Phase Stability And Elastic Properties Of Binary Iron Nitrides Compounds From First Principles

Transition metal nitrides have been widely used in industrial technology,especially materials,superhard coatings,material structure of ceramics,microelectronics,superconductivity,material reaction catalyst and photoelectric solar energy materials.This paper mainly studys the structural stability,electronic structure and elasticity properties of four transition metal binary nitrides for γ′′-FeN 、ζ-Fe₂N 、γ′-Fe₄N and α′′-Fe₁₆N₂ based on density functional theory by using first-principles.In this paper,the authors improve the method of calculated elastic constants with the strain energy-strain curve.The strain must be tested before it applied to the material.It can shorten the time of calculation and reduce the error.This method can be used to calculate the elastic constants of material unless the strain applied the material causes a small deformation than unstrained.The symmetry of the material under external high pressure might change from that of the unstrained material.The strain energy-strain curve cannot be used to calculate elastic constants in such cases.A new approach to get the elastic constants of material under external high pressure is needed.Based on the Gibbs free energy of the material,the authors find the method of calculating the elastic constants of the system under external pressure.In order to prove the correctness of the new method,this paper uses the new method to calculate the elastic constant of the iron element with ferromagnetic body cube under external high pressure,and compared with other theoretical calculations and experimental results.In this work,the disappearance of Born’s stability conditions of the ferromagnetic body cube using the new method is160 to 165 GPa.This result is closer to the experimental result,150 GPa,than theoretical calculation result,200 GPa.It indicates that the new method found in this paper is correct.The result of formation energy shows that four iron-nitrogen compounds is a thermodynamically stable phase at the ground state.They all have a larger bulk modulus associated with their atomic structure and the force between the atoms.By calculating the nitrogen content of four iron-nitrogen compounds and their formation enthalpy,the authors find that the stability of transition metal binary nitrides has a linear relationship with their nitrogen content.Their enthalpy of formation increases with increasing nitrogen content.This indicates that the thermodynamic stability is related to their nitrogen content.The third-order Birch-Murnaghan equation of state is used to calculate the volume modulus of the four iron-nitrogen compounds.This result shows that they have a large bulk modulus.By analyzing the total and partial density of states of four ironnitrogen compounds,they all are metallic and have strong hybridization existing near the Fermi level between p electrons of N atom and d electrons of Fe atom.It forms the strong covalent bonding between Fe and N atom.The further analysis of their electron local density between Fe atoms and N atoms does form a strong covalent bond.In this paper,the partial N atoms of γ′′-FeN and ζ-Fe₂ N were replaced by O atoms to study the effect of O doping on the stability and elastic properties of γ′′-FeN and ζ-Fe₂N.By calculating the enthalpy of formation of γ′′-FeN and ζ-Fe₂N after O doping,it is shown that O doping improves the structural stability of the two systems and maintains certain hardness.