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 ${\mathrm{B}}_3{\mathrm{N}}_5$ that are stable or metastable relative to solid ${\mathrm{N}}_2$ and $h$-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure ($h{\text{−}\mathrm{B}}_3{\mathrm{N}}_5$) containing hexagonal ${\mathrm{B}}_3{\mathrm{N}}_3$ layers sandwiched with intercalated freely rotating ${\mathrm{N}}_2$ molecules. At 15 GPa, a three-dimensional $C222_1$ 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, $C222_1\text{−}{\mathrm{B}}_3{\mathrm{N}}_5$ is metastable, and can be recovered under ambient conditions. Its energy density of $\sim 3.44\mathrm{kJ}/\mathrm{g}$ makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vicker’s hardness of $\sim 44\mathrm{GPa}$. Structure searching reveals a new clathrate sodalitelike BN structure that is metastable under ambient conditions.

Figure 1

Convex hull diagram for the BN system at ambient pressure. Formation enthalpy, $\mathrm{\Delta }H$, is defined as $\mathrm{\Delta }H=H({\mathrm{B}}_x{\mathrm{N}}_y)-[xH(\mathrm{B})+yH(\mathrm{N})]$. The alpha-B phase and alpha-${\mathrm{N}}_2$ phase are used in the calculation. Only stoichiometries with formation enthalpies close to the convex hull are presented.

Figure 2

Ten predicted low-enthalpy structures for BN at ambient pressure. Horizontal bars beside each structure represent formation enthalpies with respect to elemental B and N; the color represents the type of structure. Dashed vertical lines in the first six layered structures are shown to distinguish the stacking sequence of the layers.

Figure 3

Illustrations of four predicted low-enthalpy structures of ${\mathrm{B}}_3{\mathrm{N}}_5$ at ambient pressure. Dashed rectangles in the left two structures represent the different orientations of ${\mathrm{N}}_2$ molecules.

Figure 4

(a) Total energies as a function of ${\mathrm{N}}_2$ orientation at ambient pressure in the (110) and $(1\overline{1}0)$ planes of $h\text{−}{\mathrm{B}}_3{\mathrm{N}}_5$. Insets in (a) illustrate ${\mathrm{N}}_2$ rotations. Blue atoms represent rotated ${\mathrm{N}}_2$ molecules. (b) Enthalpy curves of $C222_1$ and $R3$ $m$ structures relative to $h\text{−}{\mathrm{B}}_3{\mathrm{N}}_5$ as a function of pressure.

Figure 5

(a) Calculated tensile stress-strain relations for $C222_1{\text{−}\mathrm{B}}_3{\mathrm{N}}_5$ in various directions. (b) Calculated shear stress-strain relations for $C222_1{\text{−}\mathrm{B}}_3{\mathrm{N}}_5$ in the (011) easy cleavage plane.