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Publication Detail
Investigation of high-pressure planetary ices by cryo-recovery. II. High-pressure apparatus, examples and a new high-pressure phase of MgSO₄·5H₂O
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Article
  • Authors:
    Wang W, Fortes AD, Dobson DP, Howard CM, Bowles J, Hughes NJ, Wood IG
  • Publisher:
    International Union of Crystallography
  • Publication date:
    01/06/2018
  • Pagination:
    692, 705
  • Journal:
    Journal of Applied Crystallography
  • Volume:
    51
  • Issue:
    3
  • Status:
    Published
  • Print ISSN:
    0021-8898
  • Language:
    English
  • Keywords:
    planetary ices, high pressure, cryo-recovery, epsomite, magnesium sulfate pentahydrate
  • Addresses:
    Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK.
Abstract
An apparatus is described for the compression of samples to ∼2 GPa at temperatures from 80 to 300 K, rapid chilling to 80 K whilst under load and subsequent recovery into liquid nitro­gen after the load is released. In this way, a variety of quenchable high-pressure phases of many materials may be preserved for examination outside the high-pressure sample environment, with the principal benefit being the ability to obtain high-resolution powder diffraction data for phase identification and structure solution. The use of this apparatus, in combination with a newly developed cold-loadable low-temperature stage for X-ray powder diffraction (the PheniX-FL), is illustrated using ice VI (a high-pressure polymorph of ordinary water ice that is thermodynamically stable only above ∼0.6 GPa) as an example. A second example using synthetic epsomite (MgSO₄·7H₂O) reveals that, at ∼1.6 GPa and 293 K, it undergoes incongruent melting to form MgSO₄·5H₂O plus brine, contributing to a long-standing debate on the nature of the high-pressure behaviour of this and similar highly hydrated materials. The crystal structure of this new high-pressure polymorph of MgSO₄·5H₂O has been determined at 85 K in space group Pna2₁ from the X-ray powder diffraction pattern of a sample recovered into liquid nitro­gen and is found to differ from that of the known ambient-pressure phase of MgSO₄·5H₂O (pentahydrite, space group [P {\overline 1}]), consisting of corner-sharing MgO₆—SO₄ ion pairs rather than infinite corner-sharing chains.
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