Brillouin spectroscopic evidence for a relaxation mode in ferroelectric PbHPO4 and PbDPO4

Abstract

Brillouin spectra of ferroelectric PbHPO4 (Tc=310 K) and its deuterated isomorph PbDPO4 (Tc=462 K) have been measured as a function of temperature across their transition temperatures. Certain longitudinal acoustic waves are found to exhibit anomalous propagation in the vicinity of Tc. The relaxation times, τ0, for a noninteracting dipole are evaluated from the Brillouin shifts, based on a phenomenological approach. The τ0 values are the same in both PbHPO4 and PbDPO4, indicating that the ferroelectric transition is driven by an order-disorder relaxation mode. This study also reveals that the paraelectric phase symmetry is, on a macroscopic scale, centrosymmetric. © 1995 The American Physical Society.