Under Pressure, Helium Stops Being a Bystander
(Inside Science) -- Helium is the most chemically inert element in the universe, but last year, scientists proved it could successfully form a stable compound with another element. Now researchers suggest they know why -- helium can act much like a peacekeeper, helping otherwise unruly atoms keep civil. These new findings also suggest that helium may form compounds more often than previously thought, including perhaps deep within Earth.
Helium is second only to hydrogen as the most common element in the universe, existing in abundance within stars and gas giants. However, whereas hydrogen bonds easily with other elements to form compounds such as water, helium is a member of a group of elements known as the noble gases that were long believed to not react with other elements.
Over the decades, researchers did find a few instances where helium could form highly unstable compounds with other elements. Then, in 2017, researchers synthesized a stable compound from helium and sodium known as disodium helide under the kinds of high pressures seen within gas giants, suggesting this compound might be found in nature and not just in labs.
Still, while scientists had experimentally created a stable helium compound, much remained unknown about how it could form, given helium's inertness, said study co-author Maosheng Miao, a chemist at California State University, Northridge. Now Miao and his colleagues believe they have found the explanation, and suggest that many other helium compounds are possible as well, findings they detailed online March 5 in the journal Nature Communications.
The scientists developed computer models of compounds made of ions, which are electrically charged atoms or molecules. Stable ionic compounds are often made of balanced mixes of negative and positive ions -- for instance, table salt is made of positively charged sodium ions bound to negatively charged chlorine ions.
However, in scenarios with unequal amounts of negative and positive ions -- for instance, in a compound called dilithium oxide that contains two positive lithium ions for every negative oxygen ion -- the two positive ions are both attracted to the same negative ion in the middle, but also repel each other for having the same charge. It's usually stable, but adding pressure can change the situation. "When you squeeze them by putting them under pressure they would repel each other even more," leading to unstable compounds, said study co-author Eva Zurek, a theoretical chemist at the University at Buffalo in New York.
The researchers found that when helium atoms enter these imbalanced and high-pressure scenarios, they can slip into the spaces between the more abundant ions. As such, the helium can increase the distance between these ions so they do not repel each other so much, while at the same time not chemically interacting with any other ions. In this way, helium acts like a nanny placed between kids in the back of a car, Miao said -- the helium sits between ions that do not ordinarily get along and helps keep the peace. The resulting solid compounds "are stable even though chemical bonds are not formed," Zurek said.
This model can readily explain prior work that suggested that disodium oxyhelide, a compound consisting of two positively charged sodium ions, one negatively charged oxygen ion and one neutral helium atom, is stable. However, the explanation for the existence of disodium helide, which is made of two positively charged sodium ions and one helium atom, is more complicated, since it does not possess two different kinds of ions. The key to understanding disodium helide is that at very high pressures, electrons get pushed off sodium atoms and form pairs. This leads to an imbalanced scenario with two positively charged sodium ions for every negatively charged electron pair, a situation that helium can go on to stabilize, Zurek said.
"It's very exciting work -- they have been able to explain these new helium compounds," said crystallographer Artem Oganov at the Skolkovo Institute of Science and Technology in Moscow, who helped created the first stable helium compound.
These findings suggest that helium is prone to interact with a broad range of ionic compounds at the kinds of pressures found in Earth's interior. As such, they suggested that far more helium might be stored in the planet's mantle layer than previously thought. "New chemical compounds that we previously did not consider may be important constituents of the interior of the Earth and other planets," Zurek said.
"These findings open a whole new Pandora's box when it comes to the implications for Earth's interior -- when you renounce the idea of absolute inertness, you have to revisit a lot of models regarding helium stored in Earth," Oganov said.
This potential abundance of helium deep within Earth may prove good news, since helium is scarce in Earth's crust. Helium shortages also loom because of the extensive use of the noble gas in everything from party balloons to cooling superconducting magnets, as well as the fact that it is so light, "it continuously escapes into outer space," Miao said. If helium-loaded minerals from Earth's mantle were brought up to the surface, "more likely than not, the helium could diffuse out, and we could trap or contain it," Zurek said.
Miao also noted that helium could help manufacture novel materials. For instance, one group recently suggested that helium and nitrogen could form a compound under high pressures, and when the compound was returned to normal pressure, the helium would diffuse out, leaving behind solids made of pyramids of nitrogen atoms that could serve as unprecedentedly powerful fuels or explosives. "Helium may have a chemistry that is far richer than we originally thought," Miao said.