Stable All-Nitrogen Metallic Salt at Terapascal Pressures

The phase diagram and equation of state of dense nitrogen are of interest in understanding the fundamental physics and chemistry under extreme conditions, including planetary processes, and in discovering new materials. We predict several stable phases of nitrogen at multi-TPa pressures, including a $P4/nbm$ structure consisting of partially charged $N_2^{\delta +}$ pairs and $N_5^{\delta -}$ tetrahedra, which is stable in the range 2.5–6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The $P4/nbm$ metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. The $P\mathrm{\text{−}}T$ phase diagram of nitrogen at TPa pressures is investigated using quasiharmonic phonon calculations and ab initio molecular dynamics simulations.

Figure 1

Crystal structures of the newly predicted nitrogen phases. (a) $P4/nbm$, (b) $P{2}_1$, (c) A supercell of $P{2}_1$ viewed along the $a$ axis. The colored spheres in (a) and (b) represent atoms with different charge environments, as described in the text. Atoms in different layers in (c) have been shown with different colors.

Figure 2

Enthalpy-pressure relation for N. The values in the main figure are relative to the enthalpy of $P4/nbm$. The upper inset shows the enthalpies relative to $R\overline{3}m$ at very high pressures. The lower inset shows the enthalpies at low pressures relative to $P4/nbm$.

Figure 3

Proposed $P\mathrm{\text{−}}T$ phase diagram of nitrogen. The solid-solid phase boundaries up to 4000 K were computed using the QHA. The melting temperatures were computed using the $Z$ method and ab initio molecular dynamics. The shaded blue area on the left corner is the solid molecular ${\mathrm{N}}_2$ region; the inset depicts the small stability region of $Cmca$.

Figure 4

Electron densities of polymeric nitrogen. (a) $P4/nbm$ at 2.5 TPa along [100] and [110] directions, (b) $P{2}_1$ at 8 TPa along [001] directions.