Hexagonal Layered Polymeric Nitrogen Phase Synthesized near 250 GPa.

The nitrogen triple bond dissociates in the 100 GPa pressure range and a rich variety of single-bonded polymeric nitrogen structures unique to this element have been predicted up to the terapascal pressure range. The nonmolecular cubic-gauche (cg-N) structure was first observed above 110 GPa, coupled to high temperature (>2000  K) to overcome the kinetic barrier. A mixture of cg-N with a layered phase was afterwards reported between 120 and 180 GPa. Here, by laser heating pure nitrogen from 180 GPa, a sole crystalline phase is characterized above 240 GPa while an amorphous transparent phase is obtained at pressures below. X-ray diffraction and Raman vibrational data reveal a tetragonal lattice (P4_{2}bc) that matches the predicted hexagonal layered polymeric nitrogen (HLP-N) structure. Density-functional theory calculations which include the thermal and dispersive interaction contributions are performed to discuss the stability of the HLP-N structure.