High pressure mates helium, nitrogen.

For the first time, scientists have used lighter-than-air helium gas to make a solid compound. In the past, helium would form a solid only by itself. Now, Willem L. Vos, a physical chemist at the Carnegie institution of Washington (D.C.), and his colleagues have used very high pressures to coerce room-temperature helium to combine with nitrogen.

"It's not a regular compound," says Vos. "It's a compound that exists because of packing effects." The substance lacks the covalent, ionic, or metallic bonds that typically link unlike atoms in a material; yet it still forms a solid crystal.

To make this compound, Vos and his colleagues crushed a gaseous nitrogen-helium mixture between two diamonds in a diamond anvil cell. When the pressure exceeded 77,000 times that of Earth's atmosphere, they saw a solid crystal appear.

Its crisp facets indicated that the crystal contained both types of atoms: Pure helium or nitrogen forms rounded surfaces, the researchers report in the July 2 Nature. The crystal contains 11 nitrogen molecules for every helium atom.

Vos and his colleagues consider this compound one of a new class of materials, so-called van der Waals compounds, and expect that they and others will make more of them this way. Already, Michel Jean-Louis from the University of Paris in France has made a new helium-neon solid, Vos adds.