Theoretical Study of Ferroelectric and Optical Properties in the 180� Ferroelectric Domain Wall of Tetragonal BaTiO3

The microscopic mechanism of spontaneous polarization and refractive indices in 180° ferroelectric domain walls of tetragonal Barium titanate (BaTiO3) is discussed by using a microscopic model. This model bases on the orbital approximation in correlation with the dipole–dipole interaction due to the local field acting on all constituent ions within the domain wall. It is found that the behavior of both the spontaneous polarization and the refractive indices depends on the thickness of the domain wall which was varied between 5 and 20 Å. Moreover, the spontaneous polarization shows a hyperbolic tangent shape for domain walls of a larger thickness which then vanishes at the center of the domain wall. The refractive indices suggest the domain wall to act like a biaxial crystal resulting in refractive index profiles of a Gaussian shape for domain walls of ∼20 Å. This dramatically affects optical transmission through the domain wall especially for light being polarized parallel to the domain wall.