On the Molecular Nature of the Dielectric Anomalies in Thiourea

Abstract

Of the four dielectric anomalies in thiourea, three are shown to be correlated with the disordering of the molecules in the crystal. This disordering involves both a relative rotation and a translation of the molecules between potential minima. One of the other anomalies, the high one in temperature, is shown to result from a translation of the molecules within the unit cell. Lattice parameter measurements as a function of temperature confirm that the lowest transition is of first order and show only a change in slope at the highest transition. An electrostatic model based upon the electronic resonance structures of thiourea is used to discuss the binding energy of the molecules in the crystal, the vibration of the molecules, the equilibrium orientation of the molecules in the unit cell, and the spontaneous polarization of the crystal. A simple statistical model based upon two interpenetrating nonidentical sublattices reveals a disordering of the molecules from 0°K to 178°K and yields the first-order transition together with the two ferrielectric regions of temperature.