New High Nitrogen Content High Pressure Structures of P-N Solids.

New structures of phosphorous-nitrogen (P-N) crystals under pressure are predicted using an evolutionary algorithm based upon density functional theory (DFT) calculations. The new predicted structures PN₄, PN₂, and P₃N₄ at 50 GPa are stable according to convex-hull construction and phonon dispersion calculations. PN₄ has the highest concentration of nitrogen out of all known structures of P-N solids. The predicted structures PN₄ and P₃N₄ at 50 GPa are metallic, and the band gap is 0.89 eV for PN₂ according to hybrid functional DFT calculations. The symmetry group of PN₄ is P2/m, as well as that of PN₂; P₃N₄ crystal has a C2/m symmetry group. Calculations of partial density-of-states indicate that the main contribution to the density of states at the Fermi level is related with p-electrons of N for PN₄, and for P₃N₄ p-electrons of both N and P atoms define the density-of-states at the Fermi level. The high pressure structures of P-N solids are compared with available experimental data.