Dielectric response near the permittivity maximum in (Sr_(0.8)Pb_(0.2))TiO_3 ceramic solid solution

摘要

It is well known that the weak-field relative dielectric permittivity maximum ε_(w(max)) achieved at the paraelectric-ferroelectric phase transition in ceramic ferroelectric solid solutions in the perovskite structure family is a strong function of the processing conditions used in fabrications. Heretofore, the height and sharpness of the dielectric peak have often been regarded as good qualitative indicators of the quality and homogeneity of the solid solutions. In the SrTiO_3-PbTiO_3 solid solution system at low PbTiO_3 concentrations, this sensitivity is again strongly evident. Now, however, even for samples with a very sharp ε_(w(max)) giving values >45 000 at weak-field dielectric maximum temperature T_(max) above the peak temperature, 1/ε_w (ε_w: weak-field relative dielectric permittivity) departs from Curie-Weiss behavior more than 15℃ above the Curie-Weiss temperature θ which now occurs above T_(max). The unusual condition T_(max) < θ appears to occur for quite a wide range of processing conditions. Measurements of thermal contraction also show departure from linearity for more than 15℃ above θ when measured over the same temperature range, suggesting the development of micro- or nanopolar regions as in a relaxor ferroelectric and this polar character has been confirmed by recent Raman spectroscopy. There is, however, no evidence of dielectric relaxation in the radio-frequency range as in the classic relaxors; however, earlier studies on similar compositions have shown strong dispersion in the gigahertz frequency range at temperatures near to but above T_(max). Preliminary studies of x-ray-diffraction linewidth also suggest that the highest permittivity peak samples are less homogenous, perhaps indicating that micropolarity may be associated with inhomogeneity in the lead cation distribution.