Prediction of Superhard BN_2 with High Energy Density

Considering that pressure-induced formation of short, strong covalent bonds in light-element compounds can produce superhard materials, we employ structure searching and first-principles calculations to predict a new class of boron nitrides with a stoichiometry of BN_2, which are stable relative to alpha-B and alpha-N_2 at ambient pressure. At ambient pressure, the most stable phase has a layered structure (h-BN_2) containing hexagonal BN layers between which there are intercalated N_2 molecules. At 25 GPa, a three-dimensional P4_2/mmc structure with single N–N bonds becomes the most stable. Dynamical, thermal, and mechanical stability calculations reveal that this structure can be recovered under ambient conditions. Its calculated stress-strain relations demonstrate an intrinsic superhard nature with an estimated Vickers hardness of \sim43 GPa. This structure has a potentially high energy density of \sim4.19 kJ/g.