Domain wall nucleation by impurity ions in KNbO3 single crystals

Abstract

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.

Domain morphology controlled crystal habits in PbTiO3 nanocrystals

“…Whereas the relations quoted for a model of 90 0 domain wall nucleation [28] reflect that the point domains are cubical. Taking into account of possible orientations (±x, ±y and ±z directions) of dipoles in tetragonal phase of the PbTiO 3 , BaTiO 3 or KNbO 3, we favor the cubic shape of the point domain.…”

“…The main factors involved in the nucleation process are: decrease in the electrostatic energy, increase in surface energy, change in the strain wall energy, and a factor associated with the change in entropy of the system [28]. The change in electrostatic energy for 180 0 domain is given by -2 ܲ ௦ ‫݈ܧ‬ ଷ [9,27] Thus the change in crystal energy on nucleation of domain is…”

Domain Structure in the New Ferroelectric Ba 5 Ti 2 O 7 Cl 4

“…The experience of the authors with KNbO 3 [11] has shown that the rotation of polar axes and strain dependence of domains go together. In KNbO 3 , Ingle et al [11] found very interesting strain dependence of domains in the form of nucleation of domains around strains. Looking to the large scale rotation of polar axes, experiments were performed to study the strain dependence of domains in this crystal also, and indeed very interesting results have been obtained.…”

“…Secondly, there is considerable variations of directions of polar axis in space. It is known that the possibility of rotation of polar axes has much to do with the nature of the phase transition [11,12]. The experience of the authors with KNbO 3 [11] has shown that the rotation of polar axes and strain dependence of domains go together.…”

“…It is known that the possibility of rotation of polar axes has much to do with the nature of the phase transition [11,12]. The experience of the authors with KNbO 3 [11] has shown that the rotation of polar axes and strain dependence of domains go together. In KNbO 3 , Ingle et al [11] found very interesting strain dependence of domains in the form of nucleation of domains around strains.…”