Critical slowing down and elastic anomaly of uniaxial ferroelectricCa0.28Ba0.72Nb2O6crystals with tungsten bronze structure

Abstract

The ferroelectric phase transition of uniaxial ${\mathrm{Ca}}_{0.28}{\mathrm{Ba}}_{0.72}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}$ single crystals with a moderate effective charge disorder was investigated by Brillouin scattering to clarify the dynamic properties. In the tetragonal paraelectric phase a remarkable softening of the sound velocity of the longitudinal acoustic mode and a significant increase in the sound attenuation were observed close to the Curie temperature ${T}_{\mathrm{C}}=527\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The intermediate temperature ${T}^{*}$ $\ensuremath{\sim}640\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the Burns temperature ${T}_{\mathrm{B}}$ $\ensuremath{\sim}790\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ were determined from the temperature variation in the sound attenuation. The intense broad central peak (CP) caused by polarization and strain fluctuations due to polar nanoregions was clearly observed in the vicinity of ${T}_{\mathrm{C}}$. The relaxation time determined by the CP width clearly shows critical slowing down towards ${T}_{\mathrm{C}}$, reflecting a weakly first-order phase transition under weak random fields.