First-principles study on solid nitrogen under pressure

The polymeric solid formed by nitrogen under ultrahigh pressure is a very typical, ideal and vital high-energy-density material. This paper presents the properties of several classic solid nitrogen phases in terms of physical structures, pressure-volume compression curves, electronic band gaps, densities of states (DOS) and Raman spectra under different ultrahigh pressures. Calculations reveal that the solids of cubic gauche (cg) and its similar phase, the pN phase, are metastable in the range of 0-300 GPa on account of the continuity of the Raman spectra versus pressure. The two easily-confused phases (cg and pN) can be identified by comparing the distinct peaks of Raman modes under pressure. Density functional theory calculation under pressure has gone some way toward enhancing our understanding of high-energy-density material via distinguishing similar phases.