Low-frequency dielectric dispersion in ferroelectric crystals

The dielectric permittivity function of the ferroelectric materials Rochelle salt, TGS, BaTiO3, KDP, and LiTaO3 was measured in the frequency range 1 mHz-1 kHz. The measurements were done both above and below the ferroelectric transition, except for LiTaO3, in which the highest measurement temperature was 146 degrees C. In the first four crystals, the dielectric function shows a central peak of width less than 1 Hz which disappears completely in the paraelectric phase. There are good indications that the central peak involves the diffusion of the heat generated by pyroelectric effect, but the pyroelectric coupling is not enough to account for the dispersion step. Switching of small polarization clusters with the exciting electric field is suggested to be a source of the enhanced central peak intensity. In LiTaO3, a Debye relaxation of the polarization was observed, with relaxation time decreasing very quickly with increasing temperature. That relaxation is perhaps related to a central peak observed in light scattering in the 1970's. [S0163-1829(97)03921-0].