A polymeric nitrogen N ₆ –N ₂ system with enhanced stability at low pressure

Postulated in 1992 and synthesized in 2004 above 2000 K and 110 GPa, the singly-bonded nitrogen cubic gauche crystal (cg-PN) is still considered to be the ultimate high energy density material (HEDM). The search however has continued for a method to synthesize cg-PN at more ambient conditions or find HEDMs which can be synthesized at lower pressure and temperature. Here, using ab initio evolutionary crystal prediction techniques, a simpler nitrogen-based molecular crystal consisting of N₆ and N₂ molecules is revealed to be a more favorable polynitrogen at lower pressures. The energetic gain of 534 meV/atom over cg-PN and 138 meV/atom over the N₈ molecular crystal at zero pressure makes the N₆–N₂ system more appealing. Dynamical and mechanical stabilities are investigated at 5 and 0 GPa, and vibrational frequencies are assessed for its Raman and IR spectra. The prospects of an experimental synthesis of the N₆–N₂ polymeric system compared to cg-PN is higher because the C_2h symmetry of N₆ within this crystal would be easier to target from the readily available N3- azides and the observed N3+ and N3∗ radicals.