Pressure-induced symmetry-lowering transition in dense nitrogen to layered polymeric nitrogen (LP-N) with colossal Raman intensity.

We present the discovery of a novel nitrogen phase synthesized using laser-heated diamond anvil cells at pressures between 120-180 GPa well above the stability field of cubic gauche (cg)-N. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at ∼1000 and 1300  cm^{-1} at 150 GPa), arising from two groups of highly polarized nitrogen atoms in the bulk and surface of the layer, respectively. The present result also provides a new constraint for the nitrogen phase diagram, highlighting an unusual symmetry-lowering 3D cg-N to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N (ρ=4.85  g/cm^{3} at 120 GPa).