THEORY OF TWINNING STRUCTURES IN THE ORTHORHOMBIC PHASE OF FERROELECTRIC PEROVSKITES

The microstructures of coherent 180 degrees, 90 degrees, and 120 degre es domain walls in the orthorhombic phase of ferroelectric perovskites have been studied on the basis of Landau-Ginzburg theory for the firs t-order phase transitions. It is found that for the space profile of t he polarization within an uncharged wall layer, the normal component a lways remains zero or constant, while the component parallel to the wa ll plane has two kinds of inhomogeneous configuration, one of the 'Isi ng type' and the other of the 'Bloch type'. Quasi-one-dimensional anal ytic solutions for polarization components, elastic strains, and clamp ing stresses are obtained for the Ising-type 180 degrees, 90 degrees, and 120 degrees wall interfaces. The structural characteristics and ph ysical properties of the Bloch-type walls are also illustrated and dis cussed. All of the theoretical results are functions of macroscopic qu antities for perovskite crystals, and can be applied to real systems w hen these quantities are obtained from experiments.