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.

Chemical doping is one of the major methods by which the properties of BaTiO₃ are modified to induce various device characteristics. Doping strategies are generally separated into iso- and alio-valent (donors and acceptors) and into A- and B-site species within the perovskite structure. The A_1g octahedral breathing mode at ∼800 cm⁻¹ in BaTiO₃ is Raman inactive for compositions with single B-site species. However, this mode becomes Raman active if more than one B-site species is present, including titanium vacancies, V_T′′′′. Moreover, the relative intensity of the A_1g breathing mode is qualitatively related to the concentration of the species replacing Ti in the B-site. This article illustrates some clear cases where the A_1g octahedral breathing mode can be utilized to verify the site occupancy of dopants in BaTiO₃ when used in conjunction with more conventional indirect methods such as x-ray diffraction (Vegard’s Law) and dielectric measurements as a function of temperature.