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.

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The speciation in the FLiNaK-ScF 3 and FLiNaK-ScF 3 -Li 2 O melts above 873 K was investigated by Raman spectroscopy and density functional theory (DFT) calculations. Binary scandium fluoride anions in the form of ScF 5 2− and ScF 6 3− were identified in molten FLiNaK containing 20 mol % ScF 3 , which were not affected by either temperature or alkali cations in molten fluoride. With the addition of Li 2 O into the FLiNaK-ScF 3 (20 mol %) melt, ternary scandium oxyfluoride anion Sc 2 OF 6 2− was formed and dominated the spectrum, and it was characterized to possess a linear Sc−O− Sc geometry with two ScF 3 moieties bridged by a single oxygen atom. Further increase in Li 2 O content to 40 mol % resulted in the formation of the second oxyfluoride anion Sc 2 O 2 F 6 4− containing a rhombic Sc 2 O 2 ring, while the Raman bands due to Sc 2 OF 6 2− disappeared. When the Li 2 O 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 ScF 5 2− and ScF 6 3− with O 2− in molten FLiNaK mediated by Sc 2 OF 6 2− and Sc 2 O 2 F 6 4− .