In a squeeze, nitrogen gets chunky

By applying brutal pressure, scientists have transformed a wisp of nitrogen, the most abundant gas in our atmosphere, into an opaque solid. The novel substance has remarkable electronic properties, the researchers report. Moreover, it keeps its new form even when the pressure is removed.

In the May 10 Nature, Mikhail I. Eremets and his colleagues at the Carnegie Institution of Washington (D.C.) describe how they created the new substance using a small anvil with diamond jaws. The device exerted pressures up to the equivalent of 2.4 million atmospheres. When the jaws let go, the unpressurized material remained stable as long as the researchers kept it at a frigid 100 kelvins.

The chemical bonds between nitrogen atoms in the new solid differ from the kind of bond holding ordinary nitrogen-gas molecules together. Consequently, the new substance packs tremendous energy into a small volume, notes Ho-kwang (David) Mao, a member of the Carnegie team. “If we could make enough of this material, it could be an [extraordinarily powerful] propellant or explosive,” he suggests.

The bonds rearrange because the ordinary two-atom molecules of lower-pressure nitrogen break down under such enormous pressure. The atoms then reassemble as a chunk of the element in which each atom probably bonds to several neighbors.

Signaling this changed state, electrical measurements indicated that the nitrogen had become a semiconductor. These were the first electrical measurements on a material at such high pressure. Ordinary nitrogen gas doesn’t conduct electricity.

This success with nitrogen has rekindled high-pressure researchers’ hope of reaching a long-sought goal: squeezing the lightest gas, hydrogen, into an atomic solid (SN: 8/30/97, p.143).

Solid hydrogen fascinates scientists for several reasons. For one thing, its creation in the laboratory could help reveal the nature of the interiors of giant gas planets, such as Jupiter. Also, the comparison of hydrogen with nitrogen under high pressure perplexes theorists. Of the two elements, hydrogen has the weaker molecular bonds, suggesting that it should break down into an atomic solid more easily. However, in experiments, hydrogen remains a molecular solid at far greater pressures than molecular nitrogen can endure.