Optical Phonon Modes and Dielectric Behavior of Sr_1–3x/2Ce_xTiO₃ Microwave Ceramics

Sr 1–3 x /2 Ce x TiO 3 dielectric ceramics (with x varying from 0.133 to 0.400) were investigated by Raman and infrared spectroscopies. The observed features could be related to SrTiO 3 phonon modes. The Raman spectra show first-order modes—which are modes either activated by polar defects or due to local symmetry lowering—together with classical second-order modes of SrTiO 3 . The infrared spectra are dominated by the three predicted polar phonons of SrTiO 3, although a faint tetragonal distortion confirmed by low-temperature measurements was observed for all compositions. For increasing Ce content, the infrared spectra show a continuous hardening of the TO modes and the softening of the highest frequency LO mode. Together with the broadening of the TO phonon lines, these phonon energy evolutions explain the observed dielectric behavior of the material: namely, the decreasing of the dielectric constant and temperature coefficient of the resonance frequency (τ f ) with x . The quality factor is maximized for x = 0.25. Compared to pure SrTiO 3, chemically substituted Sr 1–3 x /2 Ce x TiO 3 ceramics present more adequate dielectric properties at microwave frequencies for technological applications.