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 (C Q) and asymmetry parameter (eta), 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.