Squeezed hydrogen turns semi-metallic.

Squeezed hydrogen turns semi-metallic

For years, scientists have tried compressing hydrogen until it becomes a metal. Current theory predicts that a sufficiently large pressure should collapse the two-atom molecules of hydrogen, breaking them into a tightly packed collection of individual atoms and freeing electrons from their designated shells to conduct electricity. Achieving such a metallic state for the simplest atom would provide a great advance in basic physics. In the bargain, metallic hydrogen holds promise as a high-temperature superconductor and as a fuel for nuclear fusion.

Now, two scientists from the Carnegie Institution of Washington in Washington, D.C., report seeing the first glimmer of their long-awaited goal. Ho-Kwang Mao and Russell J. Hemley observed changes in their hydrogen samples indicating that, if not already metals, these samples have at least reached an important intermediate state as semiconductors, or semi-metals. They presented their finding earlier this month at the American Geophysical Union meeting in Baltimore.

The researchers achieve a pressure comparable to that near the center of the earth by squeezing materials between two gem-quality diamonds in a "diamond anvil." The device works by concentrating a force over a small area, the way a high-heeled shoe painfully concentrates a person's weight on a stepped-on toe.

Ten years ago, Mao and Peter M. Bell used the diamond anvil to compress hydrogen into a new kind of crystalline solid at 1.7 megabars, or 1.7 million times atmospheric pressure (SN: 3/10/79, p.156). Now they can reach 2.5 megabars. At this pressure, when the researchers peer through the transparent diamonds, they see the hydrogen turn opaque. This indicates the sample is becoming metallic, explains Mao, because electrons of semiconductors and conducting metals absorb visible light. The team cannot yet test the sample directly for electrical conductivity, but Mao says they expect to eventually develop this capability.

Mao and Hemley say they have little idea what metallic hydrogen might be like -- it could even be a liquid. Their work should improve scientists' understanding of the planets Jupiter, Saturn, Uranus and Neptune, which may have metallic-hydrogen cores. In addition, the possibilities of a superior superconductor or a new fusion source remain open. While fusion efforts have failed to produce more energy than put in, Mao says, a more compact hydrogen starting material could help boost efficiency.