Crystal structure and compressibility of magnesium chloride heptahydrate found under high pressure

The odd hydration number has so far been missing in the water-rich magnesium chloride hydrate series (MgCl₂·nH₂O). In this study, magnesium chloride heptahydrate, MgCl₂·7H₂O (or MgCl₂·7D₂O), which forms at high pressures above 2 GPa and high temperatures above 300 K, has been identified. Its structure has been determined by a combination of in-situ single-crystal X-ray diffraction at 2.5 GPa and 298 K and powder neutron diffraction at 3.1 GPa and 300 K. The single-crystal specimen was grown by mixing alcohols to prevent nucleation of undesired crystalline phases. The results show orientational disorder of water molecules, which was also examined using density functional theory calculations. The disorder involves the reconnection of hydrogen bonds, which differs from those in water ice phases and known disordered salt hydrates. Shrinkage by compression occurs mainly in one direction. In the plane perpendicular to this most compressible direction, oxygen and chlorine atoms are in a hexagonal-like arrangement.