Fig. 1. (a) Raman spectra of nitrogen heated at 143 GPa from run I (black) compared with the experimental spectra for BP-structured polymeric nitrogen (red bars) at 138 GPa, 2200 K.^[16] Inset shows the microscopic images of the sample under illumination with both reflected and transmitted light before and after laser heating. (b) Crystal structure of BP-N under 140 GPa.

Fig. 2. Raman spectra of nitrogen before and after heating at 130 GPa using TiN and Pb as laser absorber, respectively.

Fig. 3. (a) Raman spectra of BP-N during the decompression. (b) Measured pressure dependence of Raman shift (run III) of BP-N and (run II) of CG-N compared with calculations and previously reported values^{[10,13,15,16]} of polymeric nitrogen. All measurements were performed during decompression.

Fig. 4. (a) The equation of state, (b) formation enthalpy, and (c) energy density of CH-N, BP-N, and cg-N. The values of CH-N, BP-N, and cg-N phases are shown in red triangle, red circle, and blue square, respectively. The red vertical dotted line indicates the pressure (20 GPa), where CH-N becomes stable.