A cubic-tetragonal-trigonal sequence of phase transitions in the relaxor ferroelectric ${\mathrm{Na}}_{1/2}$${\mathrm{Bi}}_{1/2}$${\mathrm{TiO}}_3$ (NBT) has been studied by Brillouin scattering. Pronounced anomalies for both the hypersonic velocity and damping have been found that peak just between two phase transitions in contrast to the ordinary well-known behavior with anomalies in the vicinity of every transition point. The fact that these anomalies are centered in the intertransition region and extend beyond both transitions is attributed to the fluctuations of two coupled order parameters associated with disorder in the Na- and Bi-cation distribution in a manner characteristic of relaxor ferroelectrics. Such a phenomenon seems to be typical of relaxor materials and to be the basis of the diffuse phase-transition dynamics.

• Received 25 May 1994

DOI:https://doi.org/10.1103/PhysRevB.51.5659