A stable compound of helium and sodium at high pressure

Journal name:
Nature Chemistry
Volume:
9,
Pages:
440–445
Year published:
DOI:
doi:10.1038/nchem.2716
Received
Accepted
Published online

Abstract

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.

At a glance

Figures

  1. Thermodynamics of the Na–He system, which shows the enthalpic stability between Na2He and the mixture of elemental Na and He.
    Figure 1: Thermodynamics of the Na–He system, which shows the enthalpic stability between Na2He and the mixture of elemental Na and He.

    a, Predicted convex hulls of the Na–He system, based on the theoretical ground states of Na and He at each pressure13, 18, 22, 50. Here, ΔHformation(NaxHe1−x) = xH(Na) + (1−x)H(He)−H(NaxHe1−x), which can be considered as a criterion of thermodynamic stability. b, Enthalpy of formation of Na2He as a function of pressure. Our calculated pressures of the cI16–tI19 and tI19–hP4 transitions of Na are 151 and 273 GPa, respectively, similar to previous calculations13.

  2. Crystal structure of Na2He at 300 GPa.
    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.

  3. Experimental data on Na2He: XRD at 140 GPa.
    Figure 3: Experimental data on Na2He: XRD at 140 GPa.

    a, Integrated XRD patterns before heating and after the second and third heating to ∼1,200 and >1,500 K, respectively. Vertical ticks correspond to expected positions and intensities of XRD peaks of Na2He, tI19-Na (ref. 18), Re (gasket) and W (pressure gauge). b, Two-dimensional image after the third heating, in radial coordinates, showing single-crystal reflections of tI19-Na and Na2He, marked by light blue circles and white rectangles, respectively. This pattern was obtained during a continuous rotation of the DAC along the ω axis to collect as many single-crystal reflections of tI19-Na as possible. Semi-continuous lines are from the Re gasket. Diffraction from Na2He and W consists of many small, almost uniformly spaced spots. After the third heating, a new almost continuous line appeared near 10.5°, which we assigned to yet unidentified reaction products. The X-ray wavelength is 0.31 Å. c, Equation of state (EOS) of Na2He synthesized in a DAC at 113–155 GPa in comparison with the EOS of Na. Filled circles: experimental unit cell volumes of Na2He. Open blue circles: volumes per two Na atoms. Error bars correspond to the experimental pressure uncertainty due to pressure gradients and pressure measurements. Pressure was determined from XRD measurements of the W marker in one experiment and using Raman shift of the stressed diamonds51. The solid blue line corresponds to a superposition of the EOS of Na and He (2Na + He) determined from experimental data52. Dashed and solid lines are the results of our GGA and LDA calculations, respectively. Dotted line: extrapolated EOS of fcc Na from ref. 19. Open red circles: volumes of Na phases reported in ref. 18.

  4. Electronic structure of Na2He.
    Figure 4: Electronic structure of Na2He.

    a, Density of states (DOS) of Na2He and its Na-sublattice, showing that the insertion of He opens a bandgap (the pure Na sublattice is metallic). The vertical line is the Fermi level. b, Band structure and bandgap as a function of pressure. GW gaps (where G is the Green’s function and W is the screened Coulomb interaction) are typically accurate to within 5–10% of experimental values53. It shows that both Na and Na2He have a wide gap that increases with pressure. c, Electron density change (defined as ρ(Na2He)−ρ(Na sublattice)−ρ(He sublattice)) induced by insertion of the He sublattice. (100) sections passing through He atoms (above) and Na atoms (below) are shown at 300 GPa. NNA (non-nuclear attractors) corresponds to interstitial electron localizations. The insertion of He pushes electron density out, causing its interstitial localization.

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Author information

Affiliations

  1. School of Physics and MOE Key Laboratory of Weak-Light Nonlinear Photonics, Nankai University, Tianjin 300071, China

    • Xiao Dong,
    • Xiang-Feng Zhou &
    • Hui-Tian Wang
  2. Center for High Pressure Science and Technology Advanced Research, Beijing 100193, China

    • Xiao Dong
  3. Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA

    • Xiao Dong,
    • Artem R. Oganov,
    • Guang-Rui Qian,
    • Qiang Zhu &
    • Xiang-Feng Zhou
  4. Skolkovo Institute of Science and Technology, 3 Nobel Street, Moscow 143026, Russia

    • Artem R. Oganov
  5. Moscow Institute of Physics and Technology, 9 Institutskiy Lane, Dolgoprudny city, Moscow Region 141700, Russia

    • Artem R. Oganov &
    • Gabriele Saleh
  6. International Centre for Materials Discovery, Northwestern Polytechnical University, Xi'an 710072, China

    • Artem R. Oganov
  7. Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington DC 20015, USA

    • Alexander F. Goncharov,
    • Elissaios Stavrou &
    • Sergey Lobanov
  8. Key Laboratory of Materials Physics and Center for Energy Matter in Extreme Environments, Institute of Solid State Physics, Chinese Academy of Sciences, 350 Shushanghu Road, Hefei, Anhui 230031, China

    • Alexander F. Goncharov
  9. Lawrence Livermore National Laboratory, Physical and Life Sciences Directorate, PO Box 808 L-350, Livermore, California 94550, USA

    • Elissaios Stavrou
  10. Sobolev Institute of Geology and Mineralogy, Siberian Branch Russian Academy of Sciences, 3 Pr. Ac. Koptyga, Novosibirsk 630090, Russia

    • Sergey Lobanov
  11. Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM) e Dipartimento di Chimica, Universita’ di Milano, via Golgi 19, Milan 20133, Italy

    • Carlo Gatti
  12. Chair of Solid-State and Quantum Chemistry, RWTH Aachen University, Aachen D-52056, Germany

    • Volker L. Deringer &
    • Richard Dronskowski
  13. Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA

    • Vitali B. Prakapenka
  14. Photon Science DESY, Hamburg D-22607, Germany

    • Zuzana Konôpková
  15. Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, USA

    • Ivan A. Popov &
    • Alexander I. Boldyrev
  16. Chemistry Department, Faculty of Science, RUDN University, 6 Miklukho-Maklaya Street, Moscow 117198, Russia

    • Ivan A. Popov
  17. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

    • Hui-Tian Wang

Contributions

X.D. and A.R.O. designed the research. X.D., G.S. and I.A.P. performed and analysed the calculations. V.L.D. and R.D. carried out COHP analyses. A.G. designed experiments. S.L. and A.G. loaded the sample. A.F.G., E.S., S.L., V.B.P. and Z.K. performed the experiment. E.S. and A.F.G. analysed the experimental data. G.-R.Q., Q.Z., X.-F.Z. and A.I.B. assisted with calculations. All authors contributed to interpretation and discussion of the data. X.D., A.R.O., A.F.G., G.S., I.A.P., A.I.B. and H.-T.W. wrote the manuscript.

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The authors declare no competing financial interests.

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