Phase Transition Of Nitrogen And Its Compounds Under High Pressure—First Principles Study

In this paper,the stability and phase transformation of nitrogen elemental and nitrogen-rich compounds under high temperature and high pressure are studied by first-principles calculations.High-energy density materials have been widely used in military,aerospace,and industry fields.Searching for new materials with high-energy density and better performance has become a hot topic.For nitrogen atoms,a triple bond is usually energetically more stable than single or double bonds.In other words,the reaction is exothermic when the double/single bond transfers to the triple bond.Because of the huge bond energy differences between single bond（double bond）and triple bond,the material would harbor high-energy density when it contains lots of nitrogen-nitrogen single bonds.Searching for materials containing nitrogen-nitrogen single bonds（double bonds）becomes a breakthrough in the field of high energy density.At the same time,high temperature and high pressure conditions provide effective ways to stabilize the single bond（double bond）between nitrogen atoms.On the basis of density functional theory and density functional perturbation theory,VASP software was used to calculate the electronic properties of each structure.We found that there are stable N/N single bonds in the high-pressure polymer phase of nitrogen which is a potential candidate as a high-energy density material.Recently,a polymer nitrogen material within black phosphorus structure has been synthesized experimentally at 140 GPa and 4000 K.In this paper,different functionals（PBE,VDW-opt-B88,VDW-D2,and SCAN）are used in calculations to estimate the stable pressure region of the BP-N phase,where the effects from finite temperature are considered by adding vibration energy corrections.Nitrogen phase transition under high pressure was studied by comparing the enthalpy of different phases.Here we predicted four copper nitrogen compounds named as Pnnm-CuN₂,Ⅰ-CuN₃,Ⅱ-CuN₃,and P2₁/m-CuN₅.Our electronic calculations showed that P2₁/m-CuN₅ phase exhibits metallic properties,while the others are semiconductors.The energy densities of the four Cu/N compounds are 1.57,2.39,2.62,and 2.74kJ/g,indicating that they are potential high-energy density materials.The enthalpy values of five nitrogen high-pressure phases were calculated by using different functionals.Although the selection of functional did not change the phase transition order,the phase transition pressure points calculated by SCAN and VDW-opt-B88functionals are relatively higher.After taking the temperature effects into consideration,the nitrogen would be stabilized in the black phosphorus phase in the pressure range of 155-215 GPa,which is in good agreement with the experimental results.For the phases considered here,our comparative analysis showed that the phase transition sequence is CG-N→BP-N→Pba2→P2₁2₁2₁ at pressures ranging from 100 GPa to 400 GPa.