Résumé
Raman spectroscopy were used to study the temperature behavior of the polar optical modes of a set of strontium titanate ceramics with grain size ranging between about 20 nm and 1 µm. Diminishing the grain size yields to an enhancement of (normally forbidden) Raman polar modes with a strong reduction of dielectric permittivity. A progressive decoupling between the antiferrodistortive (AFD) order parameter and the spontaneous strain is observed. In addition, the soft mode of nano STO ceramic has been observed by Hyper-Raman spectroscopy. The obtained spectra shows no splitting of this vibration in the range between 25 K and 300 K. This is attributed to the weak tatragonality ratio c/a even at very low temperature. Moreover, the static dielectric constant calculated from the Lyddane-Sachs-Teller relation that resembles the intrinsic dielectric constant are in agreement with that obtained from dielectric measurements. These results were interpreted in comparison with the soft mode behavior of micronic grain size STO ceramic.