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Solid hydrogen at 342 GPa: no evidence for an alkali metal

Abstract

Solid hydrogen, an electrical insulator, is predicted to become an alkali metal under extreme compression, although controversy surrounds the pressure required to achieve this1,2,3. The electrical conductivity of hydrogen as a function of pressure and temperature is of both fundamental and practical interest—metallic hydrogen may be of relevance to planetary interiors4, and has been suggested as a potential high-temperature superconductor5. Calculations1,2 suggest that depairing (destruction of the molecular bond) should occur around 340 GPa, accompanied by the formation of an alkali metal at this pressure1, or at substantially higher pressures2,3. Here we report that solid hydrogen does not become an alkali metal at pressures of up to 342 ± 10 GPa, achieved using a diamond anvil cell. This pressure (which is almost comparable to that at the centre of the Earth) significantly exceeds those reached in earlier experiments—216 GPa (ref. 6) and 191 GPa (ref. 7)—at which hydrogen was found to be non-metallic. The failure of solid hydrogen to become an alkali metal at the extreme pressures reported here has implications for our current theoretical understanding of the solid-state phase.

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Figure 1: An energy dispersive X-ray diffraction pattern of tungsten adjacent to the hydrogen sample at a hydrogen pressure of 342 GPa.
Figure 2: Reflectance and transmission micrograph of the sample and tip region, and birefringence of the diamond, as viewed with polarized light at 327 GPa.
Figure 3: The effect of the ratio of the initial sample-hole diameter to the flat tip diameter.

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Acknowledgements

We acknowledge discussions with N. W. Ashcroft, B. Baranowski and R. M. Martin and thank the CHESS staff for their help. We also acknowledge the role played by B. W. Batterman in creating CHESS. This work was supported by the US NSF and, several years ago in the early stages of this project, by the Cornell Materials Science Center via support by the US NSF.

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Correspondence to Arthur L. Ruoff.

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Narayana, C., Luo, H., Orloff, J. et al. Solid hydrogen at 342 GPa: no evidence for an alkali metal. Nature 393, 46–49 (1998). https://doi.org/10.1038/29949

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