Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_3$ ceramics in the 10–300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\omega }_{\mathrm{TO}1\mathrm{}}^{-\alpha }(\alpha \approx 1.6)$ where ${\omega }_{\mathrm{TO}1\mathrm{}}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the $E_g$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_6$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller $A_{1g}-E_g$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.

The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric single-domain crystal of ${\mathrm{BaTiO}}_3$ was determined by time-domain THz spectroscopy and Fourier-transform infrared reflectivity techniques. In addition to the three well-known polar lattice modes, the experiment shows an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the $c$-axis dielectric constant. A comparison of experimental results with ab initio based effective-Hamiltonian simulations allows us to elucidate its relation to the order-disorder model of Comes, Lambert, and Guinier [ Solid State Commun. 6, 715 (1968)].

High-frequency dielectric properties of ${\mathrm{A}}_{\mathrm{5}}{\mathrm{B}}_{\mathrm{4}}{\mathrm{O}}_{15}$ (A=Ba, Sr, Mg, Zn, Ca; B=Nb, Ta) dielectric ceramics are studied by means of the microwave cavity technique, a combination of far-infrared reflection and transmission spectroscopy and time-resolved terahertz transmission spectroscopy. Microwave permittivity ε^′ and $\mathrm{Q\times f}$ factor vary, depending on the chemical composition, between 11 and 51, and 2.4 and 88 THz, respectively. The temperature coefficient ${\tau }_f$ varies between −73 and 232 ppm/°C, and in two samples ${\mathrm{|\tau }}_f|$ is less than 15 ppm/°C. It is shown that the microwave permittivity ε^′ of the ceramics studied is determined by the polar phonon contributions and that linear extrapolation of the submillimeter dielectric loss ε^″ down to the microwave region is in agreement with the microwave data of single phase samples. The relationship among phonon spectra, the crystal structure, and the unit cell volume is discussed.