Phenomenology of relaxor-ferroelectric phase transitions

We propose a phenomenological theory for the dielectric properties of relaxor ferroelectrics undergoing a `relaxor-ferroelectric' phase transition based on the assumption that the interacting polar clusters distributed in highly polarizable host lattice are responsible for relaxor behavior. Two major differences between relaxor ferroelectrics and conventional order-disorder ferroelectrics have been employed in the theory: i) the existence of the broad distribution of local fields experienced by each polar cluster; ii) the existence of the distribution of relaxation times giving rise to non-exponential non-critical kinetics which may compete with the critical slowing down near the ferroelectric phase transition temperature. We show that the theory is in good agreement with the experiment on frequency dependent permittivity in PbSc_0.5Ta_0.5O₃ undergoing the first order ferroelectric phase transition.