  Nitrogen hydrides, including ammonia (NH3), hydrazine (N2H4), hydrazoic acid
(HN3) and etc, are compounds of great fundamental and applied importance. Their
high-pressure behavior is important because of their abundance in giant planets
and because of the hopes of discoverying high-energy-density materials. Here,
we have performed a systematic investi- gation on the structural stability of
N-H system in a pressure range up to 800 GPa through evolutionary structure
prediction simulations. Surprisingly, we found that high pressure stabilizes a
series of previously unreported compounds with peculiar structural and
electronic properties, such as the N4H, N3H, N2H and NH phases composed of
nitrogen backbones, the N9H4 phase containing two dimensional metallic nitrogen
planes and novel N8H, NH2, N3H7, NH4 and NH5 molecular phases. Another surprise
is that NH3 becomes thermodynamically unstable above ~460 GPa. We found that
high-pressure chemistry is much more diverse that hydrocarbon chemistry at
normal conditions, leading to expectations that N-H-O and N-H-O-S systems under
pressure are likely to possess richer chemistry than the known organic
chemistry. This, in turn, opens a possibility of nitrogen-based life at high
pressure. The predicted phase diagram of the N-H system also provides a
reference for synthesis of high-energy-density materials.
