January 11, 2016

University of Edinburgh make precursor to metallic hydrogen

British researchers think they have come close to creating a long-sought new state for hydrogen.

They have put a sample of the familiar gas under so much pressure that it takes on a previously unseen solid crystalline form.

The team tells the journal Nature that this phase may be just a step away from so-called metallic hydrogen.
Predicted 80 years ago, this exotic substance could lead to ultra-fast computers and even super rocket fuel.

"We think we've reached a state of the material that is probably the precursor to metallic hydrogen," explained Ross Howie, formerly at Edinburgh University but now based in China.

The study (published in Nature) provides evidence that at above 350 GPa (3,500,000 atm) and at room temperature hydrogen adopts a novel structure. Interestingly the results are suggestive that this new phase could prove to be the precursor to the long sought metallic phase of hydrogen, predicted over 80 years ago by theory.

As the simplest, lightest and most abundant element of the Universe, hydrogen is of fundamental interest in many fields of science. At high pressure and low temperatures, hydrogen is predicted to transform from a molecular system to a metallic (atomic) state. This behaviour is crucial to planetary science as hydrogen is believed to be found in the centre of Jovian planets and to be the source of their exceptionally high magnetic fields. This predicted state is also believed to exhibit rich phenomena such as superconductivity and super-fluidity, which would result in many technological breakthroughs. Reaching such conditions with hydrogen in the experimental laboratory however has been a great challenge in the field of high pressure research: only 4 years ago it was limited to less than half of the pressure recorded in the study.

Through new technological breakthroughs in the containment of hot hydrogen in diamond anvil cell experiments, Philip Dalladay-Simpson, Ross Howie and Eugene Gregoryanz at the Centre for Science at Extreme Conditions report that above 350 GPa hydrogen adopts a novel phase, phase V.

Nature - Evidence for a new phase of dense hydrogen above 325 gigapascals




Through new technological breakthroughs in the containment of hot hydrogen in diamond anvil cell experiments, Philip Dalladay-Simpson, Ross Howie and Eugene Gregoryanz at the Centre for Science at Extreme Conditions report that above 350 GPa hydrogen adopts a novel phase, phase V.

Tracking this transformation over a large pressure regime up to 400 GPa and up to 450 K through Raman spectroscopy and comparing with theoretically predicted structures, the team suggest that phase V could be the precursor to the experimentally elusive, completely metallic and atomic phase of hydrogen. The work will undoubtedly reinvigorate both theoretical and experimental efforts to understand these newly discovered phases and hydrogen’s evolution to a purely metallic (atomic) structure.

The group used a set-up called a diamond anvil cell to compress its sample of molecular hydrogen.

This apparatus is essentially two gems that have been placed in opposition to each other.

Their polished tips, comparable in size to the width of a human hair, are made to press into a cavity containing the sample.

"The volume of hydrogen we use is about a micron cubed - a size that is on the order of a red blood cell," said the Nature paper's lead author, Philip Dalladay-Simpson, from Edinburgh's Centre for Science at Extreme Conditions.

Ultimately, the hydrogen atoms should stack so efficiently that their electrons become shared - just as in a metal.

However, the team does not quite see this phase, but rather something that is probably just short of it.

"This would be a mixed structure of different layers, where you might get a layer of hydrogen molecules followed by an atomic layer, and these alternate," said Dr Howie, who is now affiliated to the Center for High Pressure Science and Technology Advanced Research in Beijing.

The work puts new constraints on where the full metallic hydrogen phase might exist: possibly below 450 gigapascals at room temperature.

The ambient temperature is very significant, because if metallic hydrogen can ultimately be produced this way it opens the door potentially to a new type of perfect (zero resistance) conductor - a material to boost the performance of next-generation computers.

"It's been predicted that metallic hydrogen could be a room-temperature superconductor, which is still yet to be achieved with any material," said Dr Howie.

"However, because we are playing with such small quantities, the practical applications at this stage are not clear."

"We use brute force - a large lever arm. We apply about a tonne of force on the back of the diamonds to generate huge pressures inside the cell."

A previous study provided evidence that at above 200 GPa (2,000,000 atm) surprisingly low temperatures (480 K) are required to reach a liquid phase.

Another prediction for metallic hydrogen suggests it could form the basis of a super fuel, producing substantially more thrust than the standard super-chilled hydrogen used in today's rockets.

Scientists are also fascinated by metallic hydrogen because they think it may account for a large fraction of the internal composition of planets such as Jupiter.

The high pressures and temperatures that exist several thousand kilometres below the gas giant's cloud surface are believed to produce a fluid form of metallic hydrogen. Movement in this electrically conducting liquid is very likely the source of the world's colossal magnetic field.

SOURCES - Journal Nature, BBC News, University of Edinburgh

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