Transparent dense sodium (original) (raw)

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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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

Authors

  1. Yanming Ma
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  2. Mikhail Eremets
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  3. Artem R. Oganov
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  4. Yu Xie
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  5. Ivan Trojan
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  6. Sergey Medvedev
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  7. Andriy O. Lyakhov
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  8. Mario Valle
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  9. Vitali Prakapenka
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Correspondence toYanming 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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Editorial Summary

Li and Na show resistance

Putting solids under pressure reduces the distances between their atoms, and at extremely high pressures, as electron density increases, all materials approach an ideal metal. Under pressure, then, 'simple' metals such as lithium and sodium might be expected to become increasingly better conductors. But about 10 years ago, calculations suggested that neither element responds in such a straightforward manner. Instead, it was predicted that the alkali atoms would form pairs under pressure and yield more complex structures with insulating properties. Two groups in this issue present experimental confirmation that this is the case; lithium and sodium become not more metal-like but less metal-like as pressure is applied. Ma et al. find that under about fivefold compression (200 GPa pressure), sodium transforms into a dense insulating material that is optically transparent and lacks a metallic sheen. Takahiro Matsuoka and Katsuya Shimizu show that lithium transforms from a metal to a semiconductor at twofold compression (80 GPa).

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