Enhanced ferroelectric phase-transition temperature in perovskite-based solid solutions

We explain how the phenomena of ferroelectric phase transition temperature T_c enhancement beyond the end members in perovskite solid solution such as BiMO₃-PbTiO₃ (M = Sc, In, etc.) is related to nonlinear and spatial correlation effects. The explanation is based on the calculation of T_c in the framework of our random field theory with additional account for nonlinear effects in the above substances. We show that the maximum of T_c for certain PbTiO₃ content takes place when coefficient of nonlinearity is positive, the value of this coefficient is found from best fit between theory and experiment. This nonlinearity coefficient is the only adjustable parameter of the theory. We show that enhancement of positive nonlinearity coefficients enhances greatly the T_c maximum over its value for end members. The theory lays the foundation to calculate not only T_c for above solid solutions but virtually any equilibrium and/or nonequilibrium thermodynamic characteristics such as static and dynamic dielectric susceptibility, specific heat, etc., as a function of PbTiO ₃ content, temperature, electric field, and other external parameters.