Generated "impossible" helium connection

Sodium and helium form a salt-like crystal under high pressure

Crystal structure of the newly discovered helium compound Na2He. The green cubes represent helium atoms, the purple spheres Natium. Shown in red are localized electron pairs. © Artem R. Oganov
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"Unsocial" element tricked: researchers have for the first time produced a stable chemical compound with the noble gas helium. Previously, this element was considered too inert and stable to react with other atoms. But under extreme pressure, the chemists succeeded in producing a salt-like ionic crystal of sodium and helium. This compound is stable above 113 gigapascal pressure, according to the researchers in the journal Nature Chemistry.

Helium is the lightest among the noble gases - and one of the most "unsocial". Because unlike almost all other elements in the periodic table, helium voluntarily does not bind. Its two electrons completely fill the s orbital and make the atom "unwilling" to change it by bonding with other elements. Only under extreme conditions does helium form a so-called Efimov trimer, a loose association of three atoms.

"In recent decades, many scientists have tried to find stable helium compounds, " explain Artem Oganov of the Moscow Institute of Physics and Technology and his colleagues. But so far it has not been possible to produce stable helium molecules with true covalent bonds.

"Tricked" by high pressure

But Oganov and his colleagues have "tricked" Helium. Their idea: Under extremely high pressure, the properties of the noble gas could change so much that binding is possible. To test this, they first looked for potentially stable helium molecular structures at different pressures using an algorithm.

And indeed: "We have found a new compound: Na2He has a lower enthalpy at pressures above 160 Gigapascal than a mixture of elemental sodium and helium, " report Oganov and his colleagues. This is an indication of a stable crystal structure under these conditions. Moreover, with pressures that can be generated in the laboratory, sodium is the only element that helium uses to form such a compound. display

Ionian, salt-like crystal

But can such a helium connection also be made in reality? To test this, chemists are exposing a mixture of sodium and helium to extreme pressures in a diamond stamp press. At pressures in excess of 113 gigapascals, measurements by X-ray crystallography and Raman spectroscopy showed a change: the mixture of the two elements had become a crystalline compound.

In each empty field of the lattice formed by sodium ions (pink), there is an octentric bond (yellow) with the electron pair in the Na2He. Helium atoms are represented as white spheres. Pop Ivan Popov / Utah State University

"The connection we discovered is quite unusual, " says Oganov. In its structure, Na2He resembles an ionic, salt-like crystal. In this case, the helium atoms form a cubic lattice, similar to a 3D chessboard, in which each black field is occupied by helium. The empty fields in between are occupied by an electron pair surrounded by sodium ions.

Exotic, but a real connection

"Thus, this compound is an electron, a crystal of positively charged ion nuclei, in which strongly localized valence electrons play the role of anions, " the researchers explain. Only the presence of helium leads to this localization of the electrons and makes the exotic crystal a non-conductor.

Although Na2He is exotic, it is still a high-pressure stable, real link between helium and another element, as the scientists emphasize. That such an "impossible" connection exists, however, seemed so incredible even to the reviewers of the journal that it took two years for the journal to pass the peer review and be published.

"Our study demonstrates that very bizarre phenomena and compounds can arise under extreme conditions, " says Oganov. He and his colleagues even used their algorithm to pick up a second high pressure stable helium compound. Na2HeO resembles the Na2He in its crystal structure, except that now in each of the fields with the electron pair there is an O 2- ion. However, the existence of Na2HeO still has to be proven experimentally. (Nature Chemistry, 2017; doi: 10.1038 / nchem.2716)

(Moscow Institute of Physics and Technology, 07.02.2017 - NPO)