High-Pressure Phases of Nitrogen

Density-functional-theory calculations and a structure-searching method are used to identify candidate high-pressure phases of nitrogen. We find six structures which are calculated to be more stable than previously studied structures at some pressures. Our four new molecular structures give insight into the most efficient packings of nitrogen molecules at high pressures, and we predict two new nonmolecular structures to be stable at very high pressures.

Figure 1

Variation of the enthalpies of various phases of nitrogen with pressure. Differences in enthalpy from the $R\overline{3}c$ phase are plotted in the pressure range 0–50 GPa (a), and from the cubic gauche $I{2}_{1}3$ phase in the range 50–300 GPa (b).

Figure 2

The molecular $P{2}_1/c$ phase (top) and the molecular $P{4}_1{2}_{1}2$ phase (bottom) at 40 GPa. A fuller appreciation can be obtained by viewing the structures given in the auxiliary material [34] with a visualization package.

Figure 3

The nonmolecular $P\overline{4}{2}_{1}m$ (top) and $P{2}_1{2}_1{2}_1$ structures (bottom) at 200 GPa.