RAMAN STUDY OF THE FERROELECTRIC SOFT MODE IN LEAD GERMANATE

The soft mode Raman spectra of ferroelectric Pb5Ge 3O11 have been carefully measured as a function of temperature. These new results were analyzed to obtain the parameters of various models for the soft mode spectral response for frequencies larger than 4 cm-1. The modes employed were: (i) a Debye relaxational function for the overdamped spectra; (ii) a classically damped harmonic oscillator at all temperatures and (iii) a model which allows for coupling between the soft mode and phonon density fluctuations. It is found that the low-frequency Raman spectra are well described by the damped harmonic oscillator model for 30K<or approximately=Tc-T<or approximately=80K. Further, the resonance frequency and damping parameters obtained do not show the expected temperature dependence but diverge for Tc-T<30K. It is shown that the self-energy of this simple model does not have the appropriate frequency dependence and that a more suitable form can be obtained when coupling of the soft mode to phonon density fluctuations is considered.