Defect-driven water migration in MgCl₂ tetra- and hexahydrates

This molecular dynamics simulations study elucidates how water diffusion in MgCl₂·nH₂O (n=4 and 6) is facilitated by defects and dopants. Both the impact of a single vacancy (one water molecule was removed) and an interstitial (one water molecule was added) on long range water motion has been investigated. Spontaneous vacancy-interstitial pair defect formation was not observed, which is in line with the predicted high energy costs for defect formation: 150 and 200 kJ/mol for the tetrahydrate and hexahydrate, respectively. The vacancy defects did not show long range mobility, which we relate to the strong bonding of water in the coordination shell of the Mg-ion that prevents water molecules from shifting to neighbouring magnesium ions. On the contrary, the addition of an extra water molecule, facilitates long range motion, which was found as a sequence of hopping events. The interstitial motion is anisotropic in both tetrahydrate and hexahydrate crystals, as interstitials preferentially reside at locations of unfavourable Cl-Cl interactions. Strikingly, Mg ↔ Ca substitutional defects neither increase the mobility of vacancies nor facilitates interstitial motion in the MgCl₂ lattice. While Ca dopants slightly facilitate vacancy formation, it also stabilizes interstitial water molecules by incorporating these molecules in its coordination shell. As a consequence, the interstitial becomes trapped and loses its mobility. Therefore, Ca dopants will not increase the hydration/dehydration kinetics of MgCl₂ hydrates and cannot be used to boost the power output of MgCl₂-based heat storage devices.