Theoretical precursors to polymeric nitrogen

We predict the existence of new structures of nitrogen based on new observations in analog systems from first-principles density-functional calculations. A series of structures was examined. A structure with orthorhombic symmetry is stable relative to the $\epsilon$ and {\em cubic gauche} phases in LDA, whereas GGA shows the $\epsilon$ and the new orthorhombic structure are energetically competitive. This structure is dynamically stable at least from ambient pressure to 90 GPa and thus may be observed as a stable or metastable polynitrogen phase prior to the transition to the atomic phases of nitrogen.