| The ground-state and excited-state electronic properties and structural properties of materials are investigated in this thesis using ab initio methods. In the first chapter, we give a brief outline of the main theoretical methods used throughout the current work: the pseudopotential density functional theory for predicting ground-state properties, and the GW approximation for excited-state electronic properties. We study the ideal shear and tensile strength of bcc molybdenum and niobium in the next two chapters. The element Nb is found to be unique among the bcc metals previously studied. Its failure mode when pulled to elastic instability in the "weak" ⟨100⟩ direction is determined by the tetragonal saddle point and is associated with failure in shear rather than tension in the other bcc metals. The pressure dependence of the ideal tensile strength of niobium is also investigated. Next, we study the effects of n-doping and p-doping on the structural and electronic properties of SrTiO3. Several different doping methods are considered: introducing O vacancies, substituting V for Ti, and substituting Sc for Ti. In the following chapter, we present a study of the quasiparticle band structure of ZnS and ZnSe using the GW approximation, treating the Zn semi-core 3d states as valence states. This study demonstrates that the LDA and GGA give similar results, and that self-consistency in updating the quasiparticle energies improves the accuracy of the band gap as well as the energies of the semi-core 3d states. In chapter 6, we describe the LSDA+U method, which is used to treat the strong correlations between d electrons. Then, we perform a systematic study of doping effects on the normal state electronic and structural properties of NaxCoO 2 using the LSDA+U method, in connection to the recent discovery of superconductivity in hydrated NaxCoO 2. In particular, the doping evolution of the Fermi surface topology is studied and we find no violation of Luttinger's rule in this system, contrary to a recent suggestion. Finally, we study the splitting of zone-center TO phonons in AFII MnO and NiO. A model based on the superexchange interaction and the Heisenberg Hamiltonian is proposed to explain the phonon splitting. |
