Brillouin Scattering Studies of the Uniaxial Ferroelectric Phase Transition in TGS and TGSe

The Brillouin spectra of the ferroelectric triglycine sulfate (TGS) and its isomorphous selenate (TGSe) crystals have been observed in three cases of orientations of the longitudinal or quasi-longitudinal phonon vector q ; (1) q // c -axis, (2) q // b -axis and (3) q deviates 5° from b -axis in the (100) plane. The temperature dependence of both the frequency shift and the spectral width shows a remarkable anisotropy near T c . In the case (1) the temperature at which the width becomes maximum shifts to the lower temperature than T c which has simultaneously been determined by an accompanied measurement of dielectric constant. The polarization relaxation time is determined from this temperature shift using the phenomenological relation. In the case (2) and the case (3), the decrease in the anomaly is explained as the depolarization field effect of the long range dipole-dipole interaction which is characteristic in the ferroelectric phase transition.