Fig. 4.

The calculated enthalpy and volume per atom for the ι–N₂ structure. a Calculated PBE enthalpy differences with respect to the P4₁2₁2-candidate structure of ref.¹⁸. The computed enthalpy of ι–N₂ is more favourable than ε–N₂ above 20 GPa. Polymeric cg–N becomes favourable at 58 GPa, in agreement with previous DFT estimates. b Calculated volume per atom of ι–N₂ and the P2₁/c-candidate structure of ref.¹⁸ plotted with experimental data for ε–N₂ and ζ–N₂. The ι–N₂ volumes are ~1% larger than the P2₁/c-candidate of ref.¹⁸. The star shows the experimentally determined ι–N₂ volume per atom at 56 GPa and ambient temperature. The experimental data for ε–N₂ and ζ–N₂ are reproduced from ref.⁹ and references therein