Theory of Sinusoidal Antiferroelectricity in Thiourea

Phase transition in thiourea is discussed on the basis of a simple lattice dynamical model for the molecular rotational vibration. It is shown that a combination of the dipole-dipole interaction and the short range force can yield the soft rotational branch which has a minimum frequency at a certain :finite wave vector qo. Based on this result and the consideration on the anharmonic vibrations, a free energy expression of the Landau type is given, which describes the sinusoidal antiferroelectric phase transition and the tem­ perature dependence of the modulation wave vector. It is revealed that the temperature dependence of the modulation wave vector originates from the shift of the balance between the dipole-dipole interaction and the appropriate short range force, which is caused by the anharmonic effects of vibrations.