Carnegie Institution for
Science 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,” remarks
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 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,” says 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.”

Source: Carnegie Institution for Science