Domain wall nucleation by impurity ions in KNbO₃ single crystals

It has been shown theoretically as well as experimentally that microwalls with linear dimensions of the order of 10⁻⁶–10⁻⁸ m can be nucleated in KNbO₃ single crystals under the influence of the electric dc fields applied externally at the sites of the impurity ions. The critical field for nucleation is found to be about 8×10³ V m⁻¹. Since such low fields already exist in crystals at the nonferroelectric to ferroelectric phase transition, the mechanism is expected to be operative at that time, and a photographic evidence of it is also obtained. A model has been developed to explain the nucleation. Employing thermodynamic considerations, expressions have been derived for the critical length of the domain wall nucleated, and the activation energy of nucleation, both for 60° and 90° walls. The photomicrographs obtained after applying the dc fields show microwalls nucleated at the impurity sites, with their linear dimensions of the order given by the theory. Many other aspects of the domain wall nucleation have been made clear by the photomicrographs. Particularly, it has been found that the microwalls move after nucleation to lie linearly to reduce the strain energy of the crystal. The microwalls are not pinned at the impurities, and extend themselves so that they meet each other end to end, producing a continuous wall. It is possible to distinquish these microwalls from the microwalls nucleated by the dislocation loops. The expression for the activation energy of nucleation is also verified experimentally. It is suggested that the mechanism of domain wall nucleation can occur in other ferroelectric crystals also, and shown that the memory of domain walls, commonly observed in ferroelectrics, can be readily attributed to the impurity ions present in the crystals, and the phenomenon of domain wall nucleation operative at their sites.