Fig. 1. (a) Isostructural AlN₆ and GaN₆ of the P2₁ phase at 20 GPa. (b) and (c) Crystal structures of P-1-YN₆ at 40 GPa and P4/mnc-TiN₈ at 30 GPa, respectively. (d)–(f) geometries of channels formed by metal atoms with different types of N₄ coordination. (g) Typical poly-N42− chain and nitrogen dimer N₂.

Fig. 2. Enthalpy difference of MN_x (M = Al, Ga, Y, and Ti) relative to that of a mixture of ground-state MN (M = Al, Ga, Y, and Ti)^24,62–64 structure and bulk nitrogen phase.⁶⁵

Fig. 3. (a)–(d) Radial distribution functions (RDFs) g(r) for MN_x structures from MD simulations. The nitrogen-to-nitrogen pair (N–N) RDFs at different temperatures are shown as solid lines for (a) P2₁-AlN₆, (b) P2₁-GaN₆, (c) P-1-YN₆, and (d) P4/mnc-TiN₈. Vertical dashed lines represent the averaged distance between nitrogen atoms in the structures relaxed at 0 K. The inset graphics show the corresponding statistically averaged structures.

Fig. 4. (a)–(d) Density of states (DOS) and (e)–(h) minus projected crystal orbital Hamilton population (−pCOHP) for P2₁-AlN₆ [(a) and (e)], P2₁-GaN₆ [(b) and (f)], P-1-YN₆ [(c) and (g)], and P4/mnc-TiN₈ [(d) and (h)]. The dashed line indicates the Fermi energy level.

Fig. 5. Sketches of (a) type A and (b) type B chains for N₄–metal coordination, and fragment structures of (c) type A and (d) type B poly-N42− chains.

Fig. 6. Plots of the RDG s vs electron density multiplied by the sign of the second Hessian eigenvalue for (a) P2₁-AlN₆, (b) P2₁-GaN₆, (c) P-1-YN₆, and (d) P4/mnc-TiN₈.