Abstract
Recently, the unusual high-pressure cubic gauche structure of solid nitrogen (cg-N) was experimentally synthesized. Our first-principle calculations reveal that cg-N represents a new class of covalent solid in which the atoms are threefold-coordinated, yet the structure is stabilized by the presence of near-tetrahedral sp3 hybridized electronic states. This novel bonding scheme results in 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 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.