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.