IMF2017

Barium titanate (BaTiO₃)-based system is the most widely used ferroelectrics and the most important dielectric material for multilayered ceramic capacitors. The excellent dielectric property of BaTiO₃-based system has attracted much attention not only in the electroceramic industry but also in fundamental research. In BaTiO₃-based system, ionic and dipole polarizations predominantly determine the dielectric property. The ionic polarization is related to lattice vibrations. On the other hand, the dipole polarization is governed by the ferroelectric domain-wall contributions in multi domain crystals and ceramics. Additionally, the dielectric relaxation related to order-disorder phenomena derived from dipole clusters have been reported even at a high temperature above Currie temperature (T_C). However, these polarization mechanisms are complicated and as yet unclarified. Thus, accurate dielectric spectra in the THz region is highly desired in the analysis of the ionic polarization and order-disorder relaxation. In this study, we developed a far-infrared spectroscopic ellipsometer for measuring THz complex permittivity of high-permittivity materials, and measured the complex permittivity of BaTiO₃-based ferroelectrics in the frequency region of 30 – 700 cm^-1 (0.75 – 21 THz). THz dielectric spectra of BaTiO₃ single crystal and Ba_0.6Sr_0.4TiO₃ ceramics could not be represented by a harmonic oscillator model, which suggested anharmonicity of ferroelectric phonon mode. Based on an eight-site order-disorder model, we analyzed the THz dielectric spectra of BaTiO₃-based ferroelectrics. It was found that the order-disorder mode coupled with the Slater mode greatly contributes the high permittivity of BaTiO₃-based ferroelectrics.