Superhard materials can be divided into two main classes: compounds of boron, carbon, nitrogen and oxygen together and compounds of transition metals with boron, carbon and nitrogen. The scientists studied four systems in two simultaneously published works: hafnium-nitrogen, chromium-nitrogen, chromium-carbon and chromium-boron. Several new materials, which can be formed at relatively low pressure, were predicted. Among them there are materials with an unusual combination of very high hardness and electrical conductivity. In particular, newly predicted carbide Cr2C should even be stable at atmospheric pressure; and researchers were able to resolve for the first time the crystal structure of a known compound Cr2N.
The fact of the matter is that all good explosive compounds contain nitrogen - at the moment of explosion the nitrogen atoms form the extraordinary stable N2 molecule, releasing a vast amount of energy. The more nitrogen atoms in a compound, and the more unusual their bonding, the more energy will be released as a result of the explosion. Polymeric nitrogen was first predicted by American physicist C. Mailhiot in 1992 and then synthesized in 2004 by Russian physicist Michael Eremets under pressures exceeding one million atmospheres. At such pressures only micron-sized samples can be made, which rules out any practical applications.
Professor Oganov says: “Our group works on several projects related to metal polynitrides. This is a promising class of high-energy-density compounds, requiring much lower pressures than pure polymeric nitrogen (e.g., 5 times lower in case of HfN10, or even less for CrN4, and this is likely not the limit). Chemists have long dreamed about synthesising polymeric nitrogen in large quantities. We have proposed the compound class that can fulfil this dream. “
Two publications appeared as a result of these studies. The first author of the article published in The Journal of Physical Chemistry Letters is Alexander Kvashnin, a postdoc at Skoltech. The first author of the second article in Physical Review B is Jin Zhang, Oganov’s graduate student at Stony Brook University.
The Skolkovo Institute of Science and Technology (Skoltech) is a private graduate research university. Established in 2011 in collaboration with the Massachusetts Institute of Technology (MIT), Skoltech educates global leaders in innovation, advance scientific knowledge, and fosters new technologies to address critical issues facing Russia and the world. Skoltech conducts it work integrating the best practices of the best Russian and international educational and scientific research universities. Moreover, the university pays particular attention to entrepreneurship and innovative education. Website: http://www.skoltech.ru/
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