Helium compound produced, closing the last gap

[mfb]

Scientists created Na₂He in a diamond anvil cell. It is stable only above 113 GPa - but it forms at temperatures up to 1500 K, it is not something exotic that breaks as soon as you look at it.

Na₂He is a crystal with an unusual type of bond - it does not have regular bonds, but it has electrons in free spaces binding everything together. Removing the helium would break the crystal - the helium atoms have an active role, they are not just filling gaps of a sodium crystal.

The team also predicts the existence of Na₂HeO above 15 GPa. Which is still 100,000 the atmospheric pressure...

Helium is the last noble gas, and the last stable element in general, where a stable compound has been synthesized.

News at sciencealert.com
http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2716.html

 

[DrDu]

Fascinating paper. I cannot restrain from comparing it to the paper which appeared some weeks ago, where the observation of metallic hydrogen was anounced. The latter paper contained in the introduction some wild speculations about metallic hydrogen being eventually metastable at room temperature and eventually even usefull as a fuel for spacecraft , while the new paper on Na2He reported instead estimates on the lower bound of kinetic stability of the compound based on simulated phonon dispersion relations.

 

[TeethWhitener]

Na2He is a crystal with an unusual type of bond - it does not have regular bonds, but it has electrons in free spaces binding everything together. Removing the helium would break the crystal - the helium atoms have an active role, they are not just filling gaps of a sodium crystal.

It's interesting that they specifically mention that Na₂He isn't an inclusion compound (full disclosure: I used to work on endohedral fullerenes, including He@C₆₀, so I might be a little biased). I suppose you could call it an "exclusion compound," based on their calculations. The wavefunction of the electrons in the half-filled valence s-band in simple cubic sodium is smeared out across the entire crystal. Since helium has basically no electron affinity, introducing helium at every other octahedral hole (hole in the structural sense, not hole as in electron-hole pair) in the lattice expels the sodium valence electron density from those holes, essentially confining the valence electrons to the unoccupied octahedral holes. It's this confinement effect that causes the band gap to appear and makes Na₂He look more like a salt than a metal.

Helium is the last noble gas, and the last stable element in general, where a stable compound has been synthesized.

I wasn't aware that any stable neon compounds had been made (except, of course, for Ne@C₆₀ ).

 

[Ygggdrasil]

It's interesting that they specifically mention that Na₂He isn't an inclusion compound (full disclosure: I used to work on endohedral fullerenes, including He@C₆₀, so I might be a little biased). I suppose you could call it an "exclusion compound," based on their calculations.

The compound is an example of an electride, in which electrons make up the anions in a crystal lattice.

Here's a nice popular press article which expands upon some of the press releases and tells the story of how the research came about, the initial arxiv paper in 2013 and the three-year (!) journey through peer-review to publication in Nature Chemistry. http://www.forbes.com/sites/carmendrahl/2017/02/07/helium-sodium-compound-pressure/ [edit: just realized I forgot to insert the link]

Fascinating paper. I cannot restrain from comparing it to the paper which appeared some weeks ago, where the observation of metallic hydrogen was anounced. The latter paper contained in the introduction some wild speculations about metallic hydrogen being eventually metastable at room temperature and eventually even usefull as a fuel for spacecraft , while the new paper on Na2He reported instead estimates on the lower bound of kinetic stability of the compound based on simulated phonon dispersion relations.

It is interesting to compare the two papers in light of the criticism of the metallic hydrogen paper. That the Na2He paper took three years to get through peer review and ended up in Nature Chemistry whereas the metallic hydrogen paper got published relatively quickly in Science (the arxiv pre-print was first published October 2016) perhaps speaks to the differences between the two publishers.

 

[mfb]

I wasn't aware that any stable neon compounds had been made (except, of course, for Ne@C₆₀ ).

Hmm, looks like they are not that stable.
Van-der-Waals molecules exist, but those break quite easily.

 

[Comeback City]

Na2He is a crystal with an unusual type of bond - it does not have regular bonds, but it has electrons in free spaces binding everything together. Removing the helium would break the crystal - the helium atoms have an active role, they are not just filling gaps of a sodium crystal.

Are the electrons in this type of molecule immobile such as those of an Ionic compound, or mobile such as a metallic compound?

 

[Ygggdrasil]

Are the electrons in this type of molecule immobile such as those of an Ionic compound, or mobile such as a metallic compound?

The electrons are immobile as if they were the anions in an ionic solid. Thus, as reported in the paper, the material acts as an insulator.