Bonding and strength of solid nitrogen in the cubic gauche (cg-N) structure

Recently, the unusual high-pressure cubic gauche structure of solid nitrogen (cg-N) was experimentally synthesized. Our first-principles calculations reveal that cg-N represents a new class of covalent solids in which the atoms are threefold coordinated, yet the structure is stabilized by the presence of near-tetrahedral sp(3)-hybridized electronic states. This bonding scheme results in a strong covalency, exceptional mechanical properties, and the stabilization of cg-N as a high-energy-capacity material. The cg-N is an insulator with a wide gap that is nearly constant over a wide pressure range. The mechanical failure mode in cg-N is dominated by the shear type, and the calculated (110) < 1 $(1) over bar $0 > shear strength of 41.3 GPa (at which the lattice becomes unstable) sets an upper bound for its ideal strength. This shear-induced structural instability is associated with the structural transformation from the cg-N structure to the monoclinic structure.