Embargoed until 2 p.m. EDT
NSF PR 01-39 - May 9, 2001
Media contact:
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Amber Jones
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(703) 292-8070
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aljones@nsf.gov
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Program contact:
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Hollis Wickman, NSF
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(703) 292-4929
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hwickman@nsf.gov
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Russell Hemley, Carnegie
Institution of Washington
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(202) 478-8951
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hemley@gl.ciw.edu
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New Form of Nitrogen: A Semiconductor
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.
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