The mechanism of domain wall nucleation by impurity dipoles in KNbO 3 single crystals

Abstract

The principle underlying microdomain nucleation by impurity dipoles has been enunciated. It is shown that the impurity dipole induces the nucleation of a microdomain around it so that its dipole moment is parallel to the P s direction in the microdomain. Considering the mechanism of dipole formation in KNbO 3, it is shown that the possible directions for the dipole moment of the impurity dipole are the same as the possible P s directions in all the ferroelectric phases. Hence, it becomes possible for the impurity dipole to have its dipole moment parallel to the P s direction after the nucleation of the microdomain. The mechanism of microdomain nucleation at the phase transition at the Curie temperature as well as under the influence of the externally applied d.c. electric field is known to be that of domain wall nucleation. Hence, it is shown that in both situations the above principle is followed in microdomain nucleation. On account of strong, large distance interaction of the dipoles at the Curie temperature, the dipoles are arranged in rows in a possible P s direction of the tetragonal phase, i.e. in the pseudocubic [100] direction. Correspondingly, the microdomains are also arranged in order, and if close to each other, join each other end to end producing a large domain. Thus the mechanism of the formation of large domains by the impurity dipoles is also understood. For such a mechanism to be observable, the impurity dipole density must be of the order of 10 10 cm −3.