Fig. 1.

Thermodynamic stability of the predicted structures. Results from structure searching at 0 (a), 25 (b) and 50 (c) GPa. Convex hulls are shown as continuous lines, with (red) and without (black) the inclusion of vdW corrections. hcp-He⁵⁰ at 0, 25 and 50 GPa, α-N₂⁵¹ at 0 GPa and ε-N₂^52,53 at 25 and 50 GPa were adopted in the calculation. d Calculated enthalpy curves of the I4₁/a and P$\overline{1}$ structures with respect to the C2/c structure for HeN₄ as a function of pressure. The decomposition enthalpy (blue triangles) into hcp-He plus molecular N₂ (α-N₂ at 0 GPa and ε-N₂ above 0 GPa) was also plotted. Inset in d is an enlarged view of the decomposition enthalpy in pressure range 0–70 GPa. Black stars in inset represents the decomposition enthalpy curves of HeN₄ into (He + 4 N) as a function of pressure, where cg-N was considered at pressures above 54 GPa