Research On Electronic Structure And Optical Properties Of Ferroelectrics

Ferroelectric materials are the subjects of extensive investigation both because of their technical importance and because of the fundamental interest in the physics of their phase transition. There are many theoretical tools and models being used to study ferroelectrics. Previous theories are strongly empirical, such as Landau theory, pseudo-spin model and lattice dynamic model et al. They mainly were used to fit the experimental results and predict new experimental phenomena based on the theories. These theoretical results are not only helpful to understand the mechanism of ferroelectric character, but also important for the application of ferroelectric materials. In recent years, with the development of computer technology, first principles calculation has become one important tool in studying ferroelectrics. In this thesis, our purpose is to study the electronic structure and optical properties of CaTiO3 and the ferroelectric origin of Rb2Cd2(SO4)3, with the help of the FLAPW method based on the density functional theory (DFT). From analysis of orbital hybridization, we find that in the tetragonal CaTiO3 the Ti and O1/O2 atomic orbital have strong overlaps and exhibit the covalent character, forming two-type Ti-O1 and Ti-O2 bonds. Physically, either Ti-O bonding can be regarded as a covalent dipole, while in the cubic phase all covalent dipoles are identical due to cubic symmetry. In the same time, we also find that in the tetragonal phase dielectric function, reflectivity and absorption coefficients display a new single-peak spectra, but rather multi-peak. Our calculations show that these anomalous phenomena originate from strong orbital hybridization between Ti and O atoms, forming the covalent bonding (Ti-O dipole). It is nature that the crystal structure not only determines the dipole properties and distribution, but its symmetry also leads to optical dipole-dipole transition forbidden. In the sense, the tetragonal structures are identified as a key, causing such anomalous optical phenomena. As far as Rb2Cd2(SO4)3 be concerned, we find that orbital hybridization can form SO4 ionic groups and two-type cation ionic bonds between Cd/or Rb and O atoms with weak couplings, but the ionic bond (Cd-O) plays the dominant role. We think the macroscopic domain walls originate from such weak-coupling ionic groups, arising at the cell boundaries. In the microscopic level, the cation bonds (Rb-O and Cd-O) valences and the subsequent rotations of the SO4 tetrahedra can lead to the driving force of the ferroelectric behaviour. The predicted pyroelectric current effects were observed experimentally in the ferroelectric RbCdS langbeinite.