High-Pressure Structure Prediction And Physical Properties Of Transition Metal Pernitrides

Transition metal pernitrides have been widely concerned in experiment and theory thanks to its excellent mechanical strength and wear resistance.In this thesis,combining genetic evolution algorithm and first-principles method,the high-pressure structures of transition metal diazides are predicted.The physical properties of transition metal pernitrides,such as elastic constants,Vickers hardness and electronic structure,and their high-pressure behavior are explored.Moreover,the relationship between macroscopic mechanical properties and microscopic electronic structure of different systems is studied.Three novel high-pressure stable phases,Cmcm,P2₁/c-?and P2₁/c-?,are found in the Zr N₂ and Hf N₂ systems by searching the structure of IVB group transition metal pernitrides in the pressure range of 0-200 GPa.The elastic modulus shows that the Vickers hardness of P2₁/c-?phase is close to 20 GPa and the bulk modulus of P2₁/c-?phase is higher than 200GPa.By studying the change of N-N bond lengths with pressure,it is found that the bond lengths of N-N bonds of Cmcm and P2₁/c-?structures first increase and then decrease.The anomalous behavior of the N-N bond lengths of Cmcm and P2₁/c-?phases under pressure is related to the special configuration of two N atoms in the nitrogen-nitrogen dimer sharing one metal atom.Furthermore,we predict two metastable structures of P3₁21 and P3₂21 with chiral crystal structure in the IVB group transition metal pernitrides.According to the first-principles calculations based on density functional theory,P3₁21 and P3₂21 phases are dynamically stable at ambient pressure.Unlike the common IVB group transition metal pernitrides,P3₁21and P3₂21 exhibit unique semiconductor properties,with a band gaps of 1.076 e V for Ti N₂,1.341 e V for Zr N₂ and 1.838 e V for Hf N₂.By studying the crystal and electronic structures of I4/mcm and P3₁21,the mechanism of transition from I4/mcm metal phase to P3₁21semiconductor phase in IVB group pernitrides is discussed.The change of band gap of P3₁21phase in IVB group transition metal pernitrides with pressure is further studied.Through analyzing the orbital contribution,the reasons for different response of band gap of Ti N₂,Zr N₂and Hf N₂ under pressure are explored.In addition,the calculated mechanical properties show that P3₁21 and P3₂21 IVB group transition metal diazides have good bulk and shear moduli.Finally,we systematically search the structures of 3d periodic transition metal pernitrides(VN₂,Cr N₂,Mn N₂,Fe N₂,Co N₂,Ni N₂,Cu N₂ and Zn N₂),and find 10 novel structures in the pressure range of 0-200 GPa.The study of mechanical properties shows that among all studied systems,the phases of Cr N₂ show very high elastic modulus and Vickers hardness,among which the Cmc2₁-Cr N₂ predicted in this thesis has the highest Vickers hardness among all the studied structures.In addition,the elastic modulus and Vickers hardness of the configuration of marcasite(space group:Pnnm)in Fe N₂,Co N₂,Ni N₂ and Zn N₂ systems show the order of Fe N₂>Co N₂>Ni N₂>Zn N₂,indicating that the elastic modulus and Vickers hardness of the marcasite-type 3d transition metal pernitrides decrease with the increase of valence electrons.