Crystal structure of a high-pressure phase of magnesium chloride hexahydrate determined byin-situX-ray and neutron diffraction methods

Abstract

A high-pressure phase of magnesium chloride hexahydrate (MgCl 2 Á6H 2 O-II) and its deuterated counterpart (MgCl 2 Á6D 2 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(H 2 O) 6 octahedra. These structural features reflect the strain in the high-pressure phases of MgCl 2 hydrates.Program(s) used to refine structure: GSAS for MgCl2_6D2OII; SHELXL2014 (Sheldrick, 2015) for MgCl2_6H2OII.

Terahertz Signatures of Hydrate Formation in Alkali Halide Solutions

“…Studies of salt hydrates conducted thus far include a number of different techniques, such as nuclear magnetic resonance (NMR), , X-ray diffraction (XRD), − and neutron diffraction, − where the crystal structures, the structural environments, and the ratio of ions and water molecules of the salt hydrates can be evaluated. Infrared (IR) spectroscopy has also been widely utilized for investigations of hydrate formation in various aqueous salt solutions by cooling. − As it provides sensitivity to the high-frequency O–H vibrational information, IR spectroscopy could be a useful tool in the characterization of structural changes during hydrate formation. − , With the recent advent of a new spectroscopic technique, terahertz time-domain spectroscopy (THz-TDS), the far-infrared lower-frequency properties of materials in the range below 3.5 THz can be readily obtained. − Unlike IR spectroscopy, terahertz spectra are more sensitive to molecular conformational and structural changes. , In 2018, Ajito and coauthors captured the appearance of new terahertz absorption peaks at around 1.5–1.6 and 2.3–2.4 THz during the formation process of NaCl hydrates, and these peaks were assigned to the intermediate of NaCl hydrates .…”

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

“…The melting temperature of ice VII in the solution decreased by adding the alcohol mixture. In addition, our previous report about MgCl2•6H2O-II showed almost saturated MgCl2 aqueous solution forms MgCl2•6H2O-II over 0.9 GPa at 323 K before ice VI crystallisation 23 . This means that the crystallisation of other hydrates was disturbed by the existence of alcohol.…”

“…Its hydration number varies by temperatures and its hydration/dehydration phenomena are studied for views of application towards heat storage material and elucidation of its natural existence form. There are two high-pressure phases of MgCl2 hydrates: a high-pressure phase of MgCl2 hexahydrate (MgCl2•6H2O-II) over 0.6 GPa 23 and MgCl2•10 H2O at 2.5 GPa 5 . Interestingly, most of the MgCl2 hydrates, except for n = 1, have even hydration numbers up to n = 12 including the high-pressure phases.…”

“…Interestingly, most of the MgCl2 hydrates, except for n = 1, have even hydration numbers up to n = 12 including the high-pressure phases. Their crystal structures including H-positions have been determined up to now 5,[19][20][21][22][23] . Some hints why even hydration numbers are favoured in MgCl2 hydrates were provided from the changes in the packing scheme during the dehydration of bischofite (MgCl2•6H2O) 19 for lower hydration numbers (n ≤ 6).…”

Phase Change Materials for Renewable Energy Storage at Intermediate Temperatures

“…Schematic examples of various intermolecular interactions that can contribute to the heat of fusion, created using crystallographic data from representative materials in the literature. (A) hydrogen bonding in the crystal structure of d -mannitol, (B) metal–hydrate bonding in the crystal structure of magnesium chloride hexahydrate, (C) Coulombic interactions in the crystal structure of ammonium palmitate, and (D) van der Waals forces between alkyl chains of ammonium palmitate (produced using data from refs − ).…”