Who does not like a mystery? The universe is mysteriously rich in hydrogen. Three-quarters of its mass is hydrogen. On the other hand, our planet is mysteriously xenon-poor compared to other rocky planets such as Mars, Venus, and Mercury. In 1997, Wendel Caldwell et al. published in Science on the missing Xe. Their research combined laser-heated diamond anvil cell experiments with quantum mechanical calculations to uncover whether noble xenon is alloyed with iron in the Earth's core. No tendency was found on the part of xenon to form a metal alloy with iron under pressures to at least 100 to 150 gigapascals (GPa). In their illustration (see below) iron (blue) did "not bond" with xenon (green) even under extreme pressures.  

 

We now accept that metallization of xenon occurs at 130–150 GPa as mentioned recently by the Carnegie Institution of Washington (CIW) in Nature Chemistry. In light of the CIW approach producing positive results with xenon and hydrogen, I would suggest emphatically for us to review the previous research into the Xe-Fe system. Perhaps it is still likely that the Earth's core has served as a reservoir for primordial xenon!

 

To make another point on the pressures employed I include a phase diagram of xenon. (See below.) While the coordinates do not reach the high temperatures and pressures sought here, the notion of extreme is brought home such as 1,000 atm is only 0.1 GPa. Caldwell and colleagues were extrapolating in their thermodynamic calculations well beyond the known in this graph. This delivers us to the present topic where a CIW "study points to a new family of materials in hitherto unexplored regions of the temperature–pressure–composition space."

 

Why are we interested in the unexplored regions of temperature–pressure–composition space? Maddury Somayazulu et al. remind us of not only "the fundamental interest in forming novel, hydrogen-rich compounds as simple molecular quantum systems" but also the search for new such materials for technology use. This was my main interest in the CIW study.

We need on Earth hydrogen-rich compounds to use in a hydrogen economy. Our requirements specify practical regimes of operation for hydrogen storage and delivery. Researchers and technologists alike are joined to build a hydrogen fuel system that stores hydrogen safely and discharges hydrogen easily per demand in applications such as fuel cells or other uses.

In an elegant study that was reported online first, Maddury Somayazulu and colleagues at CIW obtained pressure-induced bonding and compound formation in xenon–hydrogen solids. What is most remarkable is the stability of the newly discovered and hydrogen-rich Xe(H2)7.

Researchers employed a diamond anvil device to squeeze together a series of gas mixtures of xenon in combination with hydrogen to high pressures. The atoms formed, at about 41,000 times the atmospheric pressure at sea level, a lattice structure rich in hydrogen but interpersed with layers of loosely bonded xenon pairs, or dimers. As the pressure increased to even much higher levels, the distances between the xenon pairs contracted to those observed in dense metallic xenon.

Here is a CIW picture of the newly discovered phenomenon. The red spheres are xenon dimers and the yellow spheres are in essence rotationally disordered (spinning) hydrogen molecules. The picture is supposed to convey two aspects, one, the dimers of xenon dumbells and, two, the packing density that occurs due to high pressures. What the researchers find is that the intermolecular forces change character gradually with increasing pressure while giving rise to novel compounds.

The picture is a structural representation of rhombohedral Xe(H2)7 showing xenon dimers coordinated to rotationally disordered hydrogen molecules and refers to the structure at about 50,000 atmospheres. The dimers exist at that pressure and all the way to 2.5 million atmospheres. The paper addresses at the end what the dimer-dimer distance would be. If that distance were below some critical value, we would have metallic xenon interspacing non-metallic hydrogen. It turns out that while the intra-dimer distance compresses rapidly, the inter-dimer distance does not. That is another mysterious behavior because hydrogen between the dimers is known to be highly compressible.

 

A structural representation of rhombohedral Xe(H₂)₇ showing xenon dimers (red) coordinated to rotationally disordered hydrogen (yellow) molecules. Photo Credit: Nature Chemistry [Historic Image]