Investigation of Structural Dynamics in KF-ZrF₄ Molten Salt Systems via High-Temperature Raman Spectroscopy and...

The KF-ZrF₄ molten salt system has emerged as a promising candidate solvent for fourth-generation reactor designs, where its physicochemical characteristics are fundamentally governed by the ionic structure within the molten fluoride matrix. A variety of complex Zr-F ions, including ZrF₅^-, ZrF₆^2-, ZrF₇^3-, and ZrF₈^4-, were identified in KF-ZrF₄ molten salts with different molar ratios through Raman spectroscopy and quantum chemical calculations. Furthermore, the Zr₂F₉^- ion with a bridging F structure was discovered in KF-ZrF₄ molten salts with molar ratios of 1 and 0.65, and the experimental Raman spectrum of this ion was obtained. At higher KF to ZrF₄ molar ratios of 5.7 and 2.3, ZrF₇^3- and ZrF₈^4- dominated, while ZrF₅^- and ZrF₈^4- were predominant at molar ratios of 1 and 1.5. At a lower ratio of 0.67, ZrF₅^- emerged as the primary species. As the KF to ZrF₄ molar ratio decreased, the system favored the formation of less-coordinated Zr-F ions. Moreover, a dynamic mutual transformation equilibrium existed among these complex ions, and this equilibrium was influenced by the zirconium fluoride content in the molten salt. Quantum electrochemical analysis revealed that ZrF₅^- was mainly involved in the cathode reaction. Zr⁴⁺ was first released from ZrF₅^-, gaining three electrons to form Zr⁺. Subsequently, Zr⁺ gained one electron, transforming into metallic zirconium at the cathode interface. These findings deepen our understanding of the physicochemical properties of molten salt reactors and have significant implications for advanced applications in metal purification, spent nuclear fuel recovery, and zirconium alloy production, thereby contributing to improved nuclear materials management and energy sustainability.