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Chemical Bonding

Helium forms stable molecules at high pressures

Findings broaden understanding of chemical reactivity, inertness, and astrochemistry

by Mitch Jacoby
February 9, 2017 | A version of this story appeared in Volume 95, Issue 7

At high pressures, Na2He forms a stable compound with a three-dimensional checkerboard-like structure. Sodiums are the purple spheres, heliums are the green cubes, and electrons are the red regions.
the image is a depiction of the structure of Na<sub>2</sub>He.
At high pressures, Na2He forms a stable compound with a three-dimensional checkerboard-like structure. Sodiums are the purple spheres, heliums are the green cubes, and electrons are the red regions.

Chemistry textbook authors may soon have to rewrite sections covering noble gases and chemical inertness. An international research team has reported the synthesis of a helium-sodium compound that’s stable at high pressures (Nat. Chem. 2017, DOI: 10.1038/nchem.2716).

Helium’s best known feature is its unwillingness to react. With a stable closed-shell electron configuration, zero electron affinity, and an ionization energy that is higher than that of all other elements, helium defines chemical inertness.

To examine the scope of the element’s low reactivity, scientists have searched for years—theoretically and experimentally—for helium-containing molecules. But they have turned up very little: for example, unusual species such as the HeH+ radical, which is stable only in its charged form, and HHeF, a metastable compound. In contrast, helium’s noble gas cousins xenon and krypton long ago showed themselves capable of forming a variety of stable compounds.

Still, a team of researchers that includes Artem R. Oganov of Skolkovo Institute of Science & Technology and Hui-Tian Wang and Xiang-Feng Zhou of Nankai University continued searching for stable helium compounds. They used a computational strategy known as evolutionary structure prediction to scan the helium-sodium interaction landscape over an enormous range of pressures. The team concluded that Na2He should be thermodynamically stable at pressures greater than roughly 115 GPa, which is about 1 million times as high as Earth’s atmospheric pressure. Then they used a diamond anvil cell to achieve this type of pressure and synthesized the compound.

On the basis of X-ray diffraction and other methods, the team reports that Na2He adopts a structure similar to that of the mineral fluorite and is electrically insulating. The material, which remains stable up to at least 1,000 GPa, is an electride—a type of crystal containing positively charged ionic cores and electrons that function as anions. The researchers also found that Na2HeO, which they have not yet prepared, should be stable at pressures greater than 15 GPa.

“This study highlights how high pressure can be used to access compounds with novel stoichiometries and electronic structures,” says Eva Zurek, a specialist in computational chemistry at the University at Buffalo, SUNY. Na2He would never be stable under atmospheric conditions, she says, but it has been synthesized in this study at roughly 40% of the pressure present at Earth’s center. The findings broaden understanding of chemical processes that may occur at great pressures inside gas giants such as Jupiter and Saturn.

Lund University inorganic chemist Sven Lidin remarks that the implications for astronomy are clearly interesting, “but regarding our perception of chemical reactivity, this is a textbook changer.”

Earlier discoveries of other noble gas compounds made it clear that inertness is a question of reaction conditions, Lidin says. But helium has been a holdout even under extreme conditions because it clings to its inner-shell electrons tightly and will not let go. These new findings, he adds, show that in some ways, “the last bastion of inertness has finally fallen.”


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