THEORY OF PHASE-TRANSITIONS IN DISORDERED FERROELECTRICS ALLOWING FORNONLINEAR AND SPATIAL CORRELATION-EFFECTS

The order parameter, ferroelectric phase transition temperature, and c ritical concentrations of random-site electric dipoles, point charges and dilatational centres in disordered ferroelectrics are calculated. The calculations are carried out by the random-field method with a ran dom-electric-held distribution function, allowing for nonlinear and sp atial correlation effects. Essential differences from the linear case are revealed. First, the aforementioned effects lead to the transforma tion of the second-order phase transition into one of first order and vice versa for alpha(3) > 0 and alpha(3) < 0 respectively, where as is the third-order nonlinearity constant. Second, for alpha(3) > 0 the p hase transition temperature T-c, has a maximum as a function of the ra ndom-field-source concentration; its maximal value appears to be large r than that in the mean-field approximation. At the same time, for alp ha(3) < 0 the maximum is absent and the T-c-value is smaller than that in the linear case (i.e. at alpha(3) = 0). This means that nonlinear and spatial correlation effects enhance the long-range order in disord ered ferroelectrics for (alpha(3) > 0 and inhibit it for alpha(3) < 0. It is shown that the critical concentrations of electric dipoles, poi nt charges and dilatational centres are the same as in the linear case . The application of the theory developed to various disordered ferroe lectrics is discussed.