Birefringence and dielectric studies on urea-doped thiourea, ${\mathrm{S}}_{0.91}$${\mathrm{O}}_{0.09}$C(${\mathrm{NH}}_2$${)}_2$, have been performed under a dc electric field. For the mixed crystal, no anomalies related to the transitions to intermediate commensurate phases of δ=1/8 and 1/9 are observed, indicating that the 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 ${\mathit{T}}_{\mathit{c}2}$≊169 K shifts driving the heating process to higher temperature with ${\mathit{dT}}_{\mathit{c}2}$/dE≊0.9 K/(kV/cm) while the other peak remains nearly fixed at ${\mathit{T}}_{\mathit{c}1}$≊${\mathit{T}}_{\mathit{c}2}$(E=0) regardless of the electric field E. The field-induced phase between the ${\mathit{T}}_{\mathit{c}1}$ and ${\mathit{T}}_{\mathit{c}2}$ is attributable to the coexistence of ferroelectric and incommensurate phases.

• Received 19 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.6899