Volume 95 Issue 7 | p. 5 | News of The Week
Issue Date: February 13, 2017 | Web Date: February 9, 2017

Helium forms stable molecules at high pressures

Findings broaden understanding of chemical reactivity, inertness, and astrochemistry
Department: Science & Technology
News Channels: Materials SCENE
Keywords: Inorganic chemistry, theoretical chemistry, helium, chemical bonding, high pressure
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.”

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Alass Salama  (Thu Feb 09 15:29:13 EST 2017)
Isn't it a state of matter so close to plasma state ?
Michael Barankin (Fri Feb 10 19:03:30 EST 2017)
No. It's electrically insulating and there is no "free electron phase" separate from the matter. It's probably a condensed phase (liquid or solid).
E. A. Henle (Wed Feb 15 18:20:49 EST 2017)
XRD shows it to be similar to fluorite; crystal structure shown at top. It's a solid.
Joseph Rubalcava  (Fri Feb 17 17:24:36 EST 2017)
well I ain't a chemist but I study the table , and in my opinion they clue u in into how it's actually made, Sven liden! The pressures! Of course there's electrons but of lithium 6.94 n they use molybdenum as a flask 95.94 to get helium isotope -3 and the temp of space is -+3 above an zero! It's been out people that went to school ! I think they add liquid nitrogen for pressure ! Just a guess idk
john tamine (Thu Feb 16 02:16:51 EST 2017)
in what way could a rigid solid crystal (with a 3-D array of repeating structural units) be considered "close" to a diffuse gas of ions and free electrons flying randomly thru mostly empty space?
Sasi (Sat Feb 11 01:40:33 EST 2017)
After formation of Na2He or some other reaction at high pressure and high temperature, What happens after it becomes normal temperature and normal pressure, it'll become inert again or stable at those medium...
E. A. Henle (Wed Feb 15 18:23:56 EST 2017)
I'm pretty sure it would gas off helium, leaving metallic sodium behind. Basically, the reaction would just reverse (as one should expect in a pressure-dependent equilibrium). If done under an oxygen atmosphere, the sodium would probably ignite.
John Doe (Sat Feb 11 12:38:54 EST 2017)
Maybe it is a kind of super state ;)
john tamine (Thu Feb 16 02:00:17 EST 2017)
would it have super delegates, too!
Noppers (Sun Feb 12 06:44:54 EST 2017)
Ron Gentry (Wed Feb 15 17:41:10 EST 2017)
It should be noted that the diatomic helium molecule was first observed in 1993 [J. Chem. Phys. 98, 3564 (1993)] and confirmed many times since. Of course it does not have a covalent bond, and it exists in only the ground vibrational and rotational states.
James Griepenburg (Wed Feb 15 18:14:24 EST 2017)
This is hard to visualize. High pressure could cause Na. to lose its electron to form a lattice ionic compound but how does the He enter the process. Wonder what happens with Na alone under those pressures. A more likely candidate in my opinion would be HeF2 Where high pressure could conceivably make a molecular bond to be of lower volume than the F2-He combination. The high IP of He would at least be partially offset by the high electron affinity of F.
john tamine (Thu Feb 16 02:28:21 EST 2017)
elemantal Na does not require pressure to form a lattice. it does so at STP. it is a prototypical example of the solid metallic state.
john tamine (Thu Feb 16 02:43:20 EST 2017)
personally, i was thrilled to read your opinion, but it's "hard to visualize" how anything can be "more likely" than established observation.
john tamine (Thu Feb 16 02:49:36 EST 2017)
not sure this should be considered a true compound. it seems more akin to a solution, comparable to the solution of H2 in Pt that occurs at ordinary temps and pressures.
john tamine (Thu Feb 16 02:50:44 EST 2017)
HeH+ is a cation, not a radical.
john tamine (Thu Feb 16 02:58:41 EST 2017)
i'm not sure "last bastion of inertness" is appropriate. the HeH+ cation (NOT radical) has been known for quite a long time. despite the unfortunate hype from the author, i personally don't see any need to re-write any textbooks over this news.
john tamine (Thu Feb 16 03:00:42 EST 2017)
a "stable compound" that exists only under extreme pressures greater than 115 GPa is not quite what the term "stable compound" connotes. under that sort of pressure, even trump supporters and trump demonstrators could be forced to remain in rigid proximity.

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