As a the least reactive member of the least reactive group of chemical elements in the periodic table, it is fair to say that helium is extremely inert. To many of us, it is perhaps best known as “the gas that fills balloons” (and in this context, its inertness is a rather useful property…). As such, it very rarely forms compounds with any other element; it is perfectly happy going solo. A few such examples are known, however these are either thermodynamically unstable or are unusual species referred to as “inclusion compounds”, in which helium does not interact, in an electronic sense, with the other element(s) present.
However, pressure has a significant effect on the chemistry of elements, with sodium being a notable and well-studied example, and researchers now claim that they have managed to successfully form a compound of helium and sodium that is stable at high pressure. To be more specific, the pressures are >113 GPa, which is more than 1 million times the value of atmospheric pressure, i.e. extremely high. The researchers first used a complex evolutionary algorithm to perform a computational search for sodium-helium compounds that might exhibit stability at high pressure. Having thus identified the compound Na2He (disodium helide? name suggestions welcome) as a possible candidate, they used specialised high-pressure apparatus to show that this compound could indeed be synthesised.
Structural analysis of this species showed that the presence of helium significantly altered the electronic structure of the material as compared to that of (high-pressure) sodium alone. Thus, in contrast to all prior helium “compounds”, this species is thermodynamically stable, at least at high pressure, and contains significant interactions between the two elements. The authors also predict the existence of a second stable helium species (Na2HeO), although they have yet to verify the existence of this experimentally.
Whilst some will still question whether the researchers have genuinely made the compound they claim – understandably, chemical analysis is somewhat challenging at 1,000,000 atm pressure – this discovery potentially changes the field of helium chemistry and could increase our understanding of chemical processes that may occur inside giant planets. Yet moreover, irrespective of any potential applications, this is simply rather cool.