Artem R. Oganov, Stony Brook University, New York, USA, Skolkovo Institute of Science and Technology, Moscow, Russia, Northwestern Polytechnical University, Xi’an, China, and colleagues have for the first time produced a stable chemical compound with helium. So far, He has been considered to be chemically inert.
The researchers found with theoretical calculations that Na2He has a lower enthalpy than a mixture of elemental sodium and helium at pressures above 160 GPa; a sign of a stable crystal structure under these conditions. In the case of pressures generated in the laboratory, sodium is also the only element with which helium contains such a compound.
The chemists exerted extreme pressure to a mixture of sodium and helium in a diamond anvil cell. At pressures above 113 GPa, the measurements showed a change by means of X-ray crystallography and Raman spectroscopy: the mixture of the two elements had become a crystalline compound.
In its structure, Na2He resembles an ionic, salt-like crystal. In this, the helium atoms form a cubic lattice, similar to a 3D chessboard, in which each black field is occupied by helium. The white fields are covered by an electron pair surrounded by sodium ions. Thus this compound is an electride, a crystal of positively charged ionic nuclei, in which highly localized valence electrons play the role of the anions. Helium leads to this localization of the electrons and makes the exotic crystal a non-conductor.
- A stable compound of helium and sodium at high pressure,
Xiao Dong, Artem R. Oganov, Alexander F. Goncharov, Elissaios Stavrou, Sergey Lobanov, Gabriele Saleh, Guang-Rui Qian, Qiang Zhu, Carlo Gatti, Volker L. Deringer, Richard Dronskowski, Xiang-Feng Zhou, Vitali B. Prakapenka, Zuzana Konôpková, Ivan A. Popov, Alexander I. Boldyrev, Hui-Tian Wang,
Nature Chem. 2017.