Study On The Magnetic Properties Of Binary Boron, Carbon And Nitride Of Transition Metal Iron

Transition metals borides,carbides and nitrides generally have high melting point,high hardness,good thermodynamic and mechanical stability and excellent chemical resistance.On the basis of these features,iron borides,carbides and nitrides also have special electric,magnetic,and catalytic properties which are extremely potential non-oxide high-temperature structural materials,electronic materials,catalytic materials,and magnetic materials.They are widely used in many fields such as cutting tools,wear parts,coating materials and electromagnetic components.In particular,the magnetic properties of iron borides,carbides and nitrides have attracted widespread attention:high saturation magnetization,low coercive force,high hardness and wear resistance make them have broad application prospects in magnetic recording materials,magnetic cores,magnetic storage materials and special magnetic materials and other fields.In addition,the non-metallic elements B,C,and N play an important role in the heat treatment of the steel.Through surface boronizing,carburizing and nitriding,the surface hardness,wear resistance,fatigue strength;corrosion resistance and high temperature oxidation resistance of the metal substrate can be greatly improved.Firstly,based on first-principles calculations,the stability,electronic structure,magnetic properties and theory hardness of binary Fe-X?X=B,C,N?compounds of the six crystal systems are investigated,including hexagonal systems:Fe₂B,???-Fe₁₆N₂,orthorhombic system:?-FeB,?-Fe₃B,?-Fe₂C,?-Fe₃C,?-Fe₂N,Fe₇C₃,cubic system:zb-FeN,rs-FeN,??-Fe₄N,?-Fe₂₃B₆ and?-Fe₂₃C₆,as well as triclinic and monoclinic Fe₅C₂,respectively.A plane wave expansion method was applied for the optimization of the crystal structures.The exchange and correlation functional was treated using the Perdew-Burke-Ernzerhof?PBE?from the generalized gradient approximation?GGA?.The ultra-soft pseudo-potential?USPP?is selected to describe the interaction potential between the ionic cores and valence electrons.The calculation results show that the lattice parameters agree well with the experimental results.The cohesive energy,formation enthalpy and the elastic constants show that all Fe-X?X=B,C,N?binary compounds are thermodynamically and mechanically stable except for hexagonal Fe₂C which was not satisfied with the criterion of mechanical stability.In addition,from the results of the cohesive energy and formation enthalpy,h-FeB₂ is the most difficult phase to synthesize in the compounds studied here.Through the analysis of the total density of states?TDOS?and the partial density of states?PDOS?,it is found that there was obvious resonance between the iron 3d orbital and non-metal atom X 2p orbital which can form covalent Fe-X bonds.The value of DOS at the Fermi level is not zero,indicating their bonding with metal and therefore has good conductivity.There are charge transfer between the metal atom Fe and the non-metal atom X?X=B,C,N?,shown that the covalent Fe-X bond contained ionicity.Therefore,the bonds of iron borides,carbides and nitrides are mainly covalent with both ionicity and metallity.And the contribution of magnetic moment contribution at different Wyckoff positions can be qualitatively analyzed by the partcial density of states.Additionally,the magnetic properties and theoretical hardness of these materials are also studied in this paper.The results show that h-Fe₂C is a paramagnetic material and the cubic FeN phase with zinc blende structure exhibits paramagnetism while the rock salt structure exhibits antiferromagnetic properties,which the magnetic moment is2.52?_B.Moreover,the magnetic behaviors of the compounds with the same chemical formula and different structures are almost the same.Especially,the triclinic and monoclinic Fe₅C₂ have similar mechanical properties,electronic structure and magnetic properties.The average iron atom magnetic moments of o-Fe₃B,c-Fe₂₃B₆ and c-Fe₂₃C₆are 2.20,2.197 and 2.123?_B,respectively,which are close to that of pure iron?2.24?_B?.However,the magnetic moments of c-Fe₄N and t-Fe₁₆N₂ are 2.51 and 2.48?_B,respectively,which are beyond pure?-Fe.High average atomic magnetic moment and saturation magnetization makes t-Fe₁₆N₂ a candidate material for high-density magnetic recording media.Then,the valence electron density,average bond length,and Mulliken population were also calculated and analyzed.The hardness results predicted by the semi-empirical models show that the hardness of the compounds was related to the bonding density or the valence electron density,the bond length and the degree of the covalent bond.From the bonding point of view,the hardness of the system is more sensitive to weak bonding strength Fe-Fe bonds.The existence of B-B bonds with high covalently did not significantly increase the hardness of iron borides.Except that the hardness of orthogonal Fe₂N is only 10.2GPa,the hardness of iron nitride is generally higher than that of iron borides and carbides.Furthermore,the hardness of Fe-B,Fe-C,and Fe-N covalent bonds has a general rule:Fe-N>Fe-C>Fe-B.