Kirichenko E. The pecularities of correlation, relaxation and nonlinear effects in disordered ferroelectric materials

A comprehensive analysis of the influence of cooperative behavior of defects and impurities ensemble on correlation, relaxation and nonlinear effects in inorganic and organic disordered ferroelectric materials has been performed. The dependence of correlation radius of ferroelectric relaxors on temperature and degree of disorder has been calculated in the framework of the random local field theory. We show that essential difference in temperature dependences of correlation radius in relaxor and ordered ferroelectrics is due to the influence of random electric fields on the frequency and damping of soft phonon mode of relaxor. Then we use above random local field method to calculate the distribution function of relaxation times in the disordered ferroelectrics. We establish the essential difference in the forms of this function in the phases with different degree of disorder and its dependence on the nonlinear contributions of random fields. The calculations of dynamic dielectric susceptibility of organic disordered ferroelectric PVDF/TrFE before and after irradiation by fast electrons have been carried out. The downward shift of the para - ferroelectric phase transition temperature and gradual transformation of the mixed FG phase into a dipole glass state with the increase of the irradiation dose was shown to be related to the self-consistent change of parameters of random field distribution function. The coincidence between calculated and measured behavior - namely, almost complete disappearance of high-temperature maxima, corresponding to the ferroelectric phase transition, their submergence into low-temperature maxima and Vogel-Fulcher law for the low-temperature maxima of absorption of the irradiated samples speaks in favor of irradiation induced relaxor behavior of P(VDF/TrFE) copolymers. A theory of nonlinear dielectric permittivity of incipient ferroelectrics with dipole impurities has been developed in the random field approach. The calculations describe the main peculiarities of temperature, electric field and concentrational dependence of observed nonlinear dielectric susceptibility. Also, we calculate the width of 180o domain walls in the ferroelectric phase of incipient ferroelectrics with dipole impurities. We obtain the temperature and concentrational dependence of domain wall width and show that this width is much wider in disordered ferroelectrics than in their ordered counterparts. The thickness increases infinitely near the phase transition temperature, which depends on the impurity dipoles concentration.