Phase Stability Of Tungsten Nitride And The Effect Of Defects On Its Properties

Transition metal nitrides have high hardness,unique electronic and chemical properties,and good thermodynamic stability.In particular,in recent years,tungsten nitride has received extensive attention both in theory and experiment because of its good physical properties,and a large amount of theoretical and experimental research has focused on nitrides having an integer stoichiometric ratio.In this paper,the particle swarm optimization algorithm and the first-principles calculation method are used to study the phase stability of tungsten nitride and the effect of defects on its properties.The main findings are as follows:?1?For W₂N₃,the ground state W₂N_3mC30₈CM and the high pressure state W₂N_3oP20₆2PNMA of W2N3 are predicted by structural prediction and phase stability analysis.Through theoretical calculations,the experimentally structural h-W₂N₃ is mechanically and dynamical stable,consistent with the experimental conclusions.The experimentally structure r-W₂N₃ is unstable in mechanics and dynamical.The W vacancy in the 3a lattice reduces the interaction between the layers in the crystal,which leads to the mechanical instability of r-W₂N₃.W₂N_3mC30₈CM and W₂N_3oP20₆2PNMA are mechanically and dynamical stable,and W₂N_3oP20₆2PNMA has the largest bulk modulus?397GPa?and shear modulus?220GPa?,it is an ultra-incompressible material.The O dopted improves the thermodynamic stability of W₂N₃,but reduces the bulk modulus and shear modulus of the structure,which in turn reduces the mechanical stability of W₂N₃.?2?For WN,through structural prediction and phase stability analysis,it is predicted that the ground state is WN_cP6₂21PM-3M,and the high pressure state is WN_mC8₈CM and WN_hP4₁94P63/MMC.WN_cP6₂21PM-3M,WN_mC8₈CM and WN_hP4₁94P63/MMC are mechanically stable.WN_hP4₁94P63/MMC has the largest bulk modulus of 428GPa,while WN_cP6₂21PM-3M and WN_mC8₈CM also have large bulk moduli,390GPa and 378GPa,respectively,which indicates WN_hP4₁94P63/MMC,WN_cP6₂21PM-3M and WN_mC8₈CM are both ultra-incompressible materials.WN_cP₂21PM-3M and WN_mC8₈CMaredynamicalstable,but WN_hP4₁94P63/MMC is dynamical unstable.For the effect of O doping on the physical properties of WN,we conclude that the doping of O improves the thermodynamic stability,but reduces the mechanical properties of WN.?3?For WN₂,through the structure prediction and phase stability analysis,it is predicted that both the the ground state and the high pressure state are WN_2hP6₁94P63/MMC.WN_2hP6₁94P63/MMC is mechanically and dynamical stable,has a very large bulk modulus?395 GPa?and shear modulus?252 GPa?it is a very ultra-incompressible material.From its energy band and density of states,a band gap appears near the Fermi level,and the band gap is 1.325 eV.WN_2hP6₁94P63/MMC is a semiconductor material.The lattice constant of the crystal structure doped with O and the volume of a single atom do not change much,but the elastic constant,bulk modulus and shear modulus become smaller,and the incorporation of O atoms reduces the mechanical stability of WN_2hP6₁94P63/MMC.?4?For WN₃,through the structure prediction and phase stability analysis,the ground state is predicted to be WN_3tP4₁23P4/MMM,and the high pressure state is WN_3tP8₁P1 and WN_3oC16₆3CMCM.WN_3tP4₁23P4/MMM,WN_3tP8₁P1 and WN_3oC16₆3CMCM are mechanically stable,and WN_3tP4₁23P4/MMM and WN_3oC16₆3CMCM express very good elastic properties in the C₃₃ direction.The phonon dispersion curve of WN_3tP8₁P1 does not converge,WN_3oC16₆3CMCM is dynamical stable,but WN_3tP4₁23P4/MMM is kinetically unstable.The incorporation of O atoms increases the thermodynamic stability of WN₃ while reducing its mechanical stability.