Crystal structure of a high-pressure phase of magnesium chloride hexahydrate determined by in-situ X-ray and neutron diffraction methods.

A high-pressure phase of magnesium chloride hexahydrate (MgCl2 ·6H2 O-II) and its deuterated counterpart (MgCl2 ·6D2 O-II) have been identified for the first time by in-situ single-crystal X-ray and powder neutron diffraction. The crystal structure was analyzed by the Rietveld method for the neutron diffraction pattern based on the initial structure determined by single-crystal X-ray diffraction. This high-pressure phase has a similar framework to that in the known ambient-pressure phase, but exhibits some structural changes with symmetry reduction caused by a subtle modification in the hydrogen-bond network around the Mg(H2 O)6 octahedra. These structural features reflect the strain in the high-pressure phases of MgCl2 hydrates.