DOMAIN FORMATION BY IMPURITY DIPOLES IN KNBO3 SINGLE-CRYSTALS

It is shown in this paper how the impurities, present in the form of dipoles, produce macrodomain structures through microdomain nucleations. Normally two stages, namely the nucleation and growth of domains, are involved in the process of domain formation. In the present case, however, where the impurity dipole concentration is high (10(16) m-3), only the nucleation stage is shown to be sufficient. At the Curie temperature (435-degrees-C) the impurity dipoles have cooperative ordering, and are arranged in the structure forming parallel linear chains in the pseudocubic [100] directions of the tetragonal phase, their mutual separation being of the order of a few microns. Each dipole can nucleate microdomains around it so that the dipole moment direction is parallel to the P(s) direction in the region around it. The various dipoles nucleate microdomains that join each other end to end producing macrodomain structures. At the tetragonal to the orthorhombic phase transition at 225-degrees-C the domain walls retain their positions, so that the 180-degrees and 90-degrees walls of the tetragonal phase serve as 90-degrees and 60-degrees walls of the orthorhombic phase, respectively. Evidence has been obtained of the cooperative ordering of the impurity dipoles, and the macrodomain structures formed by them. The domain structures worked out theoretically are found to agree with the experimentally observed domain structures. The results do not depend on the chemical nature of the impurities, which in the present case are mostly Pt ions.