Theoretical Investigation On The Defects And Defect-induced Multiferroic Properties In ABO₃-type Ferroelectrics

Various defects in ferroelectrics can influence the spontaneous polarization and the response properties of the materials, leading to fatigue and aging. On the other hand,the atomic structure and electronic structure are also altered by the point defects including doping and vacancies, and magnetism could arise in the ferroelectric material,which provides a pathway to multiferroic materials. In this thesis, using first-principles methods, we investigate the traditional ABO₃-type ferroelectrics to focus on effects of the domain walls, oxygen vacancy and dopants, and explore the way to realize multiferroics via oxygen-vacancy mediation.First, we investigate the 180 degrees ferroelectric domain walls（FDWs）, which have long been regarded as purely Ising type. But recent theoretical works suggested that they can also have N′eel- and/or Bloch-like rotations. We studied the 180 degrees FDWs on different crystallographic planes in prototypical ferroelectric perovskite BaTiO3. The polarization profiles of 180 degrees FDWs on（100） and（410） planes revealed that the（100）- and（410）-FDWs both exhibit N′eel-like character besides their intrinsic Ising character, while the（410）-FDW also simultaneously shows a Bloch-like oblique of similar to 6 nm, as a consequence of the deviation of polarization gradient from the high symmetry direction. Due to the existence of the N′eel-like component of polarization, 180 degrees FDWs in BaTiO3 exhibit a multilayer charge redistribution and thus may strongly trap charged defects. Therefore, 180 degrees FDWs on different crystallographic planes have different N′eel-like component and can thus have different bound charge densities.Then, we investigate the effects of oxygen vacancy（VO） and epitaxial strain on the ferroelectric（FE） and antiferrodistortive（AFD） properties of the [001]-oriented PbTiO₃/SrTiO₃1/1 superlattice. The perfect superlattice is shown to have polar distortion and octahedral rotation around the polar axis, while the existence of oxygen vacancies will not change the coupling between the two distortions. It is found that the oxygen vacancies form most easily when the superlattice in-plane lattice constant is between those of single-crystal PbTiO₃ and SrTiO₃. The polarization in the direction of Ti-VO-Ti chain is remarkably reduced due to the vacancy-induced local tail-to-tail polarization patterns, and consequently, the VOcan pin the polarization to certain direction. Moreover, the octahedral rotation around the direction of Ti-VO-Ti chain is also suppressed while the rotations along the other two orthogonal directions are enhanced. These results suggest a promising way to mediate the FE and AFD properties in oxygen-deficient superlattices.At last, we explore the possibility to introduce multiferroics in ABO₃-type ferroelectrics in two ways. We first attempt to introduce d⁰ magnetism in traditional B-site-drive transition metal ferroelectric BaTiO₃ and maintain the ferroelectricity simultaneously by replacing O atom with other nonmetal atoms. We dope one C atom in the system and find that the C atom can bring a magnetic moment of 2 μB. However, doping more than one C atoms into the system will lead to the dimerization of the dopants and the system now has d electrons which eliminate the ferroelectricity.Therefore, the system might be multiferroic only via single-doping.Besides, we investigate the hexagonal material YMnO₃, which is multiferroic with improper ferroelectricity and antiferromangetism. The ferroelectricity is not driven by the B-site metal and is not exclusive to magnetism. We find that in-plane oxygen vacancies are likely to form and induce Mn ions to deviate from in-plane 1/3 positions.We also predict the changing of the ground-state magnetic ordering via the VOat axial positions and a non-zero net magnetization along c axis is brought in. Therefore, the coexistence of ferromagnetism and ferroelectricity in this material might be realized by creating this kind of oxygen vacancies.