Semiconducting non-molecular nitrogen up to 240 GPa and its low-pressure stability

The triple bond of diatomic nitrogen has among the greatest binding energie s of any molecule. At low temperatures and pressures, nitrogen forms a mole cular crystal in which these strong bonds co-exist with weak van der Waals interactions between molecules, producing an insulator with a large band ga p(1). As the pressure is raised on molecular crystals, intermolecular inter actions increase and the molecules eventually dissociate to form monoatomic metallic solids, as was first predicted for hydrogen(2). Theory predicts t hat, in a pressure range between 50 and 94 GPa, diatomic nitrogen can be tr ansformed into a non-molecular framework or polymeric structure with potent ial use as a high-energy-density material(3-5). Here we show that the non-m olecular phase of nitrogen is semiconducting up to at least 240 GPa, at whi ch pressure the energy gap has decreased to 0.4 eV. At 300 K, this transiti on from insulating to semiconducting behaviour starts at a pressure of appr oximately 140 GPa, but shifts to much higher pressure with decreasing tempe rature. The transition also exhibits remarkably large hysteresis with an eq uilibrium transition estimated to be near 100 GPa. Moreover, we have succee ded in recovering the non-molecular phase of nitrogen at ambient pressure ( at temperatures below 100 K), which could be of importance for practical us e.