DOI: 10.14469/hpc/2154Metadata

1. Na2edta*2h20
2. Na2hedp
3. Helium Compound
4. Na2haso4

Created: 2017-02-10 08:48

Last modified: 2019-02-05 16:48

License: Creative Commons: Public Domain Dedication 1.0

Funding: (none given)

Description

A CIF created from the information given in the caption to Figure 2, reproduced here Figure 2 | Crystal structure of Na2He at 300 GPa. a,b, Ball-and-stick representation (a, pink and grey atoms represent Na and He, respectively) and polyhedral representation (b), where half of the Na8 cubes are occupied by He atoms (shown as polyhedra) and half by 2e (shown as red spheres). c, Electron localization function (ELF, which measures the spatial localization of electrons) plotted in the [110] plane at 300 GPa. This structure has space group Fm-3m with lattice parameter a = 3.95 Å at 300 GPa and Na atoms occupying the Wyckoff position 8c (0.25,0.25,0.25) and He atoms occupying the 4a (0,0,0) position.

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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. Na2He is what's called an electride, which is a special type of an ionic salt-like crystal. It has a positively charged sublattice of sodium ions and another negatively.

Associated DOIs

Na2edta*2h20

Na2hedp

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Moscow/Washington, DC -- Although helium is the second most-abundant element (after hydrogen) in the universe, it doesn't play well with others. It is a member of a family of seven elements called the noble gases, which are called that because of their chemical aloofness -- they don't easily form compounds with other elements. Helium, widely believed to be the most inert element, has no stable compounds under normal conditions.

Now, an international team of researchers led by Skoltech's Prof. Artem R. Oganov (also a professor at Stony Brook University and head of Computational Materials Discovery laboratory at Moscow Institute of Physics and Technology) has predicted two stable helium compounds -- Na2He and Na2HeO. The scientists experimentally confirmed and theoretically explained the stability of Na2He. This work could hold clues about the chemistry occurring inside gas giant planets and possibly even stars, where helium is a major component. The work is published by Nature Chemistry.

The authors of the study used a crystal structure-predicting tool, the first-principles evolutionary algorithm called USPEX, to conduct a systematic search for stable helium compounds. They predicted the existence of Na2He, which was then successfully synthesized in a diamond anvil cell (DAC) experiment performed at the Carnegie Institution for Science in Washington by Prof. Alexander F. Goncharov and his colleagues. The compound appeared at pressures of about 1.1 million times Earth's atmospheric pressure and is predicted to be stable at least up to 10 million times that.

'The compound that we discovered is very peculiar: helium atoms do not actually form any chemical bonds, yet their presence fundamentally changes chemical interactions between sodium atoms, forces electrons to localize inside cubic voids of the structure and makes this material insulating,' says Xiao Dong, the first author of this work, who was a long-term visiting student in Oganov's laboratory at the time when this work was done.

Na2He is what's called an electride, which is a special type of an ionic salt-like crystal. It has a positively charged sublattice of sodium ions and another negatively charged sublattice formed of localized electron pairs. Because electrons are strongly localized, this material is an insulator, meaning that it cannot conduct the free-flowing electrons that make up an electric current.

The other predicted helium compound, Na2HeO, was found to be stable in the pressure range from 0.15 to 1.1 million atmospheres. It is also an ionic crystal with a structure similar to that of Na2He. However, in place of electron pairs, it has negatively charged oxygen in the form of O²?.

Helium Compound

'This study shows how new surprising phenomena can be discovered by combination of powerful theoretical methods and state-of-the-art experiments. It shows that very weird chemical phenomena and compounds can emerge at extreme conditions, and the role of such phenomena inside planets needs to be explored,' says Oganov.

Na2haso4

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