In-situ Raman spectroscopic study on the oxidation and reduction of Yb and Eu as well as the potential-oxoacidity phase diagram of La(III) in the LiCl-KCl eutectic melt

Abstract

Molten salts, the electrolyte of pyroprocessing which can reduce the amount and radiotoxicity of used nuclear fuel, have different characteristics from aqueous solution and it is important to obtain the accurate knowledge of the chemical interactions and electrochemical behaviors of fission products in them. In this study, a home-built Raman spectroscopic system was developed for the high-temperature molten salt. The system allows the addition of reactants or simultaneous electrochemical analysis during the spectroscopic measurement for high temperature melts. The oxidation reaction was monitored in real time and the solubility products of lanthanide oxides were rapidly and accurately determined using in-situ Raman spectroscopy in combination with electrochemical techniques. The concentration of lanthanum ion was evaluated in-situ spectroscopically to study the oxidation behavior of lanthanum chloride in the molten LiCl-KCl eutectic. The potential-oxoacidity ($E-pO^{2–}$) phase diagram of lanthanum in molten LiCl-KCl was constructed based on the experimental results. The Raman spectra of trivalent and divalent ions of Eu and Yb were measured and the local structures of them were studied with DFT calculation. A new Raman peak for divalent lanthanide-chloride complex was found, and its structure was concluded to be a distorted octahedron.