The SC(NH2)2 Crystals at 161 K.

What is it about?

The residual incommensurate modulations in the ferroelectric phase of the SC(NH2)2 thiourea are investigated using single-crystal x-ray diffractometry. It is found that the structural states of the ferroelectric phase (T < 169 K) depend on the thermal prehistory of the crystal in the temperature range of existence of the incommensurate phase (202–169 K). The dielectric anomaly observed at Tx = 161 K is justified structurally.

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Why is it important?

Molecular ferroelectric crystals of the SC(NH2)2 thiourea in the paraphase (T > 202 K) and in the ferroelectric phase (T < 169 K) are characterized by equi- translational orthorhombic unit cells with symmetries Pnma and P21ma, respectively [1]. The structure revealed in the temperature range between these two phases (i.e., in the range from Ti = 202 K to Tc = 169 K) is modulated along the b axis with the wave vector q = δb*, where δ is the incommensurability parameter and b* is the reciprocal lattice vector of the initial phase. The measured physical properties of the SC(NH2)2 compound exhibit anomalies at temperatures of 169, 171, 176, 179, and 202 K which are assigned to phase transitions. However, until presently, only the phase transitions occurring at 169, 171, and 202 K had been reliably identified by structural methods. Possibly, the phase transitions at 176 and 179 K have defied identifi- cation using x-ray diffraction because the SC(NH2)2 structure is highly sensitive to x-ray radiation and defects.

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