Brillouin and Raman study of the ferroelectric phase transition in PbHPO4

Abstract
In order to investigate the dynamics of members of a novel series of ferroelectrics we have measured the Raman spectra of PbHPO4, as well as the Brillouin spectra in the paraelectric and ferroelectric phases. Mode assignment has been made. A weak underdamped soft mode has been observed below 100 cm-1. The Brillouin spectra revealed low frequency dispersion below Tc which could be fitted very well to the simple relaxation type formula. By comparison with far infrared reflectivity data we conclude that a low frequency relaxing mode and not the mode which is observed in Raman is responsible for the transition.