Local structure and dynamics of ions in LiCl-GdCl3, KCl-GdCl3 and LiCl-GdCl3-Gd2O3 melts: Ab initio molecular dynamics simulations and Raman spectroscopy

Currently, the novel technologies of gadolinium electrochemical production from raw oxide materials using molten oxide-salt media are being developed. To understand the phenomena, data on the structure of gadolinium-containing oxide-salt melts are required. In this study, using ab initio molecular dynamics (AIMD) and in situ Raman spectroscopy, we bring insight on local structure details and ion dynamics in these systems by the examples of LiCl–GdCl3, KCl–GdCl3 chloride melt and LiCl–GdCl3–Gd2O3 oxide-chloride melt with a Gd2O3 concentration up to 2 mol %. The Raman spectroscopy reveals the octahedral species [GdCl6]. According to the AIMD, the Gd-based groupings are joined by various numbers of shared chlorine anions. Gadolinium oxide in the chloride melt dissociates to form [Gd2OLi] groups, which are incorporated into the melt structure. The calculated self-diffusion coefficients of ions and the lifetimes of ion pairs showed that the local structure of the potassium containing melt is more stable as compared to that of the lithium containing melt, which is closely concurring with the divergence in conductivities of these melts. LiGdO2 and GdOCl phases were found in the solidified oxide-chloride melt LiCl–GdCl3–Gd2O3. It has been suggested that the complex oxide LiGdO2 is formed by the reaction Gd2O3 + LiCl → GdOCl + LiGdO2. Based on the obtained data, it was concluded that lithium-containing chloride melt should be considered not only as a solvent for gadolinium oxide, but also as an active reaction medium. Here, lithium is involved in ongoing chemical reactions and participate in reaction product.