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Scientists working at Diamond on the Extreme Conditions beamline (I15) have carried out experiments looking at the properties of unusual compounds of the noble gas xenon, during which they have achieved one of the highest pressures on a condensed gas generated in the UK to date. Because conditions of extreme pressure prevail deep inside both the Terrestrial and Giant planets, much of this work is connected to an understanding of chemistry of inaccessible planetary interiors and, ultimately, the properties of warm dense matter.
Simple, ‘inert’ gas atoms can solidify, alloy or bond under extreme pressures in a diamond-anvil cell (DAC) [See Fig.1] and allows scientists to explore the forced condensation of molecular solids having extreme contrast in molecular weight and size. In this study, Prof. Andrew Jephcoat, the Principal Beamline Scientist for I15, and his colleagues, Dr Annette Kleppe and Dr Mónica Amboage, studied xenon compressed with a mixture of light gases under extremely high pressures to near 150 GigaPascals (1.5 million times atmospheric pressure).
To carry out their experiments they first used facilities at the University of Oxford, Department of Earth Sciences to load xenon-helium and xenon-hydrogen mixtures into the DAC. Pressures were determined by sensitive luminescence measurements on a micron-sized chip of ruby in the sample chamber – the red “glow” of ruby is shifted to much longer wavelengths under these conditions [See Fig.2].
Helium is the lightest noble gas and xenon one of the heaviest. The noble gases have filled electron shells, and are generally considered not to form compounds, except for the reactive halogens chlorine and fluorine. However, they are subject to weak, van der Waals bonding forces, which could allow formation of molecules and solid molecular crystals under extremely high pressures. A complex morphology of crystalgrowth patterns was observed in the DAC [See Fig.3].
The DAC was brought to Diamond’s I15 beamline for structure determination with an intense and collimated X-ray beam. In the case of the xenon-helium mix, the studies found that the xenon behaves just as it does when pure, forming crystals that are face-centred cubic (fcc) at lower pressures, changing to hexagonal close packed (hcp) above 16 GPa in a continuous transformation process. Contrary to some predictions, no Xe-He molecular compound was observed. For the xenon-hydrogen mix, the story is more complicated. Two distinct H2-Xe solids were identified depending on the initial concentration of xenon in hydrogen. The precise molecular composition and crystalline form of these new compounds is still under investigation.
“We are excited by these results for a number of reasons. First, the compounds we have identified may undergo electronic transitions at extreme conditions: For example, pure xenon is known to become a metal near 140 GPa, but the search for metallic hydrogen suggests a much higher pressure, near 400 GPa is needed. The presence of xenon atoms in hydrogen-xenon molecular compounds may create a new electronic “landscape”, modifying and weakening the strong hydrogen molecular bond. In this case we observed an unexpected softening and crossing of the H-H stretch band (vibron) [See Fig.4] that was measured by laser Raman scattering. Any study where we can change material properties opens up the possibility for new materials technologies and such work takes us into uncharted territory, far beyond the regime of chemistry that has been familiar for the past century or more [Ref. N.W. Ashcroft, “Pressing some boundaries in Mendeleev’s chart, Physics 2, 65 (2009)]. Second, obtaining resolved spectra from ruby at these conditions is difficult and depends strongly on the surrounding sample, highlighting possible differences in the accepted international pressure standards. Third, such experiments are technically demanding and the subject of keen international research. They help drive forward the capabilities of Diamond’s I15 beamline, which will, with time, give scientists working in the extreme conditions field a world-class facility.”Prof Andrew Jephcoat, Diamond Light Source
Structural and Vibrational Properties of Condensed Phases in Xenon Molecular Binary Systems: He-Xe, H2-Xe Journal of Physics: Conference Series. Andrew P Jephcoat, Mónica Amboage, Annette K Kleppe.
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