  The phase diagram and equation of state of dense nitrogen are of interest in
understanding the fundamental physics and chemistry under extreme conditions,
including planetary processes, and in discovering new materials. We predict
several stable phases of nitrogen at multi-TPa pressures, including a P4/nbm
structure consisting of partially charged N2 pairs and N5 tetrahedra, which is
stable in the range 2.5-6.8 TPa. This is followed by a modulated layered
structure between 6.8 and 12.6 TPa, which also exhibits a significant charge
transfer. The P4/nbm metallic nitrogen salt and the modulated structure are
stable at high pressures and temperatures, and they exhibit strongly ionic
features and charge density distortions, which is unexpected in an element
under such extreme conditions and could represent a new class of nitrogen
materials. The P-T phase diagram of nitrogen at TPa pressures is investigated
using quasiharmonic phonon calculations and ab initio molecular dynamics
simulations.
