Mechanisms of variation of the unipolarity during thermal processing of heavily doped LiNbO$_{3}$:ZnO crystals

The phenomenological mechanisms for increasing the unipolarity under thermal processing in the short-circuiting conditions for heavily doped LiNbO$_{3}$:ZnO crystals are analyzed by comparing with the temperature behavior of nominally pure LiNbO$_{3}$:ZnO crystals with the congruent composition. It is shown that an increase in the unipolarity and, hence, the disappearance of the domain structure in heavily doped LiNbO$_{3}$:ZnO crystals is initiated by thermal decomposition of charged polar clusters stabilizing domain walls. The decomposition of polar clusters is accompanied with an abrupt jumpwise injection of extra charge carriers (Li+ cations). As a result, the conductivity of LiNbO$_{3}$:ZnO crystals at a temperature above 800 K is an order of magnitude higher than that of nominally pure LiNbO$_{3}$ crystals with the congruent composition. This leads to the degradation of the domain structure in LiNbO$_{3}$:ZnO crystals in contrast to LiNbO$_{3}$ crystals with the congruent composition.