High-Energy Density and Superhard Nitrogen-Rich B-N Compounds

The pressure-induced transformation of diatomic nitrogen into nonmolecular 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 Vicker's hardness of ~44 GPa. Structure searching reveals a new clathrate sodalitelike BN structure that is metastable under ambient conditions.