Elusive metal discovered: Nickel oxide turned into an electricity-conducting metal

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."