Temperature dependence of the Raman spectra of polycrystalline Ba1-xSixTiO3

We have studied the temperature dependence of the Raman spectra of ceramic bulk and thin-him samples of polycrystalline Ba1-xSrxTiO3 (x= 0, 0.1, 0.2, and 0.3) in the range between room temperature and 350 degrees C. The ferro electric to paraelectric phase-transition temperatures (Tc) have been estim ated from the loss of intensity for several phonon modes belonging to the t etragonal phase when the samples transform to the paraelectric cubic phase. Similar to single crystalline BaTiO3, broad first-order Raman features are found to persist well beyond Te in all the samples studied here. For bulk samples, the estimated values of Te from the Raman studies agree well with the Te values determined from the anomaly in the dielectric constant versus temperature measurements. For thin films, however, the temperature measure ments of the dielectric constant do not show any anomaly and the Raman sign ature viz., the disappearance of several phonon modes at Tc, is used for de termining Tc. We find that the values of Te in thin films are higher than t hose of the bulk samples. This is attributed to the presence of intergrain stresses in the submicron size grains in the thin films which might prevent a complete phase transformation at the expected bulk Tc. Furthermore, a ne w Raman mode has been seen at similar to 620 cm(-1) in all the thin-film sa mples but not in bulk ceramics at all temperatures. This mode has also been observed in polycrystalline bulk BaTiO3 powder under high pressure and it most probably originates from the strain in the grain-boundary regions.