Reactions of xenon with iron and nickel are predicted in the Earth's inner core

Journal name:
Nature Chemistry
Volume:
6,
Pages:
644–648
Year published:
DOI:
doi:10.1038/nchem.1925
Received
Accepted
Published online

Abstract

Studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted, a finding often referred to as the ‘missing Xe paradox’. Although several models for a Xe reservoir have been proposed, whether the missing Xe could be contained in the Earth's inner core has not yet been answered. The key to addressing this issue lies in the reactivity of Xe with Fe/Ni, the main constituents of the Earth's core. Here, we predict, through first-principles calculations and unbiased structure searching techniques, a chemical reaction of Xe with Fe/Ni at the temperatures and pressures found in the Earth's core. We find that, under these conditions, Xe and Fe/Ni can form intermetallic compounds, of which XeFe3 and XeNi3 are energetically the most stable. This shows that the Earth's inner core is a natural reservoir for Xe storage and provides a solution to the missing Xe paradox.

At a glance

Figures

  1. Chemical stabilities of Xe–Fe and Xe–Ni compounds.
    Figure 1: Chemical stabilities of Xe–Fe and Xe–Ni compounds.

    a,b, Predicted formation enthalpy of various Xe–Fe (a) and Xe–Ni (b) compounds with respect to elemental decomposition into Xe and Fe at 0 K and high pressure. c,d, Predicted Gibbs free energies of various Xe–Fe (c) and Xe–Ni (d) compounds at 250 GPa relative to elemental decomposition into Xe and Fe as a function of temperature. Dashed lines connect data points, and solid lines denote the convex hull. Compounds with enthalpy or free-energy data located on the convex hull are stable against decomposition. For clarity, we have offset the free energy plots in c and d by −0.1 eV for each 2,000 K interval.

  2. Selected structures of predicted Xe–Fe compounds.
    Figure 2: Selected structures of predicted Xe–Fe compounds.

    a, Top view of Cu3Au-type structure of XeFe3. be, Polyhedral views of Cu3Au-type structure of XeFe3 (b), P–62m structure of XeFe5 (c), P–1 structure of XeFe (d) and Pmmn structure of XeNi3 (e). Detailed structural parameters for these compounds are listed in Supplementary Tables 3 and 4. The diamond (c) and rectangles (d,e) denote the unit cells. The basic structural building blocks for these structures are closely related to different kinds of polyhedrons centred around the Xe atom. The coordination number of Xe is 7 in XeFe, but 12 in other stoichiometries.

  3. Electronic properties of XeFe3.
    Figure 3: Electronic properties of XeFe3.

    a, Electronic band structure calculated using the HSE hybrid functional for XeFe3 at 250 GPa. The dashed line indicates the Fermi energy. The high-symmetry points of X (1/2,0,0), R (1/2,1/2,1/2), M (1/2,1/2,0) and Γ (0,0,0) are used. The electronic band structure reveals the metallic nature of XeFe3. b, Projected densities of states (DOS) of Xe–5p states for XeFe3 and hypothetical XeFe0 at 250 GPa. The vertical dashed line indicates the Fermi energy. c, Difference charge density (crystal density minus superposition of isolated atomic densities) of XeFe3 plotted in the (100) plane at 250 GPa. Arrows denote the positions of Xe and Fe atoms, as indicated. Both the DOS and the difference charge density results reveal charge transfer from Xe to Fe.

  4. Phase diagram of Xe–Fe and Xe–Ni systems.
    Figure 4: Phase diagram of Xe–Fe and Xe–Ni systems.

    Dashed lines show the proposed phase boundaries and the green region presents the geotherm of the Earth's core from ref. 21. The geotherm indicates the profile of temperature versus depth in the Earth's interior. Blue and orange lines are the melting curves of Ni from ref. 43 and Fe from ref. 21, respectively. The left, grey region represents the mixture of elemental Xe with Ni and Fe. The middle, yellow region depicts the formation diagram of stable XeNi3, but not XeFe3. The right, blue area represents the region in which both XeNi3 and XeFe3 can form.

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Author information

Affiliations

  1. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

    • Li Zhu,
    • Hanyu Liu,
    • Guangtian Zou &
    • Yanming Ma
  2. Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK

    • Chris J. Pickard

Contributions

Y.M. proposed and coordinated the research. L.Z. and H.L. performed most of the calculations. L.Z., H.L., C.J.P., G.Z. and Y.M. analysed the data. C.J.P. carried out the Ab Initio Random Structure Searching structure predictions. All authors commented on the manuscript. L.Z. and Y.M. wrote the paper.

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