• Howard, Christopher Mckenzie

Abstract

<jats:p>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 (MgSO<jats:sub>4</jats:sub>·7H<jats:sub>2</jats:sub>O) reveals that, at ∼1.6 GPa and 293 K, it undergoes incongruent melting to form MgSO<jats:sub>4</jats:sub>·5H<jats:sub>2</jats:sub>O 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 MgSO<jats:sub>4</jats:sub>·5H<jats:sub>2</jats:sub>O has been determined at 85 K in space group <jats:italic>Pna</jats:italic>2<jats:sub>1</jats:sub> 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 MgSO<jats:sub>4</jats:sub>·5H<jats:sub>2</jats:sub>O (pentahydrite, space group P { 1}), consisting of corner-sharing MgO<jats:sub>6</jats:sub>–SO<jats:sub>4</jats:sub> ion pairs rather than infinite corner-sharing chains.</jats:p>

Topics

• impedance spectroscopy
• phase
• Nitrogen
• space group
• ion chromatography