Stability of new structures of phosphorous-nitrogen solids at high pressure

New structures of phosphorous-nitrogen compounds under pressure are predicted using evolutionary algorithms and density functional theory calculations. A novel predicted at 50 GPa structures of PN4 with symmetry P2/m and C2/m are stable according to convex-hull construction and phonon dispersion calculations. P2/m symmetry crystal has "dimer-like" nitrogen atoms and C2/m crystal has the oligomer of four nitrogen atoms placed between six-fold coordinated phosphorous atoms. The predicted crystals with high nitrogen content are metallic and might be considered as a new type of energetic materials with enhanced work capacity. The mechanical stability of P-N structures at 50 GPa is tested using Born criteria based on calculated elastic constants. Predicted monoclinic crystals PN4 with C2/m and P2/m symmetries, P3N4 with C2/m symmetry and previously reported monoclinic PN2, orthorhombic gamma-P3N5, and triclinic delta*-P3N5 crystal are proved to be mechanically stable at 50 GPa. Recently synthesized at high pressure delta-P3N5 crystal with C2/c symmetry is found to be thermodynamically stable at 50 GPa. Calculations of several possible structures of P-N using a method of virtual diffraction show that the best fit to experimental data gives an epsilon-phase of nitrogen mixed with the gamma-phase of P3N5.