Orientational Freedom of Molecules in Crystals. II. Higherhyphen;Order Transition by Progressive Decorrelation of Orientations. A Monte Carlo Calculation

摘要

A clearhyphen;cut example of a higherhyphen;order transition involving orientational disordering in some molecular crystals is presented. In these crystals, which are polar chloromethyl hexasubstituted benzenes, the molecules exhibit a onehyphen;axis sixfold disorder at high temperatures. On cooling, the static dielectric constant decreases, indicating the cooperative setting in of orientational order. This is confirmed by xhyphen;ray studies which indicate that the superstructure at low temperature is an antiferroelectric arrangement. A quantitative interpretation of the phenomenon is given. The energy which governs the ordering is shown to be essentially electrostatic in origin: Among 288 envisaged ordered arrangements, the electrostatic energy is the lowest for the observed (antiferroelectric) one. These calculations have been made using Coulomb interactions between charges located on the atoms since the point dipole model has been found to be too crude. A Monte Carlo calculation, which is partly dynamic and partly stochastic in nature, has been carried out for the transition region; this yields a semiquantitative agreement for the observed static polarization, and a fairly good reproduction of the shape of the experimental relaxation spectrum at different temperatures.