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 $(cg)\text{\ensuremath{-}}\mathrm{N}$. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at $\ensuremath{\sim}1000$ and $1300\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ 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 $cg\text{\ensuremath{-}}\mathrm{N}$ to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N ($\ensuremath{\rho}=4.85\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{3}$ at 120 GPa).