Atomistic synergistic mechanism of coordination compensation and structural evolution in ammonium carnallite.

CONTEXT: The preparation of anhydrous magnesium chloride is severely hindered by hydrolysis during thermal dehydration, while the ammonium carnallite route is widely adopted to mitigate this issue. However, the atomistic mechanism by which NH4Cl regulates dehydration and suppresses hydrolysis remains unclear. In this work, a unified mechanism is revealed in which Cl⁻ ions compensate for coordination vacancies in partially dehydrated magnesium species, while the collapse of the NH4Cl framework facilitates water migration and inhibits rehydration, thereby enhancing structural stability. Hydrogen bonding promotes the initial removal of coordinated water, whereas progressive structural collapse lowers migration barriers and facilitates water migration. Thermodynamic analysis and thermal experiments further confirm that most crystallization water is removed above 210 °C and that controlled heating below 240 °C effectively limits hydrolysis.

METHODS

Density functional theory (DFT) calculations were performed using the PBE functional within the generalized gradient approximation and a plane-wave basis set as implemented in VASP, with the projector-augmented wave (PAW) method and DFT-D3 dispersion corrections. Transition states were identified using the climbing image nudged elastic band (CI-NEB) method. The computational results were combined with thermodynamic analysis and thermal experiments to validate the proposed mechanism and to evaluate dehydration behavior under different temperature conditions.