A RANDOM-FIELD THEORY-BASED MODEL FOR FERROELECTRIC RELAXORS

A model for ferroelectric relaxors such as PMN, PSN and PLZT giving a quantitative description of their properties and phase diagrams is pro posed within the framework of the random field theory. In this model, the relaxors are considered as systems with random sites and orientati ons of electric dipoles, lattice vacancies, antisite ions and other de fects as well as impurities embedded into the paraelectric phase, whic h is proposed to be the 'host' lattice for these materials. The calcul ations of the temperature T-c which corresponds to the transition from the paraelectric to the ferroelectric phase is carried out as a funct ion of the concentration of lattice defects (point charges and dilatat ional centres). On the basis of these calculations, the peculiarities of the ferroelectric relaxer phase diagram are discussed. The main fea tures of the phase transition sequence when decreasing the temperature in relaxors with constant dipole and defect concentrations are descri bed. The Cross superparaelectric model and Bums temperature T-d have b een shown to appear in a natural way in the proposed model. A comparis on between calculated and experimental data has been made for the mode l ferroelectric relaxer PLZT x/65/35. Fairly good agreements between c alculated and measured T-c(x) and critical concentrations of lanthanum have been obtained from the model.