Elusive Metal
Discovered
Washington, D.C.—Carnegie scientists are the first
to discover the conditions under which nickel oxide can turn into an
electricity-conducting metal. Nickel oxide is one of the first compounds to be
studied for its electronic properties, but until now scientists have not been
able to induce a metallic state. The compound becomes metallic at enormous
pressures of 2.4 million times the atmospheric pressure (240 gigapascals). The
finding is published in Physical Review Letters.
“Physicists have predicted for
decades that the nickel oxide would transition from an insulator—a compound that
does not conduct electricity—to a metal under compression, but their predictions
have not previously been confirmed,” remarked team leader Viktor Struzhkin of
Carnegie’s Geophysical Laboratory. “This new discovery has been a goal in
physics that ranks as high as achieving metallic hydrogen, but for metal
oxides.”
The
outer shells of atoms contain what are called valence electrons, which play a
large role in electrical and chemical behavior. Metals generally have one to
three of these valence electrons, while non-metals have between five and seven.
Metals are good conductors of electricity because the valence electrons are
loosely bound, so the electrons are free to flow through the material.
Nickel
oxide is what is called a transition metal oxide, which despite its partially
filled outer shell of electrons, remains an insulator. The scientists placed
thin crystal samples, no more than one millionth of a meter (micron) thick, into
a custom-designed diamond anvil cell. Four thin foil leads were crafted to allow
the measurements. The researchers were able to measure declining electronic
resistance beginning at 1.3 million atmospheres (130 gigapascals). At 2.4
million atmospheres there was a dramatic, three-order-of-magnitude drop in
electronic resistance indicating a change from a semiconducting to a metallic
state. The metallic part of the material was located in the region of highest
compression.
“This
finding is certainly important in providing a better understanding of advanced
electronic materials,” said Alexander Gavriliuk, first author of the publication
and a visiting scientist at Carnegie’s Geophysical
Laboratory. “But it also gets us
closer to the ultimate goal of the condensed matter science—improving theory so it can predict the
properties of new materials and then guiding their preparation for practical
use.”
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The Carnegie
Institution for Science (carnegiescience.edu) is a private,
nonprofit organization headquartered in Washington, D.C., with six research
departments throughout the U.S. Since its founding in 1902, the Carnegie
Institution has been a pioneering force in basic scientific research. Carnegie
scientists are leaders in plant biology, developmental biology, astronomy,
materials science, global ecology, and Earth and planetary
science.
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