| I report ab initio simulations of nitrogen as a function of temperature and pressure that reveal new structures and basic features of the phase diagram. In particular, the simulations show there are many structures with competitive ground state energies at the volumes that a non-molecular phase is predicted. This most favorable new structure consists of hexagonally packed zig-zag metallic chains. I show several structures to be very stable, implying large barriers to transitions.{09}Furthermore, if there are competing structures with large barriers, it is possible for nitrogen to be transformed by pressure and temperature to an amorphous glassy structure. I investigate such amorphous glassy structures and find them to have similarities with the experimental data. I suggest a way to passivate defects in our amorphous structure leading to increased range of metastability, even to zero pressure at low T. I find characteristics of both the cubic gauche and hexagonally packed chain structure in the amorphous structure, and conclude that the experimental structure may be an amorphous collection of small clusters of these two structures. I have also carried out simulations as a function temperature that, in addition to qualitatively reproducing the experimental principle Hugoniot, predict the anomalous behavior of nitrogen under shock wave conditions, including “double-shock-induced cooling”. I show quenching from these high temperature phases also reveals the tendency toward structures including chains and cubic-gauche-like bonding. |
