Polymeric Nitrogen

Common nitrogen consists of diatomic molecule in which atoms hold each other with the strongest triple covalent bonds. However, in principle, nitrogen atoms can be connected with single or double bonds creating a net of atoms similar to polymeric carbon. The non-molecular nitrogen phase has been recently synthesized at very high pressures about 150 GPa, but it can be recovered to atmospheric pressure due to the huge hysteresis of the transformation. In the present work in addition to the direct polymerization of N2 molecules we explore a different approach to the nitrogen polymerization: molecular N3 anions in the lattice of NaN3 have been squeezed at pressures up to 160 GPa at temperatures 10-300 K. Raman spectra, X-ray diffraction and electrical conductivity were measured in a diamond anvil cell. As a result of pressurizing to 120-160 GPa sodium azide transforms to new structures completely different from molecular consisted of linear N3 - molecules. These new nonmolecular phases preserve at release pressure to the lowest pressures in diamond anvil cell (less than 1 GPa) and some of them can be recovered. One of these structures (phase I) starts to form at 17 GPa. Raman bands at 1670 - 1850/cm are characteristic for this phase. It appears in the pure form (without signs of molecular ion azide) after pressurizing up to 120-160 GPa (or 80 GPa with subsequent large plastic deformation) and persists in the confined cell to the lowest pressures (0-1 GPa). Probably this phase is formed of strongly coupled azide ions forming new molecules or other clusters. In the 50 to 160 GPa pressure range the amorphous phase III is formed. The phase III is conductive; it is completely black and opaque. It partly transforms back to the phase I at 50 GPa upon pressure release. This phase is most probably formed by layers of polymeric nitrogen separated by layers of sodium atoms. Other observed phases are formed by different complexes of nitrogen.