• Na2he
• Na2edta
• Na2haso4
• Disodium Helide

TIL Helium compounds actually exist. They were only discovered in the last 50 years, and are only stable at pressures hundreds of thousands of times greater than atmospheric pressure. For example, Na2He, a compound of helium and sodium, is stable at 113 gigapascals (1.115.10^6 atm). Noble Gases Noble gases—helium, neon, argon, krypton, xenon, and radon—have long been believed to be the least reactive elements on the periodic table. Helium’s composition in particular, with its.

Disodium helide^[1] (Na₂He) is a compound of helium and sodium that is stable at high pressures above 113 gigapascals (1,130,000 bar). It was first predicted^[2] using USPEX code and then synthesised in 2016.^[3]

Synthesis[edit]

Na₂He was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa. This means it should be possible to form at the higher pressure and then decompress to 100 GPa, but below that it would decompose. Compared with other binary compounds of other elements and helium, it was predicted to be stable at the lowest pressure of any such combination. This also means, for example, that a helium-potassium compound is predicted to require much higher pressures of the order of terapascals.

The material was synthesized by putting tiny plates of sodium in a diamond anvil cell along with helium at 1600 bar and then compressing to 130 GPa and heating to 1,500 K with a laser.^[3] Disodium helide is predicted to be an insulator and transparent.^[3] At 200 GPa the sodium atoms have a Bader charge of +0.599, the helium charge is −0.174, and the two-electron spots are each near −0.511.^[3] So this phase could be called disodium helium electride. Disodium helide melts at a high temperature near 1,500 K, much higher than the melting point of sodium. When decompressed, it can keep its form as low as 113 GPa.^[3] As pressure increases, the sodium is predicted to gain more positive charge, the helium to lose negative charge and the free electron density to increase. Energy is compensated by the relative shrinking of the helium atoms and the space for electrons.^[4]

Structure[edit]

Disodium helide has a cubic crystal structure, resembling that of fluorite. At 300 GPa the edge of a unit cell of the crystal has a = 3.95 Å. Each unit cell contains four helium atoms on the centre of the cube faces and corners, and eight sodium atoms at coordinates halfway between the center and each corner. Double electrons (2e⁻) are positioned on each edge and the centre of the unit cell.^{[note 1]} Each pair of electrons is spin paired. The presence of these isolated electrons makes this an electride. The helium atoms do not participate in any bonding; however, the electron pairs can be considered as an eight-centre two-electron bond.

Footnotes[edit]

1. ^Each face is shared by two cells, each edge is shared by four cells, and each corner is shared by eight cells.

References[edit]

1. ^'Under Pressure, Helium Stops Being a Bystander'. insidescience.org. 2018-03-28. Retrieved 2020-11-14. 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.
2. ^Saleh, Gabriele; Dong, Xiao; Oganov, Artem; Gatti, Carlo; Qian, Guang-rui; Zhu, Qiang; Zhou, Xiang-Feng; Wang, Hiu-tian (5 August 2014). 'Stable Compound of Helium and Sodium at High Pressure'. Acta Crystallographica Section A. 70 (a1): C617–C617. arXiv:1309.3827. doi:10.1107/S2053273314093826.
3. ^ ^abcdeDong, Xiao; Oganov, Artem R.; Goncharov, Alexander F.; Stavrou, Elissaios; Lobanov, Sergey; Saleh, Gabriele; Qian, Guang-Rui; Zhu, Qiang; Gatti, Carlo; Deringer, Volker L.; Dronskowski, Richard; Zhou, Xiang-Feng; Prakapenka, Vitali B.; Konôpková, Zuzana; Popov, Ivan A.; Boldyrev, Alexander I.; Wang, Hui-Tian (6 February 2017). 'A stable compound of helium and sodium at high pressure'. Nature Chemistry. 9 (5): 440. arXiv:1309.3827. Bibcode:2017NatCh...9..440D. doi:10.1038/nchem.2716. PMID28430195.
4. ^Wang, Hui-Tian; Boldyrev, Alexander I.; Popov, Ivan A.; Konôpková, Zuzana; Prakapenka, Vitali B.; Zhou, Xiang-Feng; Dronskowski, Richard; Deringer, Volker L.; Gatti, Carlo; Zhu, Qiang; Qian, Guang-Rui; Saleh, Gabriele; Lobanov, Sergey; Stavrou, Elissaios; Goncharov, Alexander F.; Oganov, Artem R.; Dong, Xiao (May 2017). 'A stable compound of helium and sodium at high pressure - Supplementary Information table 5'(PDF). Nature Chemistry. 9 (5): 440–445. Bibcode:2017NatCh...9..440D. doi:10.1038/nchem.2716. ISSN1755-4349. PMID28430195.

