Raman study of Ba_xSr_1-xTiO₃ films: Evidence for the existence of polar nanoregions

We have studied lattice dynamical properties of Ba_xSr_1-xTiO₃ thin films and single crystals with x=0.05, 0.1, 0.2, 0.35, and 0.5 by Raman spectroscopy in the temperature range 5-300 K. In the crystals, only the second order Raman features are observed above the phase transition temperature due to crystal symmetry. In the films the first-order phonon modes are observed both below and above the ferroelectric phase transition temperature. The soft modes in films are harder than those in single crystals and show larger overdamping over a broader temperature range near the phase transition. The overdamping is correlated with the broad peak in the temperature dependence of the dielectric constant. The lattice dynamical behaviors in the films are strikingly similar to those in relaxor ferroelectrics. The similarities are explained by the existence of polar nanoregions in the Ba_xSr_1-xTiO₃ thin films.