14 DAY TRIAL // Though scientists have been studying nickel oxide for many years, they have thus far been unable to induce the compound into a metallic state. Researchers at the Carnegie Institution for Science (CIS) recently managed to establish the conditions needed for this transformation to occur.
Details of the new study appear in a paper published in the latest issue of the esteemed journal Physical Review Letters. The reason why turning nickel oxide into a solid is so important is because the latter can efficiently conduct electricity.
What CIS investigators determined is that NiO requires pressures of up to 240 gigapascals (2.4 million times the level of atmospheric pressure at sea level) to become a metal. The fact that pressure was the key factor in turning NiO metallic was proposed years ago, but never confirmed in practice.
What is important to know about this compound is that, in its natural state, it acts as an insulator, meaning it does not allow electrons to readily flow through its inner structure. As a metal, its interior changes drastically, enabling currents to pass through it efficiently.
“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,” says Viktor Struzhkin.
“This new discovery has been a goal in physics that ranks as high as achieving metallic hydrogen, but for metal oxides,” adds the expert, who holds an appointment at the CIS Geophysical Laboratory. Struzhkin was also the leader of the team behind the new study.
He began to explain that, during the studies, electron resistance in NiO began to decline at around 130 gigapascals, and continued to do so until the compound became electrically-conductive, at 240 GPa.
The enormous amount of pressure was applied using a custom diamond anvil cell. A micron-thick NiO sample was placed inside, along four very thin foil wires, which conducted the measurements.
As soon as the pressure level reached 240 GPa, the wires reported a three-order-of-magnitude drop in electronic resistance on the metal's valence layers. This threshold marks the shift from a semiconductor to a metal, the team reports.
“This finding is certainly important in providing a better understanding of advanced electronic materials,” says Alexander Gavriliuk, the first author of the new paper.
“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,” adds the expert, who is a visiting scientist at the CIS Geophysical Laboratory.