Inelastic light scattering studies of diffuse phase transition in ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅thin films

The dynamic behavior of highly oriented ferroelectric Sr1.9Ca0.1NaNb5O15thin films has been investigated by Brillouin light scattering over a wide temperature range between 25 and 450°C. The temperature variations of the full-width at half-maximum (FWHM) of the central peak (CP) are found to exhibit marked changes around the Curie temperature (Tc= 246°C). The CP reveals a relaxational mode behavior, which is attributed to thermally activated fast relaxation of polar nanoregions (PNRs). The width of the CP decreases steadily upon cooling from ∼400°C to Tcand shows little subsequent change within the ferroelectric state, indicating an increase of the number of PNRs and a slowing down of their dynamics. By using a modified superparaelectric model, the activation energy (H0) of the relaxation processes and the Burns temperature (TB) are determined to be 2939 and 385°C, respectively. The evidence for this diffuse phase transition provided by Brillouin scattering is in agreement with results obtained from the temperature-dependent dielectric permittivity, refractive index, and Raman spectra.