Probing the Ionic Coordination in MgCl₂–KCl Mixed Salts by Quadrupolar ³⁵Cl NMR

MgCl2–KCl mixed salts have been identified as promising thermal energy storage (TES) media and heat transfer fluids (HTF) for concentrated solar power (CSP) systems and other energy storage applications. A comprehensive understanding of the ionic structure of these mixed salts is essential to elucidate the relationship between their structural arrangements and thermophysical properties. In this study, 35Cl solid-state NMR spectroscopy was employed to provide detailed insights into the local structure of MgCl2–KCl mixed salts. By analyzing the quadrupolar interactions of the coordinated Cl– ions, specifically quadrupolar coupling constant (CQ) and asymmetry parameter (η), the disruption of symmetry and the coordination of Cl– ions were characterized. The study confirmed a structural transformation from KCl to K2MgCl4, KMgCl3, and MgCl2 phases as the concentration of MgCl2 increased, resulting in changes of Cl– ionic coordination from 6-fold to 5-fold or 4-fold geometries. Such structural coordination changes will influence the lattice vibrational modes, and thus altering the thermophysical properties, such as heat capacity. These results offer a deeper understanding of the ionic structure and structural evolution of MgCl2–KCl mixed salts. Furthermore, the results highlight the utility of 35Cl NMR spectroscopy as a powerful tool for investigating the microstructure and structural evolution of solid eutectic salts.