Disodium helide

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Sodium helide
)
Disodium helide
Identifiers
3D model (
JSmol
)
  • InChI=1S/He.2Na
    Key: JTZHEFJRNNBIOK-UHFFFAOYSA-N
  • [Na].[Na].[He]
Properties
HeNa2
Molar mass 49.982141 g·mol−1
Structure[1]
Fluorite, cF12
Fm3m, #225
a = 3.95 Å at 300 GPa
Related compounds
Other cations
Lithium helium
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Disodium helide[2] (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[3] using the USPEX crystal structure prediction algorithm and then synthesised in 2016.[4]

Synthesis

Na2He was predicted to be

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

Bader charge of +0.599, the helium charge is −0.174, and the two-electron spots are each near −0.511.[4] 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.[4] 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.[1]

Structure

Disodium helide has a

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. Electron pairs (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.[4]

Footnotes

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

References

  1. ^
    S2CID 20459726
    .
  2. ^ "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.
  3. .
  4. ^ .