Abstract
Raman scattering spectra of single crystals of SC(NH 2 ) 2 were observed in the temperature range from 300 K to 77 K especially in the frequency region 0∼200 cm -1 . The previous reports that there is a soft mode whose frequency decreases as the transition point T 4 (202 K) is approached from below are confirmed. The soft mode frequency shows the scattering vector dependence in the ferroelectric phase. The incommensurate successive phase transitions in SC(NH 2 ) 2 are discussed in relation to the observed soft mode. The soft mode is inferred to be mainly the librational mode B 2u ( L ) of SC(NH 2 ) 2 molecules, whose axis of libration is parallel to the c -axis.
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