FERROELECTRIC PHASE-TRANSITION OF S0.91O0.09C(NH2)2 IN ELECTRIC-FIELDS

Birefringence and dielectric studies on urea-doped thiourea, S0.91O0.0 9C(NH2)2, have been performed under a dc electric field. For the mixed crystal, no anomalies related to the transitions to intermediate comm ensurate phases of delta = 1/8 and 1/9 are observed, indicating that t he commensurate phases are suppressed by urea impurities. When a bias field is applied to the sample the dielectric-constant anomaly related to the ferroelectric phase transition splits into two peaks. One peak at a temperature T(c2) almost-equal-to 169 K shifts driving the heati ng process to higher temperature with dT(c2)/dE almost-equal-to 0.9 K/ (kV/cm) while the other peak remains nearly fixed at T(c1) almost-equa l-to T(c2)(E = 0) regardless of the electric field E. The field-induce d phase between the T(c1) and T(c2) is attributable to the coexistence of ferroelectric and incommensurate phases.