The complex dielectric permittivity of single-domain BaTiO 3 in the region of 30-700 cm %1 (0.90-21 THz) was directly measured using a far-infrared spectroscopic ellipsometer. The dielectric responses of the Slater, Last, Axe, and O 4 torsional modes, which are all infrared-active phonon modes in BaTiO 3 were clearly observed. The resonance frequencies of the phonon modes estimated in this study were in agreement with the previous data estimated by analysis of reflection spectra using a harmonic oscillator model. These results indicate that the far-infrared ellipsometric technique is effective for analyzing the phonon modes of BaTiO 3 single crystals. Additionally, we examine the dielectric response and the damping of the soft mode with respect to the order-disorder model.

We developed a far-infrared spectroscopic ellipsometer for measuring the complex permittivity of high-permittivity materials in the terahertz (THz) region and demonstrated the dielectric function of SrTiO3 single crystal at 0.75–21 THz. The responses of the Slater, Last, and O6 displacive modes, which are all optical phonon modes of SrTiO3 at room temperature, were clearly observed. The complex permittivity measured by the ellipsometer agreed with the permittivity simulated by a damped harmonic oscillator model. The far-infrared spectroscopic ellipsometry is useful to analyze the phonon modes of high-permittivity materials, which enable us to understand the contribution of the ionic polarizations to the low-frequency permittivity.

IR active optical modes and phonon–polaritons with E(x) and A1(z) symmetries were studied in a ferroelectric congruent lithium niobate crystal. The real and imaginary parts of a dielectric constant along the a- and c-axes were accurately determined by far-infrared spectroscopic ellipsometry (FIRSP) from 40 to 700 cm−1. For the nine transverse optical (TO) modes with E(x) symmetry, it was difficult to observe the 5th E(TO5) mode at 361 cm−1 and the 9th E(TO9) mode at 665 cm−1 by Raman scattering owing to the very low Raman intensity, while these modes were clearly observed by FIRSP. In contrast, the 6th E(TO6) mode at 371 cm−1 was not observed by FIRSP owing to the very weak absorption, while it was clearly observed by Raman scattering. All the four TO modes with A1(z) symmetry were clearly observed independently by FIRSP and Raman scattering. The dispersion relations of phonon–polaritons including the damping of polaritons were determined using the real and imaginary parts of a polariton wavevector calculated from complex dielectric constants. The polariton dispersion of the lowest A1(z) mode at 254 cm−1 is in agreement with the previous forward Raman scattering experiment; however, any anticrossing predicted by the previous impulsive Raman scattering experiment was not observed.

(Sr 0.7 Bi 0.2 )TiO 3 ceramics were fabricated by a solid state reaction, and a wideband dielectric spectrum was measured for understanding the microscopic polarization mechanism of (Sr 0.7 Bi 0.2 )TiO 3 ceramics. It was revealed that the dielectric permittivity of (Sr 0.7 Bi 0.2 )TiO 3 ceramics at 25°C was determined by dipole polarization as well as ionic polarization, whereas the permittivity of SrTiO 3 is mainly determined only by ionic polarization. The temperature dependence of permittivity suggested that (Sr 0.7 Bi 0.2 )TiO 3 ceramics belongs to ferroelectric relaxor, in which off-center Bi 3+ ions contribute to the formation of polar nanoregions (PNRs). The origin of the dipole polarization is considered the dipole fluctuations of PNRs. On the other hand, the ionic polarization of (Sr 0.7 Bi 0.2 )TiO 3 ceramics was suppressed by an influence of Sr-site vacancy, compared to that of SrTiO 3 . High permittivity of (Sr 0.7 Bi 0.2 )TiO 3 ceramics is mainly due to the dipole polarization associated with dipole fluctuation of PNRs.

Rutile structured TiO 2 has very high dielectric permittivities in non-ferroelectric materials. To understand the reason why rutile TiO 2 has high ionic polarizabilities, it is essential to analyze accurate dielectric spectra of rutile TiO 2 in the THz region. In this study, the complex permittivity of rutile TiO 2 single crystal in the range 30700 cm ¹1 (0.9021 THz) was directly measured using the anisotropic far-infrared spectroscopic ellipsometer. The three E u modes and one A 2u mode, which are all infrared-active phonon modes in rutile TiO 2 , were observed. Moreover, the resonance frequencies of the phonon modes approximately were coincident with a calculation result of first-principles and the analysis data by a harmonic oscillator model. From the examination of the dielectric response, it was found that E u (1) mode, which indicates the oscillation of Ti ions against O ions in the same phase, was dominant contributor to the static permittivity for a-axis. On the other hand, the high permittivity for c-axis was due to "soft" A 2u mode.