  Cubic gauche polynitrogen (cg-N) is an attractive high-energy density
material. However, high-pressure synthesized cg-N will decompose at
low-pressure and cannot exist at ambient conditions. Here, the stabilities of
cg-N surfaces with and without saturations at different pressures and
temperatures are investigated systematically. Pristine surfaces at 0 GPa are
very brittle and will decompose at 300 K, especially (1 1 0) surface will
collapse completely just after structural relaxation, whereas the
decompositions of surfaces can be suppressed by applying pressure, indicating
that surface instability causes the cg-N decomposition at low-pressure. Due to
the saturation of dangling bonds and transferring electrons to the surfaces,
saturation with H can stabilize surfaces at ambient conditions, while OH
saturation cannot because of getting electrons from the surfaces. An acidic
environment or surface saturation with less electronegative adsorbates is more
favorable for the stability of polymerized nitrogen.
