Simulation of Plastic Crystals

Abstract

Traditionally, the name of “Plastic Crystals” has been reserved, because of their macroscopic properties, to some molecular crystals. These organic compounds have a high temperature phase, just below the melting point where the vapour pressure is relatively high and the deformation is easy. These plastic phases are highly symmetric, generally cubic and at some transition temperature, below the melting point, they transform to another crystalline phase with a lower symmetry. X-Ray diffraction studies of plastic crystals yield only a few diffracted beams and intense diffuse scattering. This is characteristic of an orientational dynamical disorder. The existence of orientational freedom is the property which distinguishes plastic crystal from other molecular solids. This orientational disorder is also observed in the high temperature phases of ionic solids containing molecular ions such as ammonium, sulphate and cyanide. On the microscopic scale the molecular motion is very similar to the one observed in plastic crystals. The essential difference is that the molecular ions are rotating within the monatomic crystal lattice of the second ion species.