Everyone knows that elemental nitrogen exists in the atmosphere as dinitrogen, N₂. There is a triple bond between the two nitrogen atoms. This is true - but under certain conditions, a fascinating N-N single bonded phase has been characterized.

In 1985 it was predicted that at high pressure, nitrogen would transform to a solid with a single-bonded crystalline structure called polymeric nitrogen. Later, it was proposed that it whould have a cubic gauche (cg-N) structure. Experimental evidence was scant however until 2004 when a team of scientists from Germany and Russia managed to make the compound directly from molecular nitrogen at temperatures above 2000 K and pressures above 110 GPa using a laser-heated diamond cell. The material was characterized by X-ray and Raman scattering methods we have identified this as the polymeric nitrogen (cg-N).

The phase is a stiff with a bulk modulus ≥300 GPa. This is characteristic of strong covalent solids. The polymeric nitrogen is metastable. The structure of N is polymeric with each nitrogen bound to three other nitrogen atoms. At a pressure of 115 GPa, each N-N bond length is 1.346 ± 0.004 Å. The N-N-N angles are all about 108.8°, very close to the ideal tetrahedral angle of just over 109°.

It did not prove possible to recover the polymeric nitrogen by releasing the pressure - in other words the polymer reverts to normal dinitrogen. The authors speculate that this form of nitrogen is a new class of single-bonded nitrogen materials that may have unique energy capacity properties (more than five times that of the most powerful energetic materials).

References

1. "Single-Bonded Cubic Form Of Nitrogen", M.I. Eremets, A.G. Gavriliuk, I.A. Trojan, D.A. Dzivenko, and R. Boehler, Nature (Materials), 2004, 3, 558. [doi:10.1038/nmat1146]
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