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Transparent dense sodium

Nature volume 458, pages 182–185 (2009)Cite this article

Abstract

Under pressure, metals exhibit increasingly shorter interatomic distances. Intuitively, this response is expected to be accompanied by an increase in the widths of the valence and conduction bands and hence a more pronounced free-electron-like behaviour. But at the densities that can now be achieved experimentally, compression can be so substantial that core electrons overlap. This effect dramatically alters electronic properties from those typically associated with simple free-electron metals such as lithium (Li; refs 1–3) and sodium (Na; refs 4, 5), leading in turn to structurally complex phases6,7,8 and superconductivity with a high critical temperature9,10,11. But the most intriguing prediction—that the seemingly simple metals Li (ref. 1) and Na (ref. 4) will transform under pressure into insulating states, owing to pairing of alkali atoms—has yet to be experimentally confirmed. Here we report experimental observations of a pressure-induced transformation of Na into an optically transparent phase at ∼200 GPa (corresponding to ∼5.0-fold compression). Experimental and computational data identify the new phase as a wide bandgap dielectric with a six-coordinated, highly distorted double-hexagonal close-packed structure. We attribute the emergence of this dense insulating state not to atom pairing, but to p–d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices. We expect that such insulating states may also form in other elements and compounds when compression is sufficiently strong that atomic cores start to overlap strongly.

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Figure 1: Raman spectra of sodium.
Figure 2: Phase transformations in Na at megabar pressures.
Figure 3: Enthalpy curves (relative to f.c.c.) as a function of pressure for cI16, CsIV, α-Ga, oP8, tI19 and hP4 structures of Na.
Figure 4: Structural and electronic properties of Na-hP4.

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Acknowledgements

We thank the Swiss National Science Foundation (grants 200021-111847/1 and 200021-116219), CSCS and ETH Zurich for the use of supercomputers. Parts of the calculations were performed on the Skif supercomputer (Moscow State University, Russia) and at the Joint Supercomputer Centre of the Russian Academy of Sciences (Moscow). We acknowledge partial support from DFG (grants Er 539/1/2-1) and the China 973 Program (no. 2005CB724400). Part of the experimental work was performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory.

Author Contributions Y.M. proposed the research and predicted the new structures. Y.M., Y.X. and A.R.O. did the calculations. M.E., I.T., S.M. and V.P. performed the experiments. Y.M., A.R.O. and M.E. contributed substantially to data interpretation and wrote the paper. A.L. wrote the latest version of the structure prediction code, and M.V. helped in data analysis. Y.M, M.E. and A.R.O contributed equally to this paper.

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Author notes

  1. Artem R. Oganov & Andriy O. Lyakhov

    Present address: Present address: Department of Geosciences and New York Center for Computational Science, Stony Brook University, Stony Brook, New York 11794-2100, USA.,

Authors and Affiliations

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

    Yanming Ma & Yu Xie

  2. Department of Materials, Laboratory of Crystallography, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland,

    Yanming Ma, Artem R. Oganov & Andriy O. Lyakhov

  3. Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany,

    Mikhail Eremets, Ivan Trojan & Sergey Medvedev

  4. Geology Department, Moscow State University, 119992 Moscow, Russia

    Artem R. Oganov

  5. Data Analysis and Visualization Services, Swiss National Supercomputing Centre (CSCS), Cantonale Galleria 2, 6928 Manno, Switzerland,

    Mario Valle

  6. Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA,

    Vitali Prakapenka

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Correspondence to Yanming Ma.

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Ma, Y., Eremets, M., Oganov, A. et al. Transparent dense sodium. Nature 458, 182–185 (2009). https://doi.org/10.1038/nature07786

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