USU chemists, from left, Professor Alex Boldyrev and doctoral student Ivan Popov, are among an international team that published pioneering findings about helium in the Feb. 6, 2017, issue of 'Nature Chemistry.'

Can helium bond with other elements to form a stable compound? Students attentive to Utah State University professor Alex Boldyrev’s introductory chemistry lectures would immediately respond “no.” And they’d be correct – if the scholars are standing on the Earth’s surface.

Na2he

But all bets are off, if the students journey to the center of the Earth, à la Jules Verne’s Otto Lidenbrock or if they venture to one of the solar system’s large planets, such as Jupiter or Saturn.

“That’s because extremely high pressure, like that found at the Earth’s core or giant neighbors, completely alters helium’s chemistry,” says Boldyrev, faculty member in USU’s Department of Chemistry and Biochemistry.

It’s a surprising finding, he says, because, on Earth, helium is a chemically inert and unreactive compound that eschews connections with other elements and compounds. The first of the noble gases, helium features an extremely stable, closed-shell electronic configuration, leaving no openings for connections.

Further, Boldyrev’s colleagues confirmed computationally and experimentally that sodium, never an earthly comrade to helium, readily bonds with the standoffish gas under high pressure to form the curious Na2He compound. These findings were so unexpected, Boldyrev says, that he and colleagues struggled for more than two years to convince science reviewers and editors to publish their results.

Persistence paid off. Boldyrev and his doctoral student Ivan Popov, as members of an international research group led by Artem Oganov of Stony Brook University, published the pioneering findings in the Feb. 6, 2017, issue of Nature Chemistry. The USU chemists’ participation in the project was supported by the National Science Foundation and the Ministry of Education and Science of the Russian Federation.

Boldyrev and Popov’s role in the project was to interpret a chemical bonding in the computational model developed by Oganov and the experimental results generated by Alexander Goncharov of the Carnegie Institution of Washington. Initially, the Na2He compound was found to consist of Na8 cubes, of which half were occupied by helium atoms and half were empty.

“Yet, when we performed chemical bonding analysis of these structures, we found each ‘empty’ cube actually contained an eight-center, two-electron bond,” Boldyrev says. “This bond is what’s responsible for the stability of this enchanting compound.”

Na2edta

Their findings advanced the research to another step.

“As we explore the structure of this compound, we’re deciphering how this bond occurs and we predicted that, adding oxygen, we could create a similar compound,” says Popov, who is one of two scholars named 2017 College of Science PhD Researcher of the Year.

Such knowledge raises big questions about chemistry and how elements behave beyond the world we know. Questions, Boldyrev says, Earth’s inhabitants need to keep in mind as they consider long-term space travel.

“With the recent discovery of multiple exoplanets, we’re reminded of the vastness of the universe,” he says. “Our understanding of chemistry has to change and expand beyond the confines of our own planet.”

Additional authors on the paper include researchers from China’s Nankai University, Center for High Pressure Science and Technology, Chinese Academy of Sciences, Northwestern Polytechnical University, Xi’an and Nanjing University; Russia’s Skolkovo Institute of Science and Technology, Moscow Institute of Physics and Technology, Sobolev Institute of Geology and Mineralogy and RUDN University; the Carnegie Institution of Washington, Lawrence Livermore National Laboratory, Italy’s University of Milan, the University of Chicago and Germany’s Aachen University and Photo Science DESY.

Related Links
“USU Chemists Observe Molecular ‘Drum’ that Beats Coordination Record,” Utah State Today
USU Department of Chemistry and Biochemistry
USU College of Science

Contacts: Alexander Boldyrev, 435-797-1630, a.i.boldyrev@usu.edu
Ivan Popov, 435-512-4261, vanekpopov@gmail.com

Writer: Mary-Ann Muffoletto, 435-797-3517, maryann.muffoletto@usu.edu

Image depicting structure formed by helium and sodium. On Earth, helium doesn't bond with other elements. Yet, under high pressure, such as conditions found at the Earth’s core or on larger planets, it does, say USU researchers. Credit: Ivan Popov.

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Na2haso4

Disodium Helide

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