Hydrogen rich compound Ar(H2)2 to 358 GPa

Caption: A hydrogen rich compound, Ar(H2)2, shows exceptional stability to ultra-high pressures. Lattice symmetry remains unchanged to 265 GPa by X-ray diffraction. Raman spectroscopy shows persistence of H2 units up to 358 GPa. Optical absorption spectroscopy suggests that Ar(H<sub>2</sub>)2 remains as an insulator up 358 GPa. The results show that adding Ar in the form of Ar(H<sub>2</sub>)<sub>2</sub> does not favor molecular dissociation of H<sub>2</sub> units and metallization of hydrogen.
Caption: A hydrogen rich compound, Ar(H2)2, shows exceptional stability to ultra-high pressures. Lattice symmetry remains unchanged to 265 GPa by X-ray diffraction. Raman spectroscopy shows persistence of H2 units up to 358 GPa. Optical absorption spectroscopy suggests that Ar(H2)2 remains as an insulator up 358 GPa. The results show that adding Ar in the form of Ar(H2)2 does not favor molecular dissociation of H2 units and metallization of hydrogen.

It has been predicted that hydrogen-rich material may promote metallization of hydrogen through chemical pressure imposed by the foreign atoms or molecules. A group utilizing HPCAT’s facility studied a hydrogen rich compound, Ar(H2)2, to ultrahigh pressures over three megabars. Ar(H2)2 is a typical van der Waals compound, in which Ar atoms and H2 molecules are ‘glued’ together by the London dispersion forces, leaving the H2 units preserved. It thus provides an opportunity to explore its crystal structure (due to the presence of heavier X-ray scatter from Ar), molecular vibrational properties (by Raman), and electronic properties (by optical absorption) at high pressures to help understand the roles of the weakly bounded ‘impurity’, Ar, in the behavior of the H2 sublattice, including addressing questions such as the stability and its crystal structure of Ar(H2)2 at ultrahigh pressures,  and whether Ar(H2)2 can promote molecular dissociation of H2 (leading to metallization) at very high pressure. By applying the X-ray beam with high brilliance and sharp focus at beam lines 16-ID-B and 13-ID-D, the group has measured the X-ray diffraction (XRD) data of Ar(H2)2 up to 265 GPa, a record high pressure for studying hydrogen-rich materials using XRD. The molecular vibrational properties and electronic properties were measured by an advanced confocal micro-Raman system with 660 nm excitation to 358 GPa. Up to the highest pressure of this study, the team’s results suggest that Ar does not facilitate the molecular dissociation and bandgap closure of H2. In contrast, it appears to work in the opposite way, suggesting a negative chemical pressure on H2 molecules brought about by doping of Ar. The new work clarifies previous controversies in crystal structures of Ar(H­2)2 at very high pressure, and provides a solid basis for future searches of hydrogen-rich materials which may facilitate metallization of hydrogen. More details in Ji et al, PNAS, doi:10.1073/pnas.1700049114