Hydrogen takes a new form

High-pressure studies reveal long-sought phase

Squeezing hydrogen at extreme pressures changes it into a mix of honeycombed atoms layered with free-floating molecules — an entirely new state of the element and the first new phase found in decades.

If confirmed, the discovery will be only the fourth known phase of hydrogen, the simplest element and one long probed for basic insights into the nature of matter.

“I think we have pretty bulletproof evidence that there is a new phase,” says Eugene Gregoryanz of the University of Edinburgh, leader of the team that will report the work in an upcoming Physical Review Letters.

Hydrogen’s first three phases pop into existence depending on the pressure and temperature. Gregoryanz and his colleagues squished room-temperature hydrogen to some of the highest pressures ever. At around 2 million times the pressure of Earth’s atmosphere, the scientists saw distinct changes in certain properties, such as how light scatters into different wavelengths when shined through the hydrogen.

The change is dramatic enough to reflect some underlying fundamental shift, Gregoryanz says, representing the long-sought “phase IV” of hydrogen.

Analysis of the scattered light suggests phase IV contains both atoms and molecules of the element. Chris Pickard, a theoretician at University College London, says the hydrogen is probably arranged in hexagonal sheets of atoms separated by unbound molecules. In a 2007 paper, Pickard and a colleague calculated that such a structure might exist for hydrogen at these pressures. The Edinburgh experiments are “exciting certainly for me,” Pickard says, since they confirm his prediction.

But like any other claim for high-pressure hydrogen, the new finding will probably attract close scrutiny. Last fall, scientists in Germany reported turning hydrogen into a metal at similar pressures and temperatures but have faced a flurry of criticism over how they interpreted their findings (SN: 12/17/11, p. 9).

Gregoryanz and his team are now working on new experiments to further pin down the existence of phase IV. “With hydrogen,” Gregoryanz says, “nothing is simple.”

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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