Single Crystal Growth of High-Pressure Phases of Ice and Salt Hydrate Far Below the Original Melting Point by Mixing Alcohol: Crystal Structure Determination of Magnesium Chloride Heptahydrate

An crystal-growth technique for single crystal x-ray structure analysis of high-pressure forms of hydrogen-bonded crystals is proposed. We used alcohol mixture (methanol: ethanol = 4:1 in volumetric ratio), which is a widely used pressure transmitting medium, inhibiting the nucleation and growth of unwanted crystals. In this paper, two kinds of single crystals which have not been obtained using a conventional experimental technique were obtained using this technique: ice VI at 1.99 GPa and MgCl₂·7H₂O at 2.51 GPa at room temperature. Here we first report the crystal structure of MgCl2·7H2O. This technique simultaneously meets the requirement of hydrostaticity for high-pressure experiments and has feasibility for further in-situ measurements.