Metastable high-pressure single-bonded phases of nitrogen predicted via genetic algorithm

Abstract

The recently proposed genetic algorithm for crystal structure prediction combined with first-principles structural optimizations is used to investigate the high-pressure structures of solid nitrogen. Starting from a population of randomly generated eight-atom structures at $80\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the evolutionary process not only recovers the four lowest-energy nonmolecular structures (CG, $C2∕c$, black phosphorus, and $Cmcm$ chain) predicted theoretically or known experimentally, but also reveals a metastable single-bonded three-dimensional structure. The stability of this structure at $80\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is established by phonon calculations. At this pressure, the enthalpy of the structure is $0.17\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$/atom higher than that of the cubic gauche phase. The energetic difference between this structure and other nonmolecular high-pressure phases is explained from analysis of the local structural motifs.