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 ScF52- and ScF63- 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 Sc2OF62- 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 Sc2O2F64- containing a rhombic Sc2O2 ring, while the Raman bands due to Sc2OF62- 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 ScF52- and ScF63- with O2- in molten FLiNaK mediated by Sc2OF62- and Sc2O2F64-.