How is hydrogen a metal

When hydrogen becomes metal

As early as 1935, it was theoretically predicted that hydrogen would turn into a metal under high pressure. With extensive high-pressure experiments, several research groups moved ever closer to this phase change. Now two physicists report in the journal “Science” that they actually generated metallic hydrogen at almost five million times the atmospheric pressure. That wouldn't just be a success for basic research. Because metallic hydrogen is considered a candidate for a superconductor that could conduct electricity without resistance even at room temperature.

Hydrogen under pressure

“The production of metallic hydrogen was one of the great challenges facing solid-state physics,” write Isaac Silvera and Ranga Dias from Havard University in Cambridge, Massachusetts. It was only a year ago that physicists in Edingburgh, Scotland, came very close to this goal when they produced a precursor of metallic hydrogen at 325 gigapascals. But it was only with an optimized diamond press cell that the two researchers were able to build up a pressure of 495 gigapascals.

The scientists installed a small chamber filled with hydrogen between two high-purity industrial diamonds. When it was deeply cooled to minus 268 degrees Celsius, the molecular hydrogen solidified and could be exposed to increasingly higher pressures. The molecular hydrogen changed into atomic hydrogen. From 335 gigapascals, the previously transparent sample darkened and the preliminary stage of metallic hydrogen was reached. And at 495 gigapascals, the sample changed its appearance again and suddenly reflected incident light. Silvera and Dias found a high reflectivity of more than ninety percent. This property is typical for metals and can therefore be interpreted as evidence for the synthesis of metallic hydrogen.

Should the synthesis of metallic hydrogen be reproduced and thus confirmed by other working groups, physicists would be able to check further theoretically predicted properties of this material. For example, the exotic metal could remain stable without permanent pressure and freezing. And maybe metallic hydrogen actually shows a way to a superconductor at room temperature.