Disodium helide

Disodium helide (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 using USPEX code and then synthesised in 2016.[1]

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. So that for example a helium-potassium compound is predicted to require much higher pressures of the order of terapascals.

Disodium helide has a cubic crystal structure, resembling 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 a quarter cell in from each face. 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.

The material was synthesised in a diamond anvil cell at 130 GPa heated to 1,500 K with a laser.[1] Disodium helide is predicted to be an insulator and transparent.[1] The sodium atoms have a Bader charge of +0.6, the helium charge is -0.15 and the two-electron spots are -1.1.[1] So this phase could be called disodium helium electride. The solid is an electrical insulator and is predicted to be transparent. 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.[1]

References

  1. 1 2 3 4 5 Dong, 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. PMID 28430195.

Footnotes

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


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