Single-bonded allotrope of nitrogen predicted at high pressure

An allotrope of nitrogen formed solely by N-N single bonds is predicted to exist between 100 and 150 GPa. The crystal structure has the Pccn symmetry and is characterized by a distorted tetrahedral network consisting of fused ${\mathrm{N}}_{8}, {\mathrm{N}}_{10}$, and ${\mathrm{N}}_{12}$ rings. Stability of this structure is established by phonon and vibrational free energy calculations at 0 K and finite temperatures. The simulated x-ray diffraction pattern of the Pccn phase is compared to the pattern of a recently synthesized nitrogen phase at the same $P\ensuremath{-}T$ conditions, which suggests that the Pccn phase is likely a minor component of the latter. The Pccn phase is expected to form above the stability field of the cubic gauche (cg) phase. The outstanding metastability of this phase is attributed to the intrinsic stability of the $s{p}^{3}$ bonding as well as the energetically favorable dihedral angles between N-N single bonds, in either gauche or trans conformation. The prediction of another single-bonded phase of nitrogen after the lab-synthesized cg phase will stimulate research on metastable phases of nitrogen and their applications as high-energy-density materials.