A polymeric nitrogen N$$_6$$–N$$_2$$ system with enhanced stability at low pressure

Abstract

AbstractPostulated 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 N$$_6$$ 6 and N$$_2$$ 2 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 N$$_8$$ 8 molecular crystal at zero pressure makes the N$$_6$$ 6 –N$$_2$$ 2 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 N$$_6$$ 6 –N$$_2$$ 2 polymeric system compared to cg-PN is higher because the C$$_{2h}$$ 2 h symmetry of N$$_6$$ 6 within this crystal would be easier to target from the readily available N$$_3^-$$ 3 - azides and the observed N$$_{3}^+$$ 3 + and N$$_{3}^*$$ 3 ∗ radicals.