At these pressures, the nitrogen is transformed into an opaque semiconducting solid. The scientists also report that, once created, the semiconducting solid can remain stable even when the pressure returns to normal.

The team, which is partially supported by the National Science Foundation (NSF) Division of Materials Research, publishes the results in today's issue of Nature.

"The fact that the major portion of the air has been turned into a semiconducting solid and brought back to be stable at ambient pressure is an important breakthrough for us," said team leader Russell Hemley.

Hemley and colleagues Mikhail Eremets, Ho-kwang Mao and Eugene Gregoryanz performed the research at Carnegie's Geophysical Laboratory, a core institution of the NSF's Science and Technology Center for High-Pressure Research.

This is the first time that scientists have been able to make electrical measurements on a condensed gas under such extreme high-pressure conditions.

The new dense form of nitrogen stores a large amount of energy and could potentially serve as a new semiconducting material. Such a high-density material formed from light elements could account for part of the cores of large gas planets such as those in our own solar system.

For years, theorists have predicted that molecular nitrogen (N2) would become either a semiconductor or a metal if subjected to pressures on the order of a million atmospheres (100 gigapascals). A similar theory holds for gaseous hydrogen, which is expected to turn into solid metallic hydrogen under similarly high pressures. Solid metallic hydrogen has yet to be produced in the laboratory.

Previous experiments have been limited in the amount of pressure that could be applied to nitrogen and in the number of measurements that could be performed on the material while under pressure. Last year, the Carnegie scientists, using optical techniques alone, reported signs of the material's transformation at room temperature.

In their recent experiment, the investigators used newly developed techniques that allowed them to measure electrical conductivity at very high pressures and various temperatures. They found that the non-molecular semiconducting form of nitrogen was stable over a remarkably wide pressure range, and some samples -- when held at low temperature -- even retained this state when decompressed to atmospheric pressure.

The observations of the new form of nitrogen suggest that other novel high-density materials -- perhaps even solid metallic hydrogen -- could be created at high pressure and recovered at ambient pressure conditions. Equally important, this work confirms theories that have been used to predict new properties such as high-temperature superconductivity in metallic hydrogen.

The National Science Foundation (Division of Materials Research, and Science and Technology Center for High-Pressure Research) and the Carnegie Institution of Washington supported this work.

The Carnegie Institution of Washington has been a pioneering force in basic scientific research since 1902. It is a private nonprofit organization with five research departments in the U.S.: Terrestrial Magnetism, Plant Biology, Observatories, Embryology and the Geophysical Laboratory.

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The Carnegie Institution of Washington

[Contact: Russell Hemley, Amber Jones, Tina McDowell]

10-May-2001