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

Abstract

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.

Quantitative Studies on the Structure of Zirconium Fluoride Anions in Molten LiF and FLiBe

“…No band was observed in the spectrum of blank LiF, consisting with previously reported alkali fluorides. [11][12][13] An intense band located at 564 cm À 1 appeared and a weak broad band was also observed at around 244 cm À 1 when 5 mol % ZrF 4 was added, and the intensity of both two bands increased with the concentration of ZrF 4 raising. The position of the intense band was blue-shifted from 564 cm À 1 to 572 cm À 1 when the concentration of ZrF 4 was increased to 20 mol %.…”

“…This is similar to the results of ThF 4 in molten FLiNaK (46.5 mol % LiF, 11.5 mol % NaF, 42.0 mol % KF) and FLiBe. [13,18] F À þ ZrF 4 $ ZrF 6 2À =ZrF 7 3À…”

“…Raman spectroscopy combined with theoretical calculation has been selected to study the structures of fluorides and oxyfluorides for a series of metal ions including transitionmetals (Sc, Ta), [11] lanthanides (La, Ce, Nd, Sm, Dy, Yb Lu), [12] and actinide (Th) [13] in different molten salt media. Furthermore, Raman spectroscopy has also been successfully used for quantification of species in molten mixtures under the premise of calibration procedure.…”

In situ high temperature Raman and DFT analysis of cerium fluoride and oxyfluoride structures in molten FLiNaK

“…[30][31][32][33][34][35] The DFT calculations used the Gaussian 09 package. [36] The DFT calculation has been proved to be effective and feasible in predicting the structure of the ions in the molten salts, [37][38][39] although part of them are electronically or thermodynamically unstable. In addition, the M06, BP86 functionals, and Hartree-Fock calculation were also employed for geometric optimization and wavenumber prediction in Figure S1.…”

Coordination Structure of Si(II) Ions in Binary Melts (LiF–KF, LiF–NaF, and NaF–KF) and Ternary Melt (LiF–NaF–KF)

“…18 The spectroscopic and computational method was also adopted to reveal the coordination structure of metallic ions in molten halides such as LiF−NaF−KF−ThF 4 and LiF− NaF−KF−LuF 3 . 19,20 However, concerning the coordination structure of Si ions, most of these investigated coordination In the electrodeposition process of silicon from Si(IV) ions in molten salt, the behavior of the divalent state of the silicon ions [Si(II)] is important because it has been reported that the Si(II) ions can be formed by comproportionation reactions between Si(IV) ions and electrodeposited silicon (Si 0 ) according to the following reaction. 7 + Si(IV) Si 2Si(II) 0 (1) The formation process of Si(II) caused the rough morphology of the top surface of the electrodeposited silicon.…”