Abstract
At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure1,2,3,4,5,6. It was predicted from theory1,2—and later confirmed by experiment3,4,5—that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states—an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.
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Acknowledgements
We thank I. Souza and N. W. Ashcroft for discussions. This work was supported by the NSERC of Canada. J.-Y.R. acknowledges support by the FNRS and the FAME NoE. E.S. worked under the auspices of the US Department of Energy at the University of California/Lawrence Livermore National Laboratory.
Author Contributions J.-Y.R., E.S. and S.A.B. contributed equally to this work. J.-Y.R. and S.A.B. designed the research. J.-Y.R. and E.S. conducted the molecular dynamics simulations. J.-Y.R., E.S. and S.A.B. performed the data analysis (S.A.B. and J.-Y.R. computed the densities of states and conductivities; E.S. performed the Wannier analysis; J.-Y.R. performed the model calculations and the ELF analysis; and S.A.B. performed the solid state calculations). J.-Y.R. and S.A.B. wrote the paper.
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Raty, JY., Schwegler, E. & Bonev, S. Electronic and structural transitions in dense liquid sodium. Nature 449, 448–451 (2007). https://doi.org/10.1038/nature06123
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DOI: https://doi.org/10.1038/nature06123
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