THEORETICAL-MODEL OF 180-DEGREES DOMAIN-WALL STRUCTURES AND THEIR TRANSFORMATION IN FERROELECTRIC PEROVSKITES

Two kinds of 180 degrees domain wall structures in the tetragonal phas e of ferroelectric perovskites, one of the ''Ising type'' with only on e polarization component and the other of the ''Bloch type'' with two polarization components within the wall layer, have been derived from the Landau-Ginzburg phenomenological theory. Temperature dependence of the interfacial energy indicates that the 180 degrees wall has its ow n ''structural transformation,'' i.e., the wall structure is possible to change from Ising type to Bloch type upon cooling. As a more stable structure than the Ising-type wall in a wide temperature region, the Bloch-type wall has special crystal symmetry and physical properties w hich are presented in the report.