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

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journal contribution
posted on 2021-02-03, 18:43 authored by Chenyang Wang, Xiuting Chen, Yu Gong
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.