First-Principles Study Of Electronic Properties VB Transition-Metal Compounds

Transition metal borides, transition metal carbides and transition metal nitrides usually havegood properties of large hardness, high melting point; wear resistance and good electrical conductivity.They are widely used in hard coating and cutting tools, and electronic applications. In this thesis, thefirst-principles was employed in study of the structural, electronic and elastic properties of thesecompounds. On the basis, all the simulated results and predict more general results were done.The bonding nature and elastic property of VB₂ have been studied. The nature of chemicalbonding can be recognized as a combination of covalent, ionic and metallic bonds from theirelectronic structures. Density of state, valence charge density and Mulliken population have also beenexplored to assess the origins of"pseudogap"and charge transfer. The calculated anisotropy factorsindicate that VB₂ are largely compression and shear isotropic and the anisotropy is weak.The electronic structures of NbN solid with the NaC1, WC, CsC1 and hcp phases have beeninvestigated. The calculated results indicate that, at the zero pressure, the order of relative stability forthe four phases is WC phase> NaC1 phase>CsC1 phase> hcp phase. As the pressure increases, thehcp phase is stable. The structural phase transitions from NaC1 phase to CsC1 phase occurs at about166 GPa, and from WC phase to CsC1 phase occurs at about 181 GPa.The effect of the vacancies on the structural and electronic properties in substoichiometric VNxand VyN is studied. A model structure of 8 and 16 atom supercell with ordered vacancies isused.When the vacancies are introduced, the formation of all types of non-stoichiometric states isenergetically unfavorable and they cause peaks in the density of state near the Fermi level. Besides,the V-N bonds are broken and new bonds are formed.The geometry and electronic structures of the (100) surface of TaC have been investigated. Theresults indicate that, after the relaxation the surface energy decreases and the TaC(100) surface leadsto the C and Ta atoms on the top layer move outward and inward, respectively, and as a result, thespace between the first and second layer is contracted. Besides, from the PDOS of Ta and C atoms,the DOS of Ta and C atoms in the first layer is similar to the DOS of the (100) surface, while the DOSof Ta and C atoms in the second layer is toward to the DOS of the bulk.