Ionic Association in LaCl3·7H2O/MgCl2·6H2O Mixed Molten Salt Hydrates and Their Contributions to Dehydration and Energy Conservation

The dynamic solvation structures of concentrated electrolyte systems, especially mixed molten salt hydrates (MSHs), govern their thermal properties and dehydration pathways. The multilevel coordination geometries formed by cations, anions, and water molecules in MSHs directly determine thermal stability and dehydration reactivity. Herein, we characterize the dehydration process of LaCl3·7H2O/MgCl2·6H2O mixed MSHs via combined experimental measurements and first-principles simulations. Three mixed molar ratios are investigated: n(MgCl2·6H2O):n(LaCl3·7H2O) = 5:1 (denoted Mg5La1), 1:1 (Mg1La1), and 1:5 (Mg1La5). Raman spectroscopy, thermogravimetric-differential scanning calorimetry (TG-DSC), vacuum in situ high-temperature X-ray powder diffraction (XRD), and ab initio molecular dynamics (AIMD) based on density functional theory (DFT) are integrated to unravel ion–ion interaction mechanisms and structural evolution during dehydration. Experimental results confirm distinct ionic association modes among La3+, Mg2+, and Cl– in mixed MSHs, which cannot be observed in single-component LaCl3·7H2O or MgCl2·6H2O melts. Elevated temperature breaks intermolecular water hydrogen bonds and increases Cl– coordination numbers around La3+; both La3+ and Mg2+ capture free Cl– to form heteronuclear bridging aggregates, which alter dehydration thermodynamics and kinetics. Coordination interactions between La3+, Mg2+, and Cl– collectively lower the dehydration onset temperature and reduce total dehydration heat absorption. All mixed MSH samples exhibit lower dehydration temperatures than single-component LaCl3·7H2O or MgCl2·6H2O. In situ vacuum XRD verifies that crystalline anhydrous LaCl3 and MgCl2 initially form at 150 °C for Mg1La5 mixtures. DSC results demonstrate that the dehydration enthalpy of Mg1La5 is merely 75% relative to that of LaCl3·7H2O on a per-unit-mass basis. The coordinated heteronuclear La–Cl–Mg cluster structure suppresses MgCl2 hydrolysis by consuming free Cl– and inhibiting HCl release, which mitigates MgO impurity formation. This structural regulation strategy provides a feasible route for low-temperature, hydrolysis-free preparation of mixed anhydrous LaCl3/MgCl2.