Fluoride Addition Effects on Voltammograms and UV-Vis Spectra of Neodymium Cation in Molten Chlorides
Authors: Matsuura Haruaki , Nezu Atsushi , Akatsuka Hiroshi , Uehara Akihiro , Fujii Toshiyuki 
Published: February 2020

Neodymium is the one of the rare earth fission products and was used as the simulant of some trans uranium elements due to its similarity of electrochemical behavior. Although molten salt should be relatively constructed by a simple structural model due to the predominant ionic species in liquid phase, electrochemical behavior has not been well explained microscopically. For a recent few years, the electrochemical behavior of rare earths including neodymium in the molten chlorides with small amount of fluorides has been focused, and structural elucidation by extended absorption fine structure and UV-vis spectroscopy of neodymium in molten salts has been performed. In this study, fluoride concentration dependence on the voltammograms of neodymium in molten LiCl-KCl and spectra of neodymium in molten NaCl-2CsCl was remeasured and the structural variation is discussed by some parameters derived from the Judd-Ofelt analysis of UV-vis spectra.

All the electrochemical experiments using molten salts have been performed in the electric furnace which is built inside a glove box filled with an argon atmosphere in high purity. Cyclic voltammetry, and differential pulsed voltammetry have been performed by using the electrodes as follows: working electrode: tungsten, counter electrode: pyrocarbon and reference electrode: silver wire dipped in molten LiCl-KCl eutectic + AgCl (1 mol%) inside the borosilicate tube, respectively. Silica glass tube was used as a crucible. To observe the fluoride addition effect, 0 to 20 times amount of LiF to the concentration of neodymium was added into the molten salt. All measurements have been performed at 773 K. UV-vis spectroscopy of neodymium in molten NaCl-CsCl-NaF at 953 K has been carried out by using the spectrophotometer. A quartz cell with 10 mm of light path was used for the molten salt container. For the calculation of oscillator strength of the hypersensitive transition and derivation of Ω
2,4,6 parameters by the Judd-Ofelt analysis, density was assumed to be the additivity of molar volume of each component, and refractive index was temporary used from the value of KCl due to the non-availability of the data of CsCl.

According to the differential pulsed voltammograms of cathodic sweeps which exhibit mainly neodymium reduction peaks, the potential of neodymium reduction to metal was not shifted positively, however, drastic negative shift depending on fluoride addition did not occur as well. More strikingly, with increasing fluoride amount, the potenital gap between Nd3+/Nd2+ and Nd2+/Nd decreased, thus disproportionation reaction may be restricted with increasing concentration of fluoride. The variation of oscillator strength of hypersensitive transition absorption of neodymium at ca. 589 nm depending on fluoride addition has been evaluated. This absorption peak has been considered to the indication of symmetric coordination around neodymium cation, i.e. 6 coordinated octahedral species, and the smaller value relates to perfect symmetry. One striking feature is that with increasing fluoride amount until ca. F/Nd = 2 in molten NaCl-CsCl-NaF, the value of oscillator strength once increases until F/Nd = 2, and decreases rapidly. While in molten LiCl-KCl-LiF, the value of oscillator strength decreases almost linearly.

This study is performed under the projects of 26P-1, 25P1-10, and 24P2-3 at Research Reactor Institute, Kyoto University.