X-Ray Study of Phase Transitions in SC(NH₂)₂

Abstract

X-ray scattering intensities from thiourea, SC(NH₂)₂, have been carefully measured in several phases. Measurements of the diffuse scattering intensity near the point (0, 6, 0) were performed above T_c=169 K for c^*-direction and b^*-direction. The result suggests that the ferroelectric transition at T_c is due to the lattice instability of the transverse optical mode. The scattering intensity around (0, 6, δ) observed just above T_m=202 K shows a strong anisotropy elongating along the c^*-direction.