Investigation of high-pressure planetary ices by cryo-recovery. II. High-pressure apparatus, examples and a new high-pressure phase of MgSO4·5H2O

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.