First-principles Study Of The Pressure Behavior And Defect Issue Of Transition Metal Oxides

The transition metal oxides (TMO) have been the hot area of researches because of the peculiar physics implied in these materials and the potential value in applications. On one hand, much attentions are paid to the influence of external parameters such as the pressure, temperature on the TMOs’properties. The external parameter pressure is a "cleaner" vari-able when compared to other parameters, since it acts only on interatomic distances. As many TMOs are magnetic, the pressure not only changes the structures of TMOs but also affects the magnetic properties, which make the TMOs’properties richer and the implied physics more peculiar. Besides, studying the pressure behavior of materials can also pro-vide useful information for the materials’preparation and usage under extreme condition. Therefore studying the TMOs’behavior under pressure has important theoretical and ap-plied significance. On the other hand, the TMOs often have lattice defects especially the defects about oxygens. These defects often have decisive effects on materials’properties. For example, the HfO2used in the Metal-Oxide-Semiconductor Field Effect Transistor (MOS-FET) acts as the gate electrode, but the MOSFET can produce leakage current due to the existence of oxygens vacancies. As another example, the HfO2based resistive random access memories (ReRAM) devices, in which the resistive switching phenomenon is closely related to the oxygens vacancies. So it is also important to study the defect issues of TMOs. In this dissertation, using the first-principles calculations which are based on density functional theory, we systematically study the pressure behavior and defect issues of TMOs.Firstly, we study the structure, elasticity, magnetism, electronic structure and the be-havior under pressure for the Tc series perovskite CaTcO3. It is found the weak correlation effects have little effects on the structural and elastic properties. The compound is found to have G-type antiferromagnetic ordering which agrees with the experiment. Band structure calculations show CaTcO3is an indirect band gap semiconductor. Using the classic Heisen-berg model, we find the magnetic superexchange strength between Tc atoms is up to～21meV, which well explain why CaTcO3has so high Neel temperature (-800K). The CaTcO3shows peculiar compression behavior under pressure, i.e., the a axis nearly keeps constant and even expands reversely after～23GPa. This phenomenon is much alike the case of the high pressure experiment on CaIrO3reported by Niwa et al. in2011. In combination with the elasticity calculations, we explain the origin of the peculiar compression behavior.Secondly, we study the structural and elastic properties of orthorhombic and tetragonal layered perovskite Ca3Mn2O7. The tetragonal phase (T-phase) is found to be antiferromag-netic (AFM) and the AFM orthorhombic phase (O-phase) is more stable than the T-phase, which agree with the experiments. The AFM O-phase has lower electrostatic interactions between ions than that of the AFM T-phase. which is the main reason why AFM O-phase is more stable than the AFM T-phase. Elasticity calculations indicate the two phases are mechanically stable against volume expansions, indicating Ca3Mn2O7can exist in the form of thin films. The AFM O-phase is found to be a ductile material, while the AFM T-phase shows brittle nature and tends to be elastically isotropic. We also investigate the influence of strong correlation effects on the elastic properties, qualitatively consistent results are ob-tained in a reasonable range of values of U. Finally, we discuss the charge transfer in the two phases by Bader analysis, and find the distributions of valence electrons have little to do with the two phases’differences of elastic properties.Then, we study the interaction between two oxygen vacancies (Vo) and the electronic structures of oxygen divacancy in the high κ material HfO2. We find the vacancy-vacancy interaction depends not only on the distance of the removed vacancies but also on the coordination of the removed oxygen atoms. The oxygen divacancy is formed energetically by the removal of two fourfold coordinated oxygen atoms (04) with a distance of about2.73A. The interaction between two04vacancies (VO4s) is attractive, indicating that the VO4S tend to form stable cluster in HfO3. The oxygen divacancy induces two in-gap defect levels, which correspond to a bonding state and an anti-bonding one. Based on the calculated results. we speculate that the conductive filaments in the "ON" state of HfO2-ReRAM are formed by the most stable oxygen divacancy which arrange along the b axis of the unit cell of HfO2.Finally, we study the influence of gadolinium (Gd) doping on the Vo of HfO2. It is found that the substitution of Gd for Hf (GdHf) would creates hole doping, and the corresponding defect level locates within the valence band. The single negatively charged GdHf-1is more stable than the neutral GdgHf0for the whole range of Fermi levels. We discuss the interactions (Eint) between GdHF and Vo from two different perspectives, and find that GdHf and Vo show strong attractive interactions. The GdHf would enhance the formation of Vo and vice versa. The most stable complex defect GdHf+Vo corresponds to the case that the GdHf and VO3form next nearest neighbor. In most of the range of Fermi levels, the single positively charged (GdHf+Vo)+1is more stable than the double positively charged (GdHf+Vo)+2, and the defect level of (GdHf+Vo)+1locates also within the valence band. In combination with the calculated results of Eint, we conclude that the experimentally observed reduction of spectrum using the photoluminescence measurement is not caused by the decrease of Vo, but because the GdHf passivates the defect states of Vo and thus change the positions of the defect levels. Therefore, the GdHf would increase the concentration of VO...