Theoretical Studies On Dielectric Properties In Complex Perovskite-type Relaxor Ferroelectrics

The dielectric properties of relaxor ferroelectrics are theoretically investigated in this dissertation.The effect of defect-induced internal field on the dielectric response of relaxor ferroelectrics is investigated by Monte Carlo simulation. Only at a small temperature range nearby the temperature of the dielectric maximum does the susceptibility decrease markedly due to the internal field. The susceptibility is almost independent ofthe internal field width at low internal field width, and then decreases linearly with enhancing internal field width. This dependence of the susceptibility on the internal field width is very similar to the relation of dielectric constant against logarithmic aging time, which probably suggests a linear dependence of the internal field on the logarithm of the aging time. The frequency dependence of the susceptibility aging is sensitive to the temperature. With increasing temperature, the curve of the susceptibility change against logarithmic frequency varies from concave to approximately linear, and then to convex, which is in agreement with the recent aging rate measurement.The dielectric properties of relaxor film are studied by Monte Carlo simulation. The dielectric constant does not vary with the film thickness if no surface effect is included. Therefore, the phenomena that the dielectric constant in the film is much smaller than that in the bulk by almost two orders of magnitude, should be attributed to the surface effect. By adding a surface layer in simulation, we has observed a decrease in the dielectric maximum and increase in temperature of the dielectric maximum as the thickness is decreased. The frequency dispersion of the dielectric constant at temperature above the temperature of the dielectric maximum has also been obtained. These results are consistent with experiments. By generalizing the eight-potential-well model by introducing isotropic random fields and considering two kinds of tunnelings, we investigated field-induced phase transition and polarization behaviour of ferroelectrics under an external field. The type of the phase transition, the first-order or second-order, is almost independent of the random field width. We found that the tunneling frequencies between wells with opposite directions prompts the field-induced phase transition while the tunneling frequencies between the nearest-neighbor wells inhibits the field-induced transition. When a moderate nearest-neighbor well tunneling frequency is considered, the behavior of average polarization with temperature at external field, including the existence of the threshold field and the corresponding temperature, can be well described for the relaxors with field-induced phase transition. Many important features of field-induced phase transition and the behavior of polarization with temperature are in agreement with the experimental measurements. The diffuse phase transition is a main characteristic of typical relaxor ferroelectrics. Dependence of the critical exponent which denote the diffuseness on the tunneling frequency has been investigated with the eight-potential-well order-disorder ferro- electric model. For low tunneling frequency, the ferroelectric phase transition is of second-order and the system conforms to the Curie-Weiss law. The critical exponent departs significantly from the Curie-Weiss value of unity and increases with increasing tunneling frequency at the region of the first-order phase transition Our work is usefulfor understanding the phenomena that some normal ferroelectrics show pressure induced crossover from normal ferroelectric to relaxor.