Theoretical study of the molecular-to-nonmolecular transformation of nitrogen at high pressures

We report calculations of the electronic charge density and the total energy of nitrogen as a function of volume in both molecular and nonmolecular crystal structures, using the local-density-functional method with an ab initio pseudopotential. At low pressures the molecular bond length is found to be 1.10 Å, in good agreement with experiment. At high pressures we find a stable distorted arseniclike structure, which is semimetallic with a very small Fermi surface and is lower in energy than simple cubic and all other simple metallic structures considered previously by McMahan and LeSar. To investigate the transition under pressure we have carried out calculations for a number of structures which are on a path that connects continuously the molecular β-${\mathrm{O}}_2$, diamond, graphitic, arseniclike, and simple-cubic structures. The calculated transition occurs at a pressure of approximately 700 kbar, with a large barrier of approximately 1 eV/atom along this path. One signature of the transition is a large decrease of the highest phonon frequency. Although there is some uncertainty in our calculated pressure, we conclude that our results predict a transition to a nonmolecular structure at experimentally accessible pressures, in apparent disagreement with recent experiments in which no such transition was found up to a reported pressure of 1.3 Mbar.