³⁺) in CaCl₂. This coordination change is validated by the EXAFS measurements, in which the CN of Cl^- around Eu increases from 5 in KCl to 7 in CaCl₂. Our simulation shows that the fewer Cl^- ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu²⁺/Eu³⁺ are related to the rigidity of their first coordination shell of Cl^-: the more rigid the first coordination shell is, the slower the solute cations diffuse.

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+ ) in Molten Salts

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La³⁺ and Nd³⁺ ) is explored in molten LiCl-KCl-LiF-LnCl₃ salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln³⁺ occur when F^- concentration increases, indicating that the F^- anions interact with Ln³⁺ via substituting the coordinated Cl^- anions, and confirm [LnCl_x F_y ]^3-x-y (y_max =3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl^- and F^- anions, which leads to the formation of four distinct Ln(III) species: [LnCl₆ ]^3- , [LnCl₅ F]^3- , [LnCl₄ F₂ ]^3- and [LnCl₄ F₃ ]^4- . Among them, the seven-coordinated [LnCl₄ F₃ ]^4- complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln-F interaction is weaker than that between transition metal and F^- ions.