A phenomenology of relaxor ferroelectrics

We propose a phenomenological theory for the dielectric properties of relaxor ferroelectrics based on the assumption that the polar clusters distributed in highly polarizable host lattice are responsible for the relaxor behavior. Two major differences between relaxors ferroelectrics and conventional order-disorder ferroelectrics have been employed for 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. We illustrate numerically predictions of the theory for the dielectric response for relaxors with incipient ferroelectric order like PMN or K_1-xLi_xTaO₃ (x < 0.022) , and for relaxors undergoing true first order ferroelectric phase transitions like PST or PLZT.