  Studies of polynitrogen phases are of great interest for fundamental science
and for the design of novel high energy density materials. Laser heating of
pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a
polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The
structure was identified in situ using synchrotron single-crystal X-ray
diffraction and further studied by Raman spectroscopy and density functional
theory calculations. The discovery of bp-N brings nitrogen in line with heavier
pnictogen elements, resolves incongruities regarding polymeric nitrogen phases
and provides insights into polynitrogen arrangements at extreme densities.
