Helium chemistry under high pressure - Dr. Xiao Dong
FEBRUARY 07, 2017
Although helium is the second (after hydrogen) most abundant element in the universe, it doesn’t play well with others. Now, an international team of researchers, including HPSTAR scientist, Dr. Dongxiao has predicted two stable helium compounds — Na2He and Na2He O. The scientists experimentally confirmed and theoretically explained the stability of Na2He. This work published by Nature Chemistry (DOI:10.1038/nchem.2716), could hold clues about the chemistry occurring inside gas giant planets and possibly even stars, where helium is a major element.
Helium is a member of a family of seven elements called the noble gases because of their chemical inertness — they don’t easily form compounds with other elements. Helium, widely believed to be the most inert element, has no stable compounds at normal conditions.
The authors of the new study used the first-principles evolutionary algorithm 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 Geophysical Laboratory 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,” said Dr. Xiao Dong, the first author of this work, also a research scientist of HPSTAR.
Na₂He is an electride, i.e., a special type of an ionic salt-like crystal. Its cation sublattice consists of sodium ions, while localized electron pairs make up the anion sublattice. 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, Na₂He O, 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 oxygen anions (O²⁻).
“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” said Prof. Oganov.
Caption: Crystal structure of Na2He, resembling a three-dimensional checkerboard. Purple spheres – sodium atoms, inside green cubes are helium atoms. Red regions inside voids of the structure show areas where localized electron pairs reside.
He和Ne是世界上最惰性的两个元素。因为1s2满壳层结构，He拥有最大的电离能和零亲和能，所以已知He仅有分子间化合物存在。通过理论预测和高压实验的方法证明He在高压下会和Na生成新的电子盐结构Na2He。亚晶格分析表明，He的占位导致电子被局域到了原子缝隙中并在Na原子核的引力下形成多中心键，从而整个体系变成了电子盐体系。该过程中，孤立电子，Na的内层电子与He的内层1s电子和外层的2s，2p轨道产生强烈的交叠。受泡利不相容原理的影响，He的1s电子密度和外层电子轨道的分布被迫发生变化导致在Na2He形成过程中He得到了0.15个电子。该工作证实了高压下He会具有弱的化学活性能够与在高压下还原性显著增强的Na形成化合物。
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