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.
Highlights
In an accompanying paper (Wood et al, 2018) we describe a new low-temperature stage for X-ray powder diffraction operating at temperatures (T) 40 T 315 K
Such materials fall into two classes: (i) those which are thermodynamically stable at atmospheric pressure (P), and (ii) those, such as high-pressure ‘planetary ices’, which are truly stable only at high P, but which may be recovered metastably to atmospheric pressure by quenching
We describe a highpressure apparatus for the synthesis of planetary ices in the range 0 P 2 GPa and 80 T 310 K, rapid chilling of the samples to 80 K whilst under load, and their recovery into liquid nitrogen
Summary
In an accompanying paper (Wood et al, 2018) we describe a new low-temperature stage (the PheniX-FL) for X-ray powder diffraction operating at temperatures (T) 40 T 315 K. A unique feature of this apparatus is that samples may be introduced into (and removed from) the stage at any temperature in the range 80–300 K and materials that are not stable at room temperature can be readily examined Such materials fall into two classes: (i) those which are thermodynamically stable at atmospheric pressure (P), and (ii) those, such as high-pressure ‘planetary ices’, which are truly stable only at high P, but which may be recovered metastably to atmospheric pressure by quenching (for example into liquid nitrogen). 51, 692–705 research papers pressure cell, removing factors such as the substantial beam attenuation and parasitic scattering from the high-pressure environment and (especially on neutron powder diffractometers with fixed detectors) geometric restrictions on the accessible data This procedure allows collection of the highest-resolution data, with good counting statistics, to be more accomplished, thereby facilitating, for example, the determination of unknown crystal structures or accurate measurements of physical properties. At the top end of the capsule is another hardened steel plug 10 mm in length (of which 8 mm fits inside the sample capsule) and a smaller annular gasket to prevent extrusion of the PTFE
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