IMF2017

Lead-oxide relaxor ferroelectrics are widely used as materials for numerous applications due to extremely large dielectric constant and unparalleled piezoelectric coefficients. It has been shown that strong quenched random local fields caused by the disordered heterovalent cations establish the relaxor phase, identify the order parameter and lead to the dramatic enhancement of piezoelectric coefficients. On the other hand, the role of random fields in lead-free analogs is not clear. In this work we analyze the results on the concentration-induced crossover from ferroelectric to relaxor state in lead-free BaTiO₃-based homovalent and heterovalent solid solutions recently obtained in our laboratory and published in the literature to underline the characteristics and microscopic mechanisms specific to each group of materials. We demonstrate, that similar to classical lead-oxide relaxors, strong quenched random electric fields suppress the macroscopic ferroelectric ordering and promote relaxor behavior in lead-free oxides. We observe different types of the crossover behavior with increasing concentration x, including a gradual transformation from a sharp ferroelectric phase transition to a diffuse phase transition and to relaxor transition. Furthermore, the relaxor behavior vanishes if the strength of random fields becomes large enough. Alongside the widely known relaxor features such as the cubic macroscopic symmetry, the Vogel-Fulcher law for the frequency dependence of T_m and the Curie-Weiss law for e(T) at T >> T_m, we verify in lead-free relaxors some other specific relaxor phenomena, such as the scaling of slim-loop polarization curves and the Vogel-Fulcher scaling of the temperature/frequency dependences of permittivity.