Some basic theories about incommensurate phase (I-phase) and commensurate-incommensurate phase transition (C-I phase transition) and phenomenological theories about incommensurate phase transition in thiourea were reviewed in some detail.
Dielectric and Raman experiments were carried out in order to study the phase transition behaviors of thiourea which is one of the old ferroelectric crystal showing incommensurate phase between normal incommensurate phase transition (N-I phase transition) temperature $T_I$ and incommensurate-commensurate of low symmetry phase transition (C-I transition) temperature $T_C$. 1) At 172K which corresponds well to the I-phase to 1/9 C-phase transition temperature $T_9$, a very small peak of dielectric constant (real part) was observed, in agreement of McKenzie and Dryden's report. 2) Low a.c. voltage dielectric constrants (real part) mainly in low temperature ferro-electric phase (C-phase) were measured at 15kHz for only two signal voltage, one is 0.7 ~ 3.2$mV_{rms}$ and the other is 40 ~ 317 $mV_{rms}$, and observed a new voltage dependence showing highly anomalous behavior at 161K and discussed briefly. 3) d.c. Electric bias effect on mainly N-I phase transition temperature was investigated by measuring dielectric constant at 10 kHz under various bias fields up to about 10 kV/cm, about N-I phase transition, we observed the bias field dependence of $T_I$ to be large in contrast to the theoretical prediction of Chapelle and could be explained in terms of the Landau expansion of free energy. 4) Both lattice phonon and molecular vibrational modes of crystal have been studied as a function of temperature by Raman scattering. Temperature dependence of the 620 $cm^{-1}$ band splitting was found to be strongly correlated with the order parameter dependence of temperature which could not be explained by the theories of Poulet and Pick but in good agreement with the results of Winterfeldt and Schaack.
