Structural Phase Transition Dynamics Studied by Real-time Measurement of Impulsive Response Function

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The purpose of this study is to clarify the ferroelectric phase transition mechanism of a potassium dihydrogen phosphate (KDP : KHィイD22ィエD2POィイD24ィエD2); the displacive type characterized by a soft mode or the order-disorder type characterized by a relaxational mode. For this purpose, we apply an ultra-short light pulse to KDP as the impulsive force and measure the impulsive response in the real-time to decide which the ferroelectric central mode observed by Raman scattering is an over-damped soft mode or a relaxational modeIn 1998, we started the measurement using the 130 fsec Ti:Sapphire pulse laser system. The ferroelectric central mode was successfully excited and its time dependence was fund to be expressed by the relaxational mode with a single relaxation time τ. Temperature dependence of τ agrees well with the ferroelectric relaxation time obtained by the dielectric measurement and Raman scattering. The phase transition mechanism seems to be of the order-disorder type at first glance. However, the vibrational behavior is estimated to be observed near 110 fsec from the results of Raman scattering if the ferroelectric central mode is an over-damped soft mode. As long as we use the 130 fsec pulse, there remains a possibility that an over-damped soft mode is also observed as a relaxational mode.In 1999, we constructed the 80 fsec pulse laser system newly and measured the impulsive response of the ferroelectric central mode at the wave vector k = 4100 cmィイD1-1ィエD1. The peak was found at about 150 fsec in the response. This means that the mode has the mechanical inertia and seems to be the over-damped soft mode. It could be concluded that the phase transition mechanism of KDP is of the displacive type. The reproducibility is being checked now.