Dielectric properties of partially disordered lanthanum-modified lead zirconate titanate relaxor ferroelectrics

Linear and third-order nonlinear dielectric susceptibilities and the dielectric polarization were measured in 6.5/65/35 lanthanum lead zirconate titanate (PLZT) hot-pressed ceramics. On cooling linear dielectric data show a transition from an ergodic to a nonergodic relaxor phase, while, on heating, a ferroelectric to ergodic relaxor phase transition appears. The third-order dielectric response is reminiscent of an ergodic to nonergodic relaxor phase transition. Below the Vogel-Fulcher temperature, at which the longest relaxation time diverges, spontaneous polarization exists, which indicates the possibility that the sample breaks up at low temperatures into relaxor glasslike and ferroelectric order-dominated regions at nonzero transition temperatures. Additional measurements of the quasistatic field-cooled-field-heated dielectric susceptibilities revealed an electric-field-temperature phase diagram, which confirmed the coexistence of both phases. The dielectric susceptibility at various bias electrical fields, obtained from polarization data, revealed a field-induced transformation between the disordered state and ordered ferroelectric state. The results were analyzed in the framework of the spherical random-bond-random-field model.