Structure and Dynamics of NaCl/KCl/CaCl 2 -EuCl n ( 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 Eu 2+ and Eu 3+ ions in CaCl 2 , NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K + to Na + to Ca 2+ , the coordination number (CN) of Cl - in the first solvation shell increases from 5.6 (Eu 2+ ) and 5.9 (Eu 3+ ) in KCl to 6.9 (Eu 2+ ) and 7.0 (Eu 3+ ) in CaCl 2 . 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 CaCl 2 . 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 Eu 2+ /Eu 3+ 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.