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

Yamashita, Keishiro*; Komatsu, Kazuki*; Hattori, Takanori   ; Machida, Shinichi*; Kagi, Hiroyuki*

So far, the odd hydration number has been missing in the water-rich magnesium chloride hydrate series (MgClHO). In this study, we have identified magnesium chloride heptahydrate, MgCl7HO (or MgCl7DO) which forms at high pressures and high temperatures of above 2 GPa and above 300 K, respectively. Its structure has been determined by a combination of single crystal X-ray diffraction at 2.5 GPa and 298 K and powder neutron diffraction at 3.1 GPa and 300 K. The results showed an orientational disorder of water molecules, which was also examined by the 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. The 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.