Electrochemical behavior and electrowinning of uranium(IV) from FLiNaK molten salt

On the Structures of Thorium Fluoride and Oxyfluoride Anions in Molten FLiBe and FLiNaK

The structures of thorium fluoride and oxyfluoride ions in molten FLiBe-ThF₄ and FLiNaK-ThF₄ were investigated by Raman spectroscopy and density functional theory calculations. Thorium fluorides are present in the form of ThF₆^2- (O_h) and ThF₇^3- (C_2v) in molten FLiNaK-ThF₄. Similar speciation was identified in FLiBe-ThF₄, and the thorium fluoride anions are in equilibrium with free F^- ions and beryllium fluoride anions, which are responsible for the red shift of the beryllium fluoride bands in the Raman spectra. With the addition of Li₂O into the FLiNaK-ThF₄ and FLiBe-ThF₄ melts, the Th₂OF₁₀^4- anion with a linear Th-O-Th geometry was formed at the expense of thorium fluoride anions. The beryllium fluoride bands in the Raman spectra of FLiBe-ThF₄ exhibit a blue shift upon Th₂OF₁₀^4- formation, which results from the release of free F^- ions that can further react with beryllium fluoride. Insoluble thorium oxides were found in the FLiNaK and FLiBe melts at a Li₂O concentration of 15 mol %, and the Th₂OF₁₀^4- anion is, therefore, a bridge connecting the soluble thorium fluorides and insoluble thorium oxides in molten fluorides.