Raman spectroscopy and quantum chemical calculation on YCl₃-KCl molten salt system

The knowledge on the ionic structure of YCl₃-KCl molten system is of guiding significance for the practical production of yttrium metals and yttrium alloys via molten salt electrolysis using this system as electrolyte. In this paper, the theoretical Raman spectra of the ionic groups which may exist in YCl₃-KCl molten system are simulated by quantum chemical calculation using Gaussian 09 and GaussView 5.0 programs based on density functional theory (DFT). Then the ionic structures of 20 mol%–60 mol% YCl₃-KCl molten salt systems are studied by comparing the Raman shift values of the bands in the theoretical Raman spectra of different ionic groups with the experimental spectra of this system. YCl₆³⁻, Y₂Cl₇⁻, Y₂Cl₈²⁻ and Y₂Cl₉³⁻ are thought to exist in the molten system. With the increase of temperature, the relative content of YCl₆³⁻ ionic groups increases while those of Y₂Cl₇⁻, Y₂Cl₈²⁻ and Y₂Cl₉³⁻ ionic groups decrease. Moreover, the "lifetime" of all ionic groups decreases within the temperature range of 692–730 °C. Meanwhile, the relative contents of Y₂Cl₇⁻, Y₂Cl₈²⁻ and Y₂Cl₉³⁻ increase with the increase of YCl₃ content, while that of YCl₆³⁻ decreases. The wave function analysis of the four ionic groups (YCl₆³⁻, Y₂Cl₇⁻, Y₂Cl₈²⁻, and Y₂Cl₉³⁻) is carried out by Multiwfn program. The net charge in each group, the direction of electron migration during the formation of each group, the sites where electrophilic and nucleophilic reactions are most likely to occur in each ionic group, and the order of bond breaking during chemical reactions for the four groups are obtained.