Polymerization of nitrogen in sodium azide

The high-pressure behavior of nitrogen in NaN₃ was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N^3- are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.