Pressure-induced bonding and compound formation in xenon–hydrogen solids

Abstract

Closed electron shell systems, such as hydrogen, nitrogen or group 18 elements, can form weakly bound stoichiometric compounds at high pressures. An understanding of the stability of these van der Waals compounds is lacking, as is information on the nature of their interatomic interactions. We describe the formation of a stable compound in the Xe-H(2) binary system, revealed by a suite of X-ray diffraction and optical spectroscopy measurements. At 4.8 GPa, a unique hydrogen-rich structure forms that can be viewed as a tripled solid hydrogen lattice modulated by layers of xenon, consisting of xenon dimers. Varying the applied pressure tunes the Xe-Xe distances in the solid over a broad range from that of an expanded xenon lattice to the distances observed in metallic xenon at megabar pressures. Infrared and Raman spectra indicate a weakening of the intramolecular covalent bond as well as persistence of semiconducting behaviour in the compound to at least 255 GPa.

High-pressure studies with x-rays using diamond anvil cells

“…Unlike the conventional diamond structure, this new allotrope consists of an interesting zeolite-type structure, which is comprised of channels with five-, six-and eight-membered silicon rings. Another interesting example of unprecedented reactivity and bonding is the reactivity of xenon at HP, forming novel xenonhydrogen compounds [99,584]. Mixtures of Xe-H2 at modest pressures (4.9, 5.4 GPa) were found to sequentially form high-hydrogen content xenon complexes Xe(H2)7 and Xe(H2)8 with intact H2 groups (Fig.…”

“…Chemical reactivity needs to be carefully considered in selecting these materials, including possible reactivity changes at HP [98,99]. Most HP experiments are conducted on polycrystalline samples.…”

New high-pressure van der Waals compound Kr(H2)4 discovered in the krypton-hydrogen binary system

“…The Xe sublattice, as determined by X-ray diffraction, consists of Xe-Xe pairs oriented along the c axis of the unit cell. Raman and IR spectra composed of multiple H 2 vibrons confirmed the presence of molecular hydrogen in Xe(H 2 ) 8 and support that its structure can be viewed as a tripled, solid H 2 hcp lattice modulated by layers consisting of Xe dimers 5 . IR spectra showed the Xe(H 2 ) 8 compound to remain an insulator to at least 255 GPa 5 while its calculated metallization pressure is around 250 GPa 14 .…”

“…Xe(H 2 ) 8 was found to be stable above 5.4 GPa with a hexagonal crystal structure 5 . The Xe sublattice, as determined by X-ray diffraction, consists of Xe-Xe pairs oriented along the c axis of the unit cell.…”

“…The first reported, pressure-stabilized, stoichiometric van der Waals (vdW) solid was reported in the He-N 2 system, He(N 2 ) 11 1 . Since then a variety of stoichiometric vdW compounds have been synthesised under pressure for example, in the systems He-Ne 2 , Ar-H 2 3 , CH 4 -H 2 4 , and most recently Xe-H 2 5,6 . The study of hydrogen-rich binary mixtures has attracted particular attention because they are of interest to planetary science and astronomy, are potentially relevant technologically for hydrogen storage 7 , and, last but not least, offer the tantalising prospect to promote the pressure-induced insulator-metal transition in hydrogen at a lower pressure than required for pure hydrogen (e.g., 3,8).…”