Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.
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This work was supported by the China Scholarship Council (grant no. 201206200030), NSAF (grant no. U1530402), National Science Foundation (grant no. EAR-1114313), DARPA (grant no. W31P4Q1210008), Russian Science Foundation (grant no. 16-13-10459), National 973 Program of China (grant no. 2012CB921900) and Foreign Talents Introduction and Academic Exchange Program (grant no. B08040). X.F.Z. acknowledges funding from the National Science Foundation of China (grant no. 11674176). Calculations were performed at the Tianhe II supercomputer in Guangzhou and the supercomputer of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the US Department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC02-98CH10086. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR-1128799) and Department of Energy – Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. PETRA III at DESY is a member of the Helmholtz Association (HGF). The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007- 2013) under grant agreement no. 312284. The work of E.S. was performed under the auspices of the US Department of Energy by Lawrence Livermore National Security under contract no. DE-AC52-07NA27344. A.F.G. acknowledges support from the National Natural Science Foundation of China (grant no. 21473211), the Chinese Academy of Sciences (grant no. YZ201524) and the Chinese Academy of Sciences visiting professorship for senior international scientists (grant no. 2011T2J20) and Recruitment Program of Foreign Experts. S.L. was partly supported by state assignment project no. 0330-2016-0006. A.I.B. acknowledges the support of the National Science Foundation (CHE-1361413 to A.I.B.). I.A.P. acknowledges the support of the Ministry of Education and Science of the Russian Federation (agreement number 02.a03.21.0008).
The authors declare no competing financial interests.
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Dong, X., Oganov, A., Goncharov, A. et al. A stable compound of helium and sodium at high pressure. Nature Chem 9, 440–445 (2017). https://doi.org/10.1038/nchem.2716
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