Optical phonon modes and dielectric behavior of Sr1-3x/2CexTiO3 microwave ceramics

Sr1-3x/2CexTiO3 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 SrTiO3 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 SrTiO3. The infrared spectra are dominated by the three predicted polar phonons of SrTiO3, although a faint terragonal 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 7 observed dielectric behavior of the material: namely, the decreasing of the dielectric constant and temperature coefficient of the resonance frequency (tau(f)) with x. The quality factor is maximized for x = 0.25. Compared to pure SrTiO3, chemically substituted Sr1-3x/2CexTiO3 ceramics present more adequate dielectric properties at microwave frequencies for technological applications.