Reentrant Phenomenon in Barium Titanate Zirconate-Based Relaxor Ferroelectrics

Reentrant phenomena are rare occurrences in relaxor ferroelectrics, and their underlying mechanisms are still unknown. In this paper, we report on the reentrant relaxor behavior in the perovskite system (1 - x)Ba(Ti_0.85Zr_0.15)O₃-xBi(Zn_2/3Nb_1/3)O₃ with x  = 0, 0.015, 0.02, 0.04, 0.06, and 0.15. For all compositions, the temperature dependences of the dielectric permittivity exhibit characteristic reentrant features: a frequency-independent maximum ≈ 340 K (T_m) associated with the ferroelectric phase transition, and a frequency-dependent anomaly (T_R) at lower temperatures indicating relaxor behavior. Raman spectroscopy shows that the correlation length of the B-cation displacements is suppressed by doping resulting in a cross-over from macroscopic to mesoscopic polar order. On the other hand, correlated antiferrodistortive rotations of the oxygen octahedra give rise to nanometer size clusters of antipolar shifted A-site cations distributed within the polar matrix. Slowing down of the dynamics of these clusters leads to low-temperature dielectric anomaly and reentrant relaxor behavior. Although competition between polar modes and antiferrodistortive modes is a common feature of perovskite ferroelectrics, it is shown that it can be the origin of the reentrant relaxor behavior.