A model of the paraelectric phase of thiourea

Abstract

As a contribution to the understanding of the incommensurate phase transitions in thiourea, we present a theoretical study of the crystallographic details of the parael ectric phase. A model intermolecular potential is developed, which includes a reasonable distribution of electrostatic multipole interactions as well as the standard dispersive and repulsive interactions. The model gives a satisfactory prediction of the structure of the para electric phase, and in particular explains the occurrence of the hydrogen-bond network. Calculations of phonon-dispersion curves predict a soft-phonon branch in the b direction with the same symmetry as that observed experimentally. Computer simulations predict reasonable values for the vibrational amplitudes, and show the existence of large-amplitude fluctuations of an harmonic quantities at incommensurate wave vectors. However, although the model displays a strong tendency towards incommensurate and lock-in ordering, it does not in fact give a phase transition at a finite temperature. This failure is attributed to the neglect of molecular polarizability, and it is concluded that this feature provides the mechanism that stabilizes the low-temperature phases.

For the interested reader, full details of the molecular dynamics simulation technique using parallel processing are presented here. In particular, a method of extracting normal-mode eigenvectors from the results of the simulations is described.