Helium as the most inert elements in the periodic table, under atmospheric conditions, helium is widely believed that it is impossible to form compounds. The International Research Group, which was attended by the Beijing High Voltage Science Center and Nankai University, discovered sodium-helium compounds under high pressure conditions. This breakthrough finding has aroused strong repercussions in academia. The results were published in the February 6 issue of "Nature - Chemistry".

The second element on the periodic table He is a very magical element. It is the second element of the universe content, is one of the six rare gas elements. He has the largest ionization energy of all elements, which means that He is the most difficult to give electrons in all the elements. While He's affinity is almost impossible to observe, which means that He is also difficult to get electrons. To date, argon, krypton, xenon, radon have the presence of confirmed compounds in the six rare gas elements, but neon and helium have no confirmed compounds, and only the inclusion complex formed by van der Waals force. Helium and neon are truly "inert" gases.
The strong stability of helium is due to its closed-shell electronic configuration: its shell is in a complete state, with no space and other atoms being combined by common electrons. But this is the case of the Earth's surface environment.

As the second rich element of the universe, helium plays an important role in the composition of stars and giant gas planets. In the outer space or the extreme conditions of the earth, it may follow an unusual law. Today, researchers have just verified this strange phenomenon.

"Very high pressure, such as the core of the earth or other giant stars, can completely change the chemical properties of helium," said Alex Boldyrev, co-author of the Utah State University article.

Beijing Institute of High Pressure Science and Nankai University, Dr. Dong school through the "crystal structure prediction" model calculations found that under extreme pressure, a stable helium sodium compounds can be formed. And then they in the diamond pressure chamber experiment really created an unprecedented compound: Na2He. Experiments can provide helium and sodium atoms equivalent to 1.1 million times the Earth's atmospheric pressure (equivalent to the Earth's mantle pressure or the middle of the Jupiter atmosphere) conditions.

Professor Wang Huitian of Nankai University, a co-author of this study, said: "The compounds found are very peculiar: helium atoms usually do not form any chemical bonds, and the presence of new substances fundamentally changes the chemical interaction between sodium atoms , Forcing the electrons to concentrate in the cubic space of the structure, with the ability to insulate.
The crystal structure of Na2He, alternating from sodium atom (purple) and helium atom (green), is common in electron (red) in the region between them.
"Na2He is actually a special phase named electron salt, strongly repelled by the inner electrons, where electrons are localized into the spatial voids of the system. In this case, isolated electrons without nuclei can be seen as Is anion, and in simple terms, Na2He is actually an ion compound consisting of Na, He and isolated electron pairs, "Artem R. Oganov, co-author of this paper.

"But the actual bonding situation is much more complex than the above, isolated electrons and Na and He have a strong interaction and charge transfer, and when we analyze the inside of He's special key way, we accidentally found very serious The problem is that the electronic structure under high pressure is distorted by external forces, leading to a lot of common sense and chemical theory at atmospheric pressure that no longer apply.We have tried different keying methods and found that using different approximations would be completely different "The essence of this problem is that the chemist does not give a clear definition of the chemical bond, which is not a problem at atmospheric pressure because most of the atmospheric pressure chemical bonds are typical of covalent The bond between the bond and the bond between the bond and the polarization becomes blurred, and the problem becomes serious after increasing the amount of external pressure such as a continuously adjustable amount at high pressure It actually tells us that we should re - examine our basic chemical theory and define whether it is applicable under high pressure. This unexpected result, the researchers spent more than two years to persuade reviewers and editors.
"We can confirm that Na2He is not a conventional inclusion in the conventional sense, and that the electronic structure does not change when you take away the inclusion in the inclusion," Na said, "that Na2He is not the case." In Na2He , If you remove the helium atoms, the structure will not be able to remain stable. Sublattice analysis shows that the occupancy of He leads to electrons being localized to the atomic gap and forming a multi-center bond under the gravitational force of the Na nucleus, so that the whole system becomes an electron salt system. In this process, the inner electrons of Na, the inner layer of Na, and the inner layer 1s of He and the 2s, 2p orbital of the outer layer are strongly overlapped.
"In fact, the biggest difference between Na2He and conventional clathrate is that, in this process, the original Na sublattice free electron gas is forced to become local electrons while giving He a strong reaction. Dong Jing stressed. Under this reaction, the distribution of the original electron density and the outer electron orbital of He is forced to change significantly due to the principle of forgiveness, which leads to 0.15 electrons in the formation of Na2He. This work confirms that He will have weak chemical activity under high pressure and can form compounds with Na that is significantly enhanced at high pressure.
"This work proves that Na becomes the most active metal element under high pressure, and Ne becomes the most inert element under high pressure. Under high pressure, the most active element and the most inert element are only a difference in the number of nuclear charges. Can only be said that the magic of nature, there are Chinese textbooks on the reduction of metal elements called "metal", but in fact the high pressure under the "metal" the strongest element Na precisely because the formation of the electronic salt state and change Become insulators, this can only explain the unpredictable nature, "Dong school commented.
"This new compound may be present in the atmosphere of Jupiter, and its interaction with the (metal) hydrogen is related to the material cycle of the giant planet." Our study shows that under high pressure we have to re-examine what we have under normal pressure Chemical laws, and even may be necessary to rewrite the chemistry textbook, "Dong said.
Although the recent breakthrough on metal hydrogen research has been very questioned, but this article from the United States Carnegie Institute of experimental data to be a lot of solid. Henry Rzepa, a physicist from the Imperial College in London, said in contrast to the findings of this study and metal hydrogen: "This is a more reliable science, and helium compounds are a major breakthrough."

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