Evidence for a New Extended Solid of Nitrogen

A new extended solid nitrogen, referred to as post-layered-polymeric nitrogen (PLP-N, or Panda-N), was observed by further heating the layered-polymeric nitrogen (LP-N) to above 2300 K at 161 GPa. The new phase is found to be very optically transparent and exhibits ultra-large d-spacings ranging from 2.8 to 4.9 Å at 172 GPa, suggesting a lower-symmetry large-unit-cell 2D chain-like or 0D cluster-type structure with wide bandgap. However, the observed x-ray diffraction pattern and Raman scattering data cannot match any predicted structures in the published literature. This finding further complicates the phase diagram of nitrogen and also highlights the path dependence of the high-pressure dissociative transition in nitrogen. In addition, the phase transition from cubic gauche nitrogen (cg-N) to LP-N is observed at 157 GPa and 2000 K.