Weakly coupled relaxor behavior of BaTiO₃-BiScO₃ ceramics

The structural and dielectric properties of (1-x)BaTiO₃- xBiScO₃ (x=0-0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi₂O₃ and BiScO₃ substitutions in a BaTiO₃ solid solution. The solubility limit for BiScO₃ into the BaTiO₃ perovskite structure was determined to be about x=0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x=0.05-0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO₃ to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO₃. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel-Fulcher model, which yielded an activation energy of 0.2-0.3 eV. The attempt characteristic frequency was 10¹³ Hz and the freezing temperature, T_f, ranged from -177° to -93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO₃-BaTiO₃ solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.