Solid hydrogen resists becoming metal.

At sufficiently high pressure, solid molecular hydrogen should, in principle, become metallic. In other words, the material should undergo a transition to a state in which electrons roam about freely and conduct electricity as they do in metals such as copper. Researchers have succeeded in explosively squeezing liquid hydrogen briefly into a metallic state (SN: 4/20/96, p. 250). Despite a number of determined efforts, however, they have failed to crush solid hydrogen into a metal.

Calculating the behavior of solid hydrogen at ultrahigh pressures, Neil W. Ashcroft and B. Edwards of Cornell University have now uncovered theoretical evidence of a subtle effect that may explain why this element stubbornly resists turning into a metal. The physicists describe their findings in the Aug. 14 Nature.

A hydrogen molecule consists of two hydrogen atoms bound together, with two protons sharing two electrons. At high pressures, the two electrons prefer to stay close to just one of the protons, producing an uneven distribution of charge known as an electric dipole. Interactions among the molecules stabilize that polarized electric charge distribution. The presence of this partially ionic state frustrates the transition to the long-sought metal.