Recoverable high-energy compounds by reacting methane and nitrogen under high pressure

The search for the stable polymeric forms of solid nitrogen is of great importance in view of its potential application as a high-energy-density material. Using first-principles calculations combined with an effective crystal structure search method, we demonstrate that nitrogen can react with methane under relatively moderate pressures. The structures and the stability strongly depend on the composition. ${\mathrm{CN}}_2{\mathrm{H}}_4$ and ${\mathrm{CN}}_4{\mathrm{H}}_4$ compounds become stable in $Pca{2}_1$ and $I\text{−}42d$ phases under pressures of 11 and 41 GPa, respectively. Especially, ${\mathrm{CN}}_4{\mathrm{H}}_4$ is recoverable as a metastable high energy material at ambient condition, and can release enormous amount of energy ($6.43\mathrm{kJ}{\mathrm{g}}^{-1}$) while decomposing to molecular nitrogen and methane.

Figure 1

Phase stabilities of ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds. (a) The enthalpies of formation for various ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds under a series of pressures. The dotted lines connect the data points, and the solid lines denote the convex hull. The data show that ${\mathrm{C}}_2{\mathrm{NH}}_8$, ${\mathrm{CNH}}_4$, ${\mathrm{CN}}_2{\mathrm{H}}_4$, and ${\mathrm{CN}}_4{\mathrm{H}}_4$ are the stable stoichiometries, and ${\mathrm{CN}}_2{\mathrm{H}}_4$ is the most stable stoichiometry at high pressures. vdW interactions and ZPE corrections are considered during our calculations. (b) Predicted stable pressure ranges for ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds, different colors present different phases in the corresponding stable pressure ranges. (c) Division of ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ phase diagram into regions of molecular compounds (amines, diamines), polymeric compounds, and the most interesting HEDM region. Red squares are our predicted stable structures.

Figure 2

Crystal structures of ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds. (a) $P1$ phase at 10 GPa for ${\mathrm{C}}_2{\mathrm{NH}}_8$. This phase can be seen as a mixture of molecular units: ${\mathrm{C}}_3{\mathrm{NH}}_9+{\mathrm{NH}}_3+{\mathrm{CH}}_4$. (b) $P{2}_1$ structure for ${\mathrm{CNH}}_4$ at 50 GPa. This phase can be seen as a molecular phase with formula units ${\mathrm{C}}_2{\mathrm{N}}_2{\mathrm{H}}_8$. (c) The polymeric $Pca{2}_1$ phase of ${\mathrm{CN}}_2{\mathrm{H}}_4$ at 20 GPa. (d) $I\text{−}42d$ phase of ${\mathrm{CN}}_4{\mathrm{H}}_4$ at 50 GPa.

Figure 3

Electronic properties and phonon dispersion curve of $I\text{−}42d$ phase for $p\text{−}{\mathrm{CN}}_4{\mathrm{H}}_4$ at 0 GPa. (a) Electronic band structure and density of states. (b) phonon spectrum. (c) Calculated crystalline orbital Hamiltonian population and integrated crystalline orbital Hamiltonian population of C-C and N-N pairs. The horizontal lines present the Fermi levels. (d) Three-dimensional valence electron localization functions with isosurface value of 0.85.

Figure 4

Formation mechanism, phase diagram and energy densities of ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds. (a) Internal energies (top panel), $PV$ energy terms (middle panel), and formation enthalpies (bottom panel) for ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ compounds relative to ${\mathrm{CH}}_4+{\mathrm{N}}_2$ at 50 GPa. (b) Temperature versus pressure phase diagram of ${\mathrm{CH}}_4\text{−}{\mathrm{N}}_2$ system. Dashed lines show the proposed phase boundaries. (c) Calculated energy densities of the polymeric ${\mathrm{CN}}_4{\mathrm{H}}_4$ phase at ambient pressure in relation to the pressure required to stabilize them. The green scale represents the w.t. of nitrogen.