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 ThF62- (Oh) and ThF73- (C2v) 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 Th2OF104- 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 Th2OF104- 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 Th2OF104- anion is, therefore, a bridge connecting the soluble thorium fluorides and insoluble thorium oxides in molten fluorides.