LINEAR IMPURITY STRUCTURES AND DIPOLAR IMPURITIES IN KNBO3 SINGLE-CRYSTALS

It is shown that linear impurity structures that are observed in pseudo-cubic [100] directions in KNbO3 single crystals are formed by impurity dipoles that are present in crystals near the ferroelectric phase-transition temperature. These dipoles align themselves in the direction of spontaneous polarization of the tetragonal phase (i.e. in the pseudo-cubic [100] direction), and are frozen to room temperature without subsequent movement. Many other dipoles relax producing ions that get scattered in the structure. These ions can be re-excited to the dipolar state by the application of external d.c. field to the crystal containing the impurities. The dipoles so formed are found to arrange themselves in the pseudo-cubic [110] direction, the direction of Pδ in the orthorhombic phase. It is further shown that domain-wall nucleation is caused by impurities when they are in the dipolar state, and not in the ionic state. Thus the dipolar impurities present in the crystal near the ferroelectric transition temperature (435°C) play a signficant role in domain formation and switching. © 1990 Taylor & Francis Group, LLC.