Lattice dynamics in Ba_xSr_(1-x)TiO_3 thin films studied by Raman spectroscopy

摘要

We present a Raman spectroscopy study of the lattice-dynamical properties of the Ba_xSr_(1-x)TiO_3 thin films with x=0.05, 0.1, 0.2, 0.35, and 0.5 in the temperature range of 5-300 K. Soft phonon modes were observed, and the temperature dependence of their frequencies and Raman intensities were investigated. The Raman spectra of the Ba_xSr_(1-x)TiO_3 films are compared with the single crystals of the same compositions. The essential differences in the lattice-dynamical properties of the thin films and single crystals are observed, such as the forbidden first-order Raman scattering in the films well above the phase-transition temperature in bulk, hardening of the soft phonon modes in films compared to crystals, and significantly larger range of the soft-mode overdamping in the films. On the other hand, the lattice-dynamics behavior in the Ba_xSr_(1-x)TiO_3 films is found similar to that of the relaxor ferroelectrics. Analogous to the relaxors, the presence of the polar nanoregions existing in the films at the temperatures above the bulk ferroelectric phase transition explains the specific lattice-dynamical properties of the Ba_xSr_(1-x)TiO_3 thin films.