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 [ Time:2012/10/10 ]
Hydrogen Takes a Mixed Atomic-molecular Phase at High Pressures
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CEMEE chief scientist Prof. Alexander Goncharov made an important contribution to a collaborative work of the Centre for Science at Extreme Conditions at University of Edinburgh (CSEC) and Carnegie Institution of Washington. The researchers have discovered a new phase of solid hydrogen under pressures of 240 GPa and at room temperature. The researchers find that at the highest pressure reached (320 GPa) some of atoms are at least 2 times weaker bounded in molecules than in the ordinary molecules. The material becomes opaque at these conditions. This is only the forth known phase of solid hydrogen, and it is perhaps the closest one to the metallic state. The further compression of this state is expected to produce two-dimensional metallic graphite-like layers along with the layers of almost unbounded molecules. Previously, hydrogen was expected to become metallic in either a monatomic or purely molecular states. As such, this discovery provides unforeseen previously insight into the nature of metallic hydrogen, with implications for the interiors of giant planets.


Figure. Left: The structure of the Pbcn phase IV of hydrogen at 250 GPa with the atomic positions from the DFT calculations performed by Chris Pickard. The figure credit is to Maddury Somayazulu.
Right two panels: optical microscopy images illustrating transformation of hydrogen to nontransparent material.

Ross T. Howie, Christophe L. Guillaume, Thomas Scheler, Alexander F. Goncharov, and Eugene Gregoryanz, Mixed molecular and atomic phase of dense hydrogen. 
Phys. Rev. Lett., 108, 125501 (2012).

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