Critical slowing down and elastic anomaly of uniaxial ferroelectric Ca0.28Ba0.72Nb2 O6 crystals with tungsten bronze structure

Abstract

The ferroelectric phase transition of uniaxial Ca0.28Ba0.72Nb2O6 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 TC=527K. The intermediate temperature T* ∼640K and the Burns temperature TB ∼790K 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 TC. The relaxation time determined by the CP width clearly shows critical slowing down towards TC, reflecting a weakly first-order phase transition under weak random fields. © 2014 American Physical Society.