The First-Principles Study Of The Defects And The Associated Performance In LiNbO₃ Doping TiO₂ System

Point defects are inherent internal structures in solid material such as crystal, and thin film. The presence of point defects will significantly affect the physical properties of material including the conductibility, magnetism, and optical performance, and so on. On the other hand, the first-principles calculation method is an outstanding theoretical method for studying the structures and the performance of material, and it can explore the source of microstructures affecting the properties of material in the electronic structure level. Presently, the research method of point defects based on the first-principles has become one of the important scientific research frontiers.General method determining the defect stability and studying the associated properties of material was obtained by analyzing the previous reported first-principle study on defects. Defect stability in Lithium Niobate (LiNbO3) crystal was firstly studied in order to determine which defect model is the most probable for depicting the defect structures in LiNbO3 crystal. The applicability of the first-principle study on defects under non-perfect condition such as trigonal crystal and high-charged state also was discussed. Furthermore, the origin of unexpected magnetism in doping titanium dioxide (TiO2) system was analyzed in detail to investigate the mechanism of special magnetism.Firstly, the defect stability in LiNbO3 crystal was studied by the first-principles. The calculated results of un-relaxed formation energy show that lithium-vacancy model is the most probable defect model for non-stoichiometric LiNbO3 crystal. However, the configurational stability cannot be fully estimated by full-relaxed formation energy calculation. In such cases, we can qualitatively determine the defect stability from the un-relaxed results.Based on the defect stability analysis, the special magnetism in copper doped titanium dioxide (Cu-doped TiO2) was investigated. The stability of isolated defects in Cu-doped TiO2 crystal was firstly analyzed by the first-principles study. It is found that the most stable defect under reduction atmosphere is transformed from Cu4+o to Cu3+o with against the Fermi-levels increasing in the band gap. Similarly, the isolated defect with the lowest formation energy is transformed from 4+OCu to , 3+OCu1-TiCu or 2-TiCu under the oxidation atmosphere depending on the Fermi-levels. It can also be found that the isolated defects can not contribute the magnetic moment, but 2-TiCu defect can. In addition, the defect can get the lowest formation energies only when the Fermi-levels locate at the conduction-band-bottom. Therefore, it explains why the room temperature magnetism generation caused by isolated defects is difficult. The calculated formation energies of the neutral defect complexes show non-magnetic Cu2-TiCuTi-CuO defect complexes have the lowest formation energy, which means that no magnetic source was provided for the system. Thus, this explains theoretically the reasons for no magnetism generation in Cu-doped TiO2 crystal. For Cu-doped TiO2 thin film with the same element as in Cu-doped TiO2 crystal, the origin of magnetism was thought that originates from the defect enhancement caused by the non-equilibrium growing condition of thin film. The promising defect structure that can provide the magnetism for thin film is CuTi-VO defect complexes.Finally, the mechanism of special magnetism generation in Cu-doped TiO2 thin film was studied. The origin of magnetism was focused on CuTi-VO defect complex. The spin density analysis shows that the magnetic interaction is localized in Cu ion and the neighboring four oxygen atoms, and they formed a special CuO4 complex. This complex CuO4 is similar to CuO4 complex in Cu-doped ZnO films and MgN4 complex in Mg-doped AlN films, and it can provide the magnetism for thin film. It can be concluded that the origin of magnetism for CuO4 complex originates from the following factors by analyzing the energy levels split: 1) the hybridization of p-d orbitals between the Cu and O ions near the band gap of energy level, and 2) the spin polarization of the 3d orbitals of Cu and 2p orbitals of O ions.On the other hand, the first-principles calculations were carried out to investigate the energies and defect structures of C-doped TiO2 rutile. The effect of the crystal growing condition on the defect stabilities has been analyzed using the defect formation energies. Results show that the C substituting on an O site appears under reduction condition, and C substituting on a Ti site appears under the oxidation condition. The density of states calculations show the C substituting on an O site can provide the better photocatalysis performance. Furthermore, the electronic structure and the associated magnetism of carbon-doped rutile TiO were investigated using the first-principles generalized gradient approximation (GGA) method. We found the carbon substitutional oxygen ions can provide the magnetic moment of 1.997?B/C, but the carbon substitutional titanium can not provide any magnetism. The spin density graphics show that the magnetism originates from the structure distortion in (110) plane of primitive TiO around the carbon substitutional oxygen ions. 22Our results suggest that the magnetism results from the special mechanism in the C-doped TiO2 rutiles, and the C-doped TiO2 could be a significant candidate of DMS.