Electrochemical Formation of Divalent Samarium Cation and Its Characteristics in LiCl–KCl Melt

Abstract

The electrochemical reduction of trivalent samarium in a LiCl-KCl eutectic melt produced highly stable divalent samarium, whose electrochemical properties and electronic structure in the molten salt were investigated using cyclic voltammetry, UV-vis absorption spectroscopy, laser-induced emission spectroscopy, and density functional theory (DFT) calculations. Diffusion coefficients of Sm²⁺ and Sm³⁺ were electrochemically measured to be 0.92 × 10^-5 and 1.10 × 10^-5 cm²/s, respectively, and the standard apparent potential of the Sm^2+/3+ couple was estimated to be -0.82 V vs Ag|Ag⁺ at 450 °C. The spectroelectrochemical study demonstrated that the redox behavior of the samarium cations obeys the Nernst equation ( E°' = -0.83 V, n = 1) and the trivalent samarium cation was successfully converted to the divalent cation having characteristic absorption bands at 380 and 530 nm with molar absorptivity values of 1470 and 810 M^-1 cm^-1, respectively. Density function theory calculations for the divalent samarium complex revealed that the absorption signals originated from the 4f⁶ to 4f⁵5d¹ transitions. Additionally, laser-induced emission measurements for the Sm cations in the LiCl-KCl matrix showed that the Sm³⁺ ion in the LiCl-KCl melt at 450 °C emitted an orange color of fluorescence, whereas a red colored emission was observed from the Sm²⁺ ion in the solidified LCl-KCl salt at room temperature.