First Principles Study Of Series Materials Of Transition Metal Sulfide

The electronic structure of orthorhombic perovskite BaVS₃ compound is studied by first-principles calculations based on the ab initio pseudopotential and plane wave basis. Its lattice dynamics is studied using the density-functional perturbation theory by the linear response method.Phonon spectrum throughout the Brillouin zone is obtained and the interatomic force constants are analyzed.By comparing the different behaviors of lattice dynamics between hexagonal phase and orthorhombic phase,we believe that the soft E_2u¹ mode is responsible for the observed hexagonal-orthorhombic phase transition.We also calculate the electronic structure and phonon frequency atΓof BaVS_3-xSe_x compounds.The results show that the hexagonal structure of BaVS_3-xSe_x compounds becomes more unstable with the increase of Se doping,which is consistent with the experiment.Finally,we study the electronic structure of BaV_1-xTi_xS₃ with x=0.8.By using the frozen-in phonon method,the mode which involves the opposite vibration of V/Ti atoms in planar is studied.The imaginary frequency which appears in BaVS₃ doesn't appear in BaV_0.2Ti_0.8S₃,convincing that BaV_0.2Ti_0.8S₃ in hexagonal phase is stable.There is no phase transition from hexagonal to orthorhombic.There are three chapters in our work.In chapter 1,the formerly research and the primary content of this thesis is introduced;in chapter 2,we present the basic theories employed in this study,such as adiabatic approximation,density functional theory, density functional perturbation theory,local density approximation and generalized gradient approximation,self-consistent field calculation,plane wave pseudopotential method and the used ABINIT package as well;the computational results and discussion are given in chapter 3.