Evolution of relaxor behavior and high-field strain responses in Bi(Mg1/2Ti1/2)O3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ferroelectric ceramics

A composition-induced phase transition from a diffuse-type ferroelectric to a relaxor ferroelectric and then to a weak relaxor ferroelectric was observed in a new (0.68-x)Bi(Mg1/2Ti1/2)O3-0.32PbTiO3-xPb(Ni1/3Nb2/3)O3 (BMT-PT-xPNN) ternary system. Meanwhile, a gradual evolution process of nonergodic-ergodic-nonergodic realxor states was identified at room temperature from the BMT-rich side to the PNN-rich side. Two strain maxima of ∼0.42% and ∼0.29% under 7 kV/mm were obtained in the x = 0.2 and x = 0.65 samples, respectively. It is worthy to note that the high-field strain of the x = 0.2 composition was thermally stable but seriously hysteretic. By comparison, the x = 0.65 composition exhibited a weakly hysteretic strain and a relatively small threshold electric field. This phenomenon was attributed to larger sizes and slower dynamics of polar nanoregions (PNRs) in BMT-rich compositions than those in PNN-rich compositions. Moreover, it was believed that the strain hysteresis of BMT-PT-xPNN relaxor ferroelectrics would be dominated by the growth process of PNRs into ferroelectric microdomains during loading and their subsequent dissociation process during unloading.