New structures of dense nitrogen: Pathways to the polymeric phase

The existence of new polyatomic structures of nitrogen at high density has been examined based on recent observations for analog systems using first-principles density-functional theory. A structure with Immm symmetry and two molecules per unit cell is found to be stable relative to the known high density diatomic and polymeric phases of nitrogen. The structure is dynamically stable from ambient pressure to at least 90 GPa and thus may represent a new class of observable polynitrogen phases at high pressures and temperatures. Other newly predicted structures may be observed experimentally as stable or metastable phases in different pressure–temperature ranges.