A Raman scattering study of the antiferroelectric phase transition in ${(\mathrm{S}\mathrm{r}}_{0.70}{\mathrm{Ca}}_{0.30}{)\mathrm{T}\mathrm{i}\mathrm{O}}_3$

The Raman scattering measurements on ceramic pellets of ${\mathrm{Sr}}_{0.70}{\mathrm{Ca}}_{0.30}{\mathrm{TiO}}_3$ (SCT 30) have been carried out as a function of temperature from 290 K to 8 K. It is shown that below $(236\mathrm{}\pm 5\mathrm{K}),$ two additional lines appear around 79 and 128 ${\mathrm{cm}}^{\mathrm{-}1}$. Further, there is enhancement in the intensity of the modes near 170 and 541 ${\mathrm{cm}}^{\mathrm{-}1}$ below $(236\mathrm{}\pm 5\mathrm{K}).\mathrm{}$ The temperature at which these changes occur in the Raman spectra of SCT 30 correlates very well with the temperature at which the antiferroelectric superlattice reflections appear in the x-ray diffraction patterns. We propose that the additional modes are due to $q\ne 0$ phonons that have become Raman active due to the folding of the corresponding special points into the zone center below the cell-doubling antiferroelectric phase-transition temperature.