A polymeric nitrogen N_6–N_2 system with enhanced stability at low pressure

Abstract

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 N_6 and N_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 molecular crystal at zero pressure makes the N_6–N_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–N_2 polymeric system compared to cg-PN is higher because the C_{2h} symmetry of N_6 within this crystal would be easier to target from the readily available N_3^- azides and the observed N_{3}^+ and N_{3}^* radicals.