  The existence of polar nanoregions is the most important characteristic of
ferroelectric relaxors, however, the size determination and dynamic of PNRs
remains uncertain. We reveal a re-entrant relaxor behavior and
ferroelectric-paraelectric transition coexists in complex perovskite oxide
0.6Bi(Mg1/2Ti1/2)O3-0.4PbTiO3. Two dielectric anomalies (i) the low-temperature
re-entrant relaxor transition and (ii) the high-temperature diffuse phase
transition (DPT) were described by the phenomenological statistical model. The
sizes of the two kinds of polar nanoregions (PNRs) corresponding to two
ferroelectric states were obtained. The dynamic of PNRs were analyzed using
isothermal electrical modulus, which shows three critical temperatures
associated with the diffuse phase transition, the formation and freezing of
PNRs, respectively. The temperature evolution of the PNRs evolution depends on
the stoichiometry of bismuth. The results provide new insights into the dynamic
behavior of PNRs and the modification way of re-entrant relaxor behavior.
