Investigation of high-pressure planetary ices by cryo-recovery. II. High-pressure apparatus, examples and a new high-pressure phase of MgSO4·5H2O.
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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 nitro-gen 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 nitro-gen and is found to differ from that of the known ambient-pressure phase of MgSO4·5H2O (pentahydrite, space group ), consisting of corner-sharing MgO6-SO4 ion pairs rather than infinite corner-sharing chains.
SUBMITTER: Wang W
PROVIDER: S-EPMC5988006 | biostudies-literature | 2018 Jun
REPOSITORIES: biostudies-literature
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