Structure Prediction,Electronic,and Mechanical Properties Studies Of New V_xN（x=1,2）

In recent years,transition metal nitrides have been widely used in industrial production because of there strong incompressibility and ultra-high hardness,such as catalysts,cutting tools,abrasives,and solar control coatings.Different from bulk materials,2D transition metal nitrides have attracted much attention due to their unique physical and chemical properties and their potential as anode materials for ion batteries.In this paper,vanadium nitride is selected as the research object,and the structure of V-N compounds crystals with different stoichiometric numbers is predicted by CALYPSO software.The potential stable phase was identified by energy analysis（convex hull）,and its stability,electronic and mechanical properties were calculated using Material Studio.In addition,the structure of two-dimensional monolayer V2N was predicted by CALYPSO software,and its stability,electronic properties,and adsorption concentration for Li/Na ions were calculated.The main results of the paper are as follows:1.Different stoichiometric ratios will form different coordination,which will affect the properties of compounds.In this study,the crystal structure of VxNy（x=1-4,y=1-6）was searched by the CALYPSO program.And the stable phases of VN5,VN6,V2N,VN,and V4N under ambient conditions were determined.VN and V2N with the lowest energy were mainly analyzed.The research shows that the stable structure of VN is P6m2 and the stable structure of V2N is Pnnm.The analysis of phonon dispersion and thermal stability shows that VN and V2N are kinetically stable,and VN and V2N can maintain thermodynamic stability at 1500k and 1000K,respectively.The band structure and electronic density of states show that that VN and V2N are metals.The calculation of elastic modulus shows that both VN and V2N have high bulk modulus,shear modulus and Young’s modulus,and the G/B of VN and V2N are 0.8 and 0.6,respectively.Both of them are brittle materials,showing good compressive and deformation resistance,and good mechanical properties.2.The space group of 2D monolayer V2N predicted by CALYPSO software is C2/m,the binding energy is 9.15eV/atom,and the phonon dispersion has no imaginary frequency,indicating that the structure is stable and can be synthesized.Thermodynamic analysis showed that the monolayer V2N can maintain the structural stability until 500K.The energy band structure of monolayer V2N is metallic.Furthermore,the adsorption concentrations of Li/Na ions of the monolayer V2N surface are Li4.5V2N and Na2V2N,respectively.The lithium storage capacity and sodium storage capacity are 1546 mA h g-1 and 687 mA h g-1,respectively,indicating that the monolayer V2N has the potential to act as anode material for lithium/sodiumion batteries.