Diffuse and sharp ferroelectric phase transitions in relaxors

Using x-ray diffraction, differential scanning calorimetry, polarizing microscopy and dielectric spectroscopy, we study the phase transition from an ergodic relaxor state to a ferroelectric phase in the single crystals of Pb(Sc_1/2Nb_1/2)O₃ (PSN) and Pb(Zn _1/3Nb_2/3)O₃ (PZN) complex perovskites and compare it with the behavior of the canonical relaxor Pb(Mg_1/3Nb _2/3)O₃ (PMN). It is shown that the presence of polar nanoregions (PNRs) in the high-temperature phase is the common feature of all three relaxors. However, they exhibit distinct behaviors in terms of structural phase transition, i.e. PMN does not show macroscopic ferroelectric phase transition, PZN exhibits a diffuse transition into a ferroelectric phase and PSN undergoes a sharp transition into a ferroelectric phase. Different behaviors can be understood based on the different kinetics of the development of PNRs, namely, (i) order-disorder type ferroelectric transition through alignment of PNRs into single direction in PSN, (ii) comparatively abrupt growth of PNRs at the ferroelectric phase transition without changing the directions of their dipole moments in PZN, and (iii) undeveloped PNRs in PMN.