Description of ferroelectric phase transitions in solid solutions of relaxors in the framework of the random-field theory

A model based on the random-field theory is proposed for calculating the properties of solid solutions of ferroelectric relaxors. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is calculated taking into account the contribution of nonlinear and correlation effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature Tc on the concentration of solid solution components is analyzed. Numerical calculations are performed for the lead scandoniobate and lead scandotantalate solid solutions (PbSc1/2Nb1/2O3)1−x(PbSc1/2Ta1/2O3)x with different degrees of ordering and the lead magnoniobate and lead titanate solid solution (PbMg1/3Nb2/3O3)1−x(PbTiO3)x. It is shown that the higher transition temperature for more disordered solid solutions of the composition (PbSc1/2Nb1/2O3)1−x(PbSc1/2Ta1/2O3)x in the range 0≤x<0.5 is associated with the larger nonlinearity coefficient for PbSc1/2Nb1/2O3 as compared to that for PbSc1/2Ta1/2O3. The theory provides a means for calculating the region of the coexistence of the phases with different symmetry groups in the temperature-composition phase diagram of the (PbMg1/3Nb2/3O3)1−x(PbTiO3) solid solution. Numerical calculations with the use of the fitting parameters obtained from the known transition temperatures Tc for the solid solution components adequately describe the experimental phase diagrams for the aforementioned solid solutions of ferroelectric relaxors.