  Nitrogen represents an archetypal example of material exhibiting a pressure
driven transformation from molecular to polymeric state. Detailed
investigations of such transformations are challenging because of a large
kinetic barrier between molecular and polymeric structures, making the
transformation largely dependent on kinetic stimuli. In the case of nitrogen,
additional complications occur due to the rich polymorphism in the vicinity of
the transition. Here, we report the observation of both molecular (\theta) and
polymeric (BP) phases, crystallized upon temperature quenching of fluid
nitrogen to room temperature at 97-114 GPa. Synchrotron single-crystal X-ray
diffraction, Raman spectroscopy, and first-principles theoretical calculations
have been used for diagnostics of the phases and determination of their
structure and stability. Molecular \theta-nitrogen is the most stable among
molecular phases bordering the stability field of polymeric phases, partially
settling a previously noted discrepancy between theory and experiment
concerning the thermodynamic stability limit of molecular phases.
