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Hydrogen takes a mixed atomic-molecular phase at high pressures

structure of the Pbcn phase IV of hydrogen at 250 GPa

Search for metallic hydrogen state is the Holy Grail of Condensed Matter Science. The material is expected to show extraordinary properties such as ultra-high energy density and high-temperature superconductivity. 

Hydrogen is the most abundant element in the Cosmos. The great majority of this material resides in high-density forms as they are gravitationally compressed in Giant planets and Stars. But not much is known about these states as the experiments are very challenging and the theoretical calculations are prohibitively expensive to explore all the possibilities.    

The research team of the Centre for Science at Extreme Conditions at University of Edinburgh (CSEC) and the Geophysical Laboratory has discovered a new phase of solid hydrogen under pressures exceeding the atmospheric by 2.4 million times and at room temperature. The researchers find that at the highest pressure reached (which is 3.2 million times larger than atmospheric) 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.

Alexander Goncharov of the Geophysical Laboratory said that “this finding has become possible as the results of new technological breakthroughs in diamond anvil cell technique accomplished at CSEC and also in theoretical structure predictions performed by Chris Pickard & Richard Needs in 2007.” The results will stimulate further research in this area, which will give an ultimate answer to the long standing problem. “The element number one still possesses secrets. The discovery of phase IV of hydrogen with atoms of two different sorts is the great example of complex behavior of this putatively simple material” – Goncharov said. 

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