Superhard: Calculating a nitrogen fix
- Published: Feb 15, 2017
- Author: David Bradley
- Channels: Chemometrics & Informatics
An international team has used chemical computation to predict the existence of a novel superhard material that also has unusual electrical properties.
Artem Oganov of Stony Brook University and the Moscow Institute of Physics and Technology and his colleagues have used theoretical methods to take a new look at the classical chemistry of nitrides of hafnium and chromium with the chemical formulae HfN10 (and its zirconium analogue ZrN10) and CrN4. They suggest that these compounds might be made at relatively low pressures and would contain high-energy groups of nitrogen atoms. The team explains that pure polymeric nitrogen would be the high-energy norm for this element if it were not for the fact that it can only exist at super high pressures because it would pack so much energy per unit volume and be incredibly explosive. Indeed, 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 these pressures only microscopic samples can be made.
However, Oganov's new study reveals that nitrogen can polymerize at much lower pressures in the presence of metal ions and so might find real-world applications. The team predicts a range of novel hafnium nitrides as well as chromium nitrides, carbides and borides some of which have an odd combination of properties - extreme hardness, electrical conductivity, and toughness.
"Our group works on several projects related to metal polynitrides," Oganov says. "This is a promising class of high-energy-density compounds, requiring much lower pressures than pure polymeric nitrogen (i.e. five 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."
Superhard materials fit into two separate classes: compounds that combine boron, carbon, nitrogen and oxygen and compounds of transition metals with boron, carbon and nitrogen. The international team investigated four systems and have published details simultaneously in two papers: the papers discuss the existence of hafnium-nitrogen, chromium-nitrogen, chromium-carbon and chromium-boron compounds. Several of the new materials, which theory suggests can be squeezed into existence with far less extreme pressures than polymeric nitrogen, are 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. The researchers were also able to resolve for the first time the X-ray crystal structure of a known compound Cr2N. The most interesting finding is the chemical compound with the formula HfN10 – here, there are ten nitrogen atoms per hafnium atom. Its structure is very peculiar from a chemical point of view: The hafnium atoms and N2 molecules are sandwiched between infinite chains of nitrogen atoms. Such a structure forms, the team reports, at the relatively mild pressure of 0.23 megabar, about a quarter of a million, as opposed to a million atmospheres.
Pent up energy
In terms of practical applications, it is worth noting that almost all powerful, conventional explosives contain nitrogen. The pent up energy in their chemical bonds is released and dinitrogen gas formed at the moment of explosion. The more nitrogen atoms in a given compound, the more energy that can be released per unit mass.
J Phys Chem Lett 2017, online: "Computational Search for Novel Hard Chromium-Based Materials"
Phys Rev B 2017, 95, 020103(R): "Pressure-stabilized hafnium nitrides and their properties"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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