Predicting the crystal structure of N ₅AsF ₆ high energy density material using ab initio evolutionary algorithms

N ₅AsF ₆ is the first successfully synthesized salt that has a polymeric nitrogen moeity (N5+). Although 12 other N5+ salts followed, with N ₅SbF ₆ and N ₅Sb ₂F ₁₁ being the most stable, the crystal structure of N ₅AsF ₆ remains unknown. Currently, it is impossible to experimentally determine the structures of N ₅AsF ₆ due to its marginal stability and explosive nature. Here, following an ab initio evolutionary prediction and using only the stoichiometry of N ₅AsF ₆ as a starting point, we were able to reveal the crystal structure of this high energy density material (HEDM). The C _2V symmetry of the N5+ cation, as suggested from earlier investigations, is confirmed to be the symmetry adopted by this polymeric nitrogen within the crystal. This result gave full confidence in the validity of this crystal prediction approach. While stability of the N5+ within the crystal is found to be driven by electronic considerations, the marginal stability of this HEDM is found to be related to a partial softening of its phonon modes.