New phase of dense hydrogen discovered

Hydrogen (left) and deuterium (right) samples at very high pressures in reflected and transmitted light as seen through the diamonds.
Hydrogen (left) and deuterium (right) samples at very high pressures in reflected and transmitted light as seen through the diamonds.

Obtaining metallic hydrogen, the Holy Grail of high pressure science, has been a long-standing problem in Condensed Matter Science. The high pressure behaviour of hydrogen has major implications for the interiors of the Jovian planets (the gas giants: Jupiter, Saturn, Neptune and Uranus). High pressure experiments have been driven over the past 70 years not just by the desire to convert the most abundant element in the universe metallic, but through predictions of exotic properties the material may exhibit, such as superfluidity and high temperature superconductivity.

Through new technological breakthroughs in diamond anvil cell experiments, a team of researchers at the Centre for Science at Extreme Conditions at the University of Edinburgh and Geophysical Laboratory, Washington DC have not only set a pressure record for the compression of hydrogen at room temperature but discovered a new phase - one of only four known phases - at conditions that exceed 2.2 million times atmospheric pressure.

"This interesting new phase is exemplary of how pressure can introduce complexity in an element otherwise thought as simple and provides a continuous, elegant pathway to the metallization of hydrogen. These are exciting times in high pressure hydrogen research and the discovery of a new phase will without doubt stimulate further studies and drive experiments in obtaining the elusive metallic state." PhD student Ross Howie, the first author on the Physical Review Letter where the discovery is reported.

The new phase consists of 'honeycomb' layers between which unbound molecules intersperse. Such a structure is highly unusual, exhibiting a mixture of both atomic and molecular properties. Predictions (Pickard and Needs, 2007) suggest this phase will evolve if further compressed, electrically conducting along the honeycomb layers much like graphite. Despite reaching 3.2 million times atmospheric pressure at room temperature, the metallic state is still to be attained. Nonetheless, as it was previously thought that hydrogen would become conducting in either the pure atomic or pure molecular state, this study is crucial in the understanding of the nature of metallic hydrogen.