Abstract
A model for ferroelectric relaxors such as PMN, PSN and PLZT giving a quantitative description of their properties and phase diagrams is proposed within the framework of the random field theory. In this model, the relaxors are considered as systems with random sites and orientations of electric dipoles, lattice vacancies, antisite ions and other defects as well as impurities embedded into the paraelectric phase, which is proposed to be the `host' lattice for these materials.
The calculations of the temperature 
which corresponds to the transition from the paraelectric to the ferroelectric phase is carried out as a function of the concentration of lattice defects (point charges and dilatational centres). On the basis of these calculations, the peculiarities of the ferroelectric relaxor phase diagram are discussed. The main features of the phase transition sequence when decreasing the temperature in relaxors with constant dipole and defect concentrations are described.
The Cross superparaelectric model and Burns temperature 
have been shown to appear in a natural way in the proposed model.
A comparison between calculated and experimental data has been made for the model ferroelectric relaxor PLZT x/65/35. Fairly good agreements between calculated and measured 
and critical concentrations of lanthanum have been obtained from the model.