Study of Transformation of Ferroeletric Phase Transition Mechanism by Farーinfrared Fourier Spectroscopy

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The ferroelectrics Li_2Ge_7O_<15> undergoes peculiarly interesting phase transition, where both the dielectric critical slowing-down characteristics of the order-disorder type and the soft phonon mode characteristic in the disp laciveone. In this kind of phase transiton it could be considered the following two cases : (i) The soft phonon becomes overdampened and transforms to the relaxational mode.(ii) There exists a relaxational mode with an independent degree of freedom as well as the soft phonon mode and the character of softening transfers from the phonon to the relaxational mode. This is an important problem to understand the dynamics of structural phase transition.In the present study we attempt to measure both the soft phonon and the relaxational mode simultaneously by an infrared Fourier spectrometer particularly designed and to observe the transformation of the phase transition mechanism. In the first year a cryostat for optical measurements was purchased by means of the present Grant-in-Aid. The temperature dependence of the far-infrared transmission spectra have been measured with the cryostat and a handーmade furnace. In the paraelectric phase B phonon with the lowest frequency has been confirmed to show softening as the transition temperature was approached from above. This result shows good agreement with that of Raman scattering measurment.In the second year, to observe both phonon and relaxational mode in the lowerwave number region the improvement of the interferometer has been performed.By means of the interferometer with a high sensitive Si : GA bolometer, the spectra in the frequency region to 4cm^<-1> can be measured. The study is now in progress.