Layered single-bonded nonmolecular phase of nitrogen from first-principles simulation

Abstract

A phase of nonmolecular nitrogen, found by first-principles density functional theory simulations at high pressures is reported. The phase has a $P{2}_{1}∕m$ symmetry and consists of two-dimensional layers of fused ${\mathrm{N}}_{6}$-rings where the rings are all in the boat conformation. The phase has been designated the layered boat or LB phase. Properties of LB are presented in comparison with other computationally studied polymeric phases of nitrogen. In particular, the structure of LB is compared to two other layered phases, black-phosphorus (BF) and $\ensuremath{\alpha}$-arsenic $(A7)$. A calculated enthalpy versus pressure phase diagram reveals that LB lies between BF and $A7$ in enthalpy at pressures less than $200\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. At pressures higher than this, the enthalpy of LB edges above that of $A7$. The metastability of LB at low pressures is probed by a few simple tests and the band structure is calculated.