Studies on the phase transitions in thiourea

Srinivasan, M. R. ; Rama Rao, A. H. ; Shabanova, L. A. ; Bhat, H. L. ; Narayanan, P. S. (1978) Studies on the phase transitions in thiourea Ferroelectrics, 21 (1). pp. 539-541. ISSN 0015-0193

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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.

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