Studies on the phase transitions in thiourea

Abstract

Thiourea (CS(NH₂)₂) is one of the few examples of molecular crystals exhibiting ferroelectric properties. The dielectric constant along the ferroelectric axis [100] shows maxima at 169, 177 and 202 K. An inflection point occurs at 170.5 KZ Following Goldsmith and White the phases are named as I (F.E. below 169 K), II (A.F.E. 169 K<T<176 K), I11 (F.E. 176 K<T<180 K), IV (A.F.E. 180 K<T<202 K) and V (P.E. Above 202 K). The appearance of satellite spots in the X-ray and neutron scattering indicates the presence of structural modulation in phases 11, 111 and IV. The effect of structural modulation is to destroy the local n glide symmetry, though the overall structure is Pnma. Thiourea also exhibits thermoluminescence after being irradiated with x-rays which is accounted for by defects like NH₃⁺ and NH^- formed by the breaking of hydrogen bonds.