**First-principles Investigation on Structures and Physical Propreties of Group VB Transition Metal Nitrides**

Author:Chen Lei

Supervisor:jiang zhen yi

Database:Doctor

Degree Year:2019

Download:53

Pages:122Size:7095K

Keyword:crystal structures，Elastic properties，electronic structures，Ideal strength，Nitrides

With the development of science and technology,the demand for new hard materials is increasing day by day.Transition metal nitrides are widely used due to their generally high hardness,good wear resistance,corrosion resistance,and high chemical stability.In a wide variety of industrial applications,such as abrasives,cutting tools,coatings,etc.,it has become a research hotspot in the field of materials.The first-principles calculation method is used to study the changes of material structure and physical properties in the large pressure range,which is of great significance for guiding experiments.Based on the first-principles calculation,CALYPSO code is employed for crystal structural prediction fo the transition metal-nitrogen of Group VB.The structures of mononitrides,dinitrides,and ternary-nitrides are explored in the large pressure ranges.The stability and mechanical properties of the structures are also studied to provide guidance for future experimental synthesis.The following innovative results have been obtained:First,the study of the binary mononitrides of V,Nb,Ta shows that the P-6m2 structure is the ground state phase of VN and NbN,which is the most stable structure in the range of 0 GPa to 100 GPa.The ground state phase of TaN adopts the P-62m structure.The B1structure is defined as the high temperature phase which is unstable at 0 K and cannot satisfy the stability of lattice dynamics.By comparison with nine neighboring mononitrides,the number of valence electrons in f.u.VEC is a key factor affecting the mechanical properties.As the hardest materials in these nine mononitrides VN and TaN have the best VEC values and the same d-orbital electrons.The calculations of elastic anisotropy and strength anisotropy show that the mechanical anisotropy of the P-6m2 structure is much smaller than that of B1 structure.The calculated results from empirical formula and ideal indentation shear strength show that the theoretical hardness of P-6m2 structure VN and TaN is about 30 GPa.From the analysis of electronic structure,the bonding nature between transition metal and nitrogen is a mixture of ionic,covalent and metallic bond,and the covalent action is the source of high hardness and strength.Second,the study of binary dinitrides shows that VN2 has a hexagonal phase with the space group of P63/mmc,an orthorhombic phase with the space group of Cmc21 and a tetragonal phase with the space group of I4/mcm existing in different pressure range.The two structures of NbN2 and TaN2 are the orthorhombic phase with the space group of Amm2 and the I4/mcm structure.We have verified the thermodynamics,lattice dynamics and mechanical stability of these structures.The results show that these structures are thermodynamically stable at 0 K,and are also lattice dynamics and mechanically stable.The results of elastic anisotropy and ideal strength show that the elastic anisotropy of the hexagonal phase and the orthorhombic phase is large.The tetragonal phase has the smallest elastic anisotropy,and the hexagonal phase of VN2 shows a better ideal strength.The mechanical property is greatly improved in the comparison of the B1 structure.Third,studies on ternary mononitrides V0.5Ta0.5N,V0.5Nb0.5N and Nb0.5Ta0.5N show that they have two orthorhombic phases with the space groups of Pmm2,Cmc21 and one cubic structure with the space group of Fm-3m existing in different pressure range.The verification of thermodynamic,lattice dynamics and mechanical stability shows that these structures are thermodynamically stable at 0 K,and are also lattice dynamics and mechanically stable.There is an obvious band gap near the Fermi level in the calculated electronic structures of cubic phase of V0.5Ta0.5N and V0.5Nb0.5N by HSE06 hybrid functional method.Further analysis shows that the formation of the band gap is mainly due to the difference of atomic radius in the lattice.The results of elastic anisotropy and ideal strength show that the orthorhombic phase has less elastic anisotropy and the cubic phase structure has greater elastic anisotropy.Since the two orthorhombic phases only have the difference of metal atom ordering,the difference in enthalpy is formed.There may be a competitive relationship during experimental synthesis.The orthorhombic phase has better ideal strength and minimum strength anisotropy.The indentation shear strength and Vickers hardness of Cmc21 phase of V0.5Ta0.5N reach up to 30 GPa,which can be used as a candidate for hard engineering materials.