New form of nitrogen: a semiconductor

May 08, 2001

Scientists at the Carnegie Institution of Washington reported today they have created a new form of nitrogen by subjecting ordinary nitrogen gas, which makes up about 75 percent of the earth's atmosphere, to pressures of up to 2.4 million times the atmospheric pressure at sea level. At these pressures the nitrogen is transformed to an opaque, semiconducting solid. The scientists also reported 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 the May 10 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 reported signs of the material's transformation at room temperature, using optical techniques alone.

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 from 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.
-end-
NSF is an independent federal agency which supports fundamental research and education across all fields of science and engineering, with an annual budget of about $4.5 billion. NSF funds reach all 50 states, through grants to about 1,800 universities and institutions nationwide. Each year, NSF receives about 30,000 competitive requests for funding, and makes about 10,000 new funding awards.

Media contact:
Amber Jones
(703) 292-8070/aljones@nsf.gov

Program contacts:
Hollis Wickman, NSF
(703) 292-4929/hwickman@nsf.gov
Russell Hemley, Carnegie Institution of Washington
(202) 478-8951/hemley@gl.ciw.edu

National Science Foundation

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