  The pressure-induced transformation of diatomic nitrogen into non-molecular
polymeric phases may produce potentially useful high-energy-density materials.
We combine first-principles calculations with structure searching to predict a
new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are
stable or metastable relative to solid N2 and h-BN at ambient pressure. The
most stable phase at ambient pressure has a layered structure (h-B3N5)
containing hexagonal B3N3 layers sandwiched with intercalated freely rotating
N2 molecules. At 15 GPa, a three-dimensional C2221 structure with single N-N
bonds becomes the most stable. This pressure is much lower than that required
for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More
importantly, C2221-B3N5 is metastable, and can be recovered under ambient
conditions. Its energy density of 3.44 kJ/g makes it a potential
high-energy-density material. In addition, stress-strain calculations estimate
a Vickers hardness of 44 GPa. Structure searching reveals a new clathrate
sodalite-like BN structure that is metastable under ambient conditions.
