Inelastic light scattering studies of diffuse phase transition in ferroelectric Sr _(1.9)Ca _(0.1)NaNb _5O _(15) thin films

摘要

The dynamic behavior of highly oriented ferroelectric Sr _(1.9)Ca _(0.1)NaNb _5O _(15) thin 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 (T _c = 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 T _c and 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 (H _0) of the relaxation processes and the Burns temperature (T _B) 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.