There is experimental evidence that nitrogen forms a polymeric solid under pressure. In this study, we compare various solid nitrogen phases by means of ab initio calculations based on density-functional theory to describe solid nitrogen at different pressures. We show that two phases with covalent bonds in all three dimensions are energetically the most favored at pressures of $50–200\mathrm{GPa}$ and that the layered black phosphorus phase becomes lower in enthalpy at pressures exceeding $200\mathrm{GPa}$, at least at low temperatures. However, according to our results, black phosphorus is unstable at pressures close to $200\mathrm{GPa}$. We further present band structure and density of states calculations for the relevant known phases at several pressures and show that black phosphorus is metallic when metastable, whereas the lower-pressure phases remain dielectric with pressure-dependent band gaps. Also the questionable experimental relevance of several of the previously proposed structures is addressed.

• Received 7 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.144109