Probing the Ionic\nCoordination in MgCl<sub>2</sub>–KCl Mixed Salts by Quadrupolar <sup>35</sup>Cl NMR

MgCl<sub>2</sub>–KCl mixed salts have been identified\nas\npromising thermal energy storage (TES) media and heat transfer fluids\n(HTF) for concentrated solar power (CSP) systems and other energy\nstorage applications. A comprehensive understanding of the ionic structure\nof these mixed salts is essential to elucidate the relationship between\ntheir structural arrangements and thermophysical properties. In this\nstudy, <sup>35</sup>Cl solid-state NMR spectroscopy was employed to\nprovide detailed insights into the local structure of MgCl<sub>2</sub>–KCl mixed salts. By analyzing the quadrupolar interactions\nof the coordinated Cl<sup>–</sup> ions, specifically quadrupolar\ncoupling constant (<i>C</i><sub>Q</sub>) and asymmetry parameter\n(η), the disruption of symmetry and the coordination of Cl<sup>–</sup> ions were characterized. The study confirmed a structural\ntransformation from KCl to K<sub>2</sub>MgCl<sub>4</sub>, KMgCl<sub>3</sub>, and MgCl<sub>2</sub> phases as the concentration of MgCl<sub>2</sub> increased, resulting in changes of Cl<sup>–</sup> ionic\ncoordination from 6-fold to 5-fold or 4-fold geometries. Such structural\ncoordination changes will influence the lattice vibrational modes,\nand thus altering the thermophysical properties, such as heat capacity.\nThese results offer a deeper understanding of the ionic structure\nand structural evolution of MgCl<sub>2</sub>–KCl mixed salts.\nFurthermore, the results highlight the utility of <sup>35</sup>Cl\nNMR spectroscopy as a powerful tool for investigating the microstructure\nand structural evolution of solid eutectic salts.