First Principles Study The Defect Properties In Tetragonal BaTiO₃ Ferroelectric

Defects always exist in ferroelectric materials, and play an important role in determining the mechanical, electronic, and the optical properties of ferroelectric materials. Therefore, the defect properties in ferroelectric materials have attracted wide attention and become the focus of study and research. Unfortunately, there are still not enough research papers to report the microscopic mechanism of defect in ferroelectric materials. In this thesis, the first-principles calculations were carried out systematically to reveal some defects and their properties in perovskite tetragonal BaTiO₃. The main results are as follows.1. The lattice constants, bulk modulus, and band structure of tetragonal BaTiO₃ have been calculated. The 3 ? 3?3 supercell model has been built for defects calculation after convergence test of total energy. After introduction intrinsic defects such as vacancies, Frenkel pairs, Schottky defects into BaTiO₃, the formation energies of defects were calculated by first-principles total energy method. The relationships between formation energies and atomic chemical potentials, charged defect and electron chemical potentials have been showed. We also have compared the results with experimental data. According to our calculation results, the formation energies of defects sharply varied with atomic chemical potential. Taking formation energies of oxgen vacancy as an example, it shows highest in oxidate environment, but lowest in reduction environment. It means that, annealing treatment can effectively suppress the formation of oxgen vacancy and to prevent fatigue process. Because of the lower formaton energy of V_Ba - V_O di-vacancies vary every atomic chemical potential, V Ba ? VO di-vacancies is regarded as the research focus. Conversely, the Frenckel pairs show the highest formation energies. Meanwhile, Full valence charged defect is more stable than neutral species.2. The microscopic mechanism of imprint has been discussed in atomic scale with dipole model. The formation energies of 1st, 2^nd, 3^rd and 4^th-neighbor VB a ? VO pairs and all the possible relative orientations within each pair in 3×3×3 supercell have been studied. The lowest defect construction has been found. We have presented a new method combined with modern theory of polarization to calculate and analyse the spontaneous polarization of supercell containing V_Ba-V_O pairs. The result matched pretty well with the classical molecular dynamics method. It is found that the 2nd-neighbor complexes that are aligned along the direction of the spontaneous polarization have the lowest formation energies. The symmetry between the up and down polarization states is broken, which is manifested by a vertical shift in the polarization-voltage hysteresis loop. Systems containing randomly distributed V_Ba-V_O pairs were also studied and the shift in the hysteresis loop is negligible, which suggests that the alignment of polar V_Ba-V_O divacancies is responsible for the polarization imprint in ferroelectric BaTiO₃.