A team of international researchers succeeded in creating a compound of sodium and helium which is stable from a thermodynamic point of view. They published their results in the journal Nature Chemistry.
The compound they synthesized, Na2He, has a structure of a fluorite type and is stable at high pressures and temperature. Helium is the second most abundant element in the universe, immediately after hydrogen. It is present in quite high quantities in stars or in gas planets such as Jupiter and Saturn.
Also, helium, together with neon, is one of the most inert element in the periodic table. This is given by the fact that the helium atom has the highest ionization potential but its electron affinity is zero. Its electron configuration is extremely stable and closed. Thus, it leaves no space for connections.
Alex Boldyrey, professor at Utah State University, is one of the co-authors of the study. Given the properties of helium, the scientists started an extensive search for possible stable compounds that could contain the element. They tried to form compounds with several elements, including hydrogen, oxygen, sodium, potassium, magnesium and so on, and see how they acted.
The results showed that the only element helium could form a compound with at the pressure typical for static experiments was sodium. They found that the compound Na2He has a lower enthalpy than a basic mixture of Na and He. This occurs at pressures above 160 gigapascals (GPa).
Other calculations showed that Na2He is stable above 100 GPa. This means that, after the formation of the compound, it can be decreased to 100 GPa, but now lower, since then it might become unstable. That was the moment when they proved that sodium and helium can bond at high pressures.
Such a discovery was quite unexpected. Boldyrey admitted that it was so unexpected that they needed about two years to convince scientists and other researchers that such a combination was possible.
The bond that is actually responsible for the functionality of this compound is an eight-center two-electron bond present in the sodium cubes.
This magical compound might not be the only one that contains helium. The researchers suspect that the existence of Na2HeO is also possible with a similar structure and at a pressure of 15 GPa.
Image Source: Wikimedia Commons