Unusual electric response of the dislocations and the impurity dipoles in KNbO3 single crystals

Abstract

A highly unusual electric response of the dislocations and the impurity dipoles in KNbO3 single crystals has been found. The etch pits representing these defects move in the structure carrying these defects with them. This shows that the defects have associated with them structural units that are orders of magnitude larger in physical size. Naturally these units are also associated with stress fields and electric effects on account of the ferroelectric nature of the crystal. The nature of the stress fields could be determined by studying the movement of the etch pits in response to the externally applied electric fields. It is found that the impurity dipoles in the observed pseudocubic (001) surface have stress fields similar to those of screw dislocations, and the dipoles in the front and side faces have stress fields similar to those of edge dislocations. The external dc field causes rotation of the dislocation loops and, hence, changes the domain structure. It may order or disorder the dislocations, the effect being reversible. Similar effects have been observed for impurity dipoles. Several consequences of the stress fields have been considered theoretically and observed experimentally. For example, the stress fields of impurity dipoles and dislocation loops lead to their mutual exclusion. Hence, at a given place in the crystal the domain structure is nucleated either by the dislocation loops or by the impurity dipoles. The conjoint domain nucleation, sometimes possible by an impurity dipole and a dislocation loop worked out on theoretical considerations, agrees with experimental observation. Large distance mutual interaction, bulk ordering, dependence of bulk ordering on the density of defects, and domain nucleation have all been explained on the basis of the structural units associated with the impurity dipoles and the dislocation loops.