The photo is sourced from rscf.ru
Nickel monoxide (NiO) is a crystalline substance varying in colour from light to dark green or black, depending on the production method and heat treatment. The NiO main application area still remains the glass and ceramics industry, where it is used as a green pigment.
The NiO feature is its inability to conduct electric current at room temperature and atmospheric pressure: the electrons around nickel atoms repel one another with great force, which does not allow them to travel long distances. and thereby, ensure electric current flow. Back in 1937, Neville Mott, a physicist and a future Nobel Prize laureate predicted the ability of nickel monoxide to transform into a metallic state and conduct electricity, at high pressure.
It was possible to experimentally prove the described transition only in 2012, when the physicists from the Institute for Nuclear Research of the Russian Academy of Sciences discovered it at the pressure of 240 gigapascals (GPa), which is 2.4 million times atmospheric pressure. Under such conditions, a nickel monoxide crystal turned from transparent to black, and its ability to prevent current flow “weakened” by three orders of magnitude.
The scientists from the Institute for Nuclear Research of the Russian Academy of Sciences, in their new work, described structural changes occurringin nickel oxide during transition from insulator (the substance blocking the electric current flow) to metal. For this purpose, the research authors created an experimental setup in the form of a miniature chamber of less than a square centimetre, with the diamond anvils on its both sides. A sample of nickel oxide placed inside the chamber was compressed with the anvils up to ultrahigh pressures of several million atmospheres. The sensors installed in the chamber allowed the scientists to measure the NiO oxide resistance.
The experiment recorded a structural change of the NiO sample: its volume first gradually decreased by about 33% with an increase in pressure from atmospheric to 225 GPa, and then, decreased sharply by another 2.7% at the moment of transition. Since the pressure level was close to the one observed in the 2012 research, the scientists drew a conclusion that the structural transition relates to the transition from insulator to metal and can explain the latter.
“The obtained results are very important, firstly, for understanding the fundamental properties of this kind of insulators, and, secondly, for geophysics and studies of the Earth’s structure. According to modern concepts, nickel, along with iron, is part of the Earth’s core in an amount of about 9% in relation to iron. This means that the high pressure properties of nickel and its monoxide must be taken into account when creating a model of the structure of the Earth’s inner layers and core,” the Russian Science Foundation quotes Alexander Gavrilyuk, PhD in Physics and Mathematics, senior researcher at the Institute for Nuclear Research of the Russian Academy of Sciences.