A Raman study of the ferroelectric soft mode in lead germanate

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Abstract
The soft mode Raman spectra of ferroelectric Pb5Ge3O11 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 30Kc-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.