Raman and IR study of high-pressure atomic phase of nitrogen

Atomic phase of nitrogen has been studied up to pressure 250 GPa and temperature 3300 K using a shear diamond anvil cell. This phase was synthesized both from azide NaN3 and molecular N2. The atomic phase has been interpreted as a cubic gauche (CG) structure by means of Raman and IR absorption spectroscopy procedures. The phase transition to CG begins at pressure 50 GPa and room temperature for NaN3 and at 127 GPa for N2. Observed pressure dependencies and degeneration of phonon modes, the selection rules for IR and Raman spectra, as well equilibrium pressure between molecular N2 and atomic phase of nitrogen agree well with theoretical predictions for CG.