Plastic crystallinity

Plastic crystallinity is a phenomenon exhibited by many materials that form molecular solids with relatively weak interaction between the molecules, usually just van der Waals forces.

Like liquid crystals, plastic crystals can be considered a transitional stage between real solids and real liquids and can be considered "soft matter". Another common denominator is the simultaneous presence of order and disorder. Both types of phases are usually observed between the true solid and liquid phases on the temperature scale:

: true crystal -> plastic crystal -> true liquid: true crystal -> liquid crystal -> true liquid

The difference between liquid and plastic crystals is easily observed in X-ray diffraction. Plastic crystals possess strong long range order and therefore show sharp Bragg reflections. Liquid crystals show no or very broad Bragg peaks because the order is not long range. The molecules that give rise to liquid crystalline behavior often have a strongly elongated or disc like shape. Plastic crystals consist usually of almost spherical objects. In this respect one could see them as opposites.

The name plastic crystal refers in large part to the soft plastic mechanical properties of such phases. They resemble waxes and are easily deformed.

In a plastic crystal there is a true solid lattice with long range translation symmetry inside of which there is considerable disorder. It was thought in the past that this disorder had to be of a rotational nature, hence the alternative term 'rotor phase' that was introduced. It was thought that molecules in these phases were more or less free to rotate around their center so that they would act as stacked spherical objects. This may be true for some plastic phases but certainly not for all. In carbon tetrabromide e.g. the molecules are restricted to only six possible more or less fixed orientations [Coupled orientational and displacive degrees of freedom in the high-temperature plastic phase ofthe carbon tetrabromide α-CBr4 Jacob C. W. Folmer, Ray L. Withers, T. R. Welberry, and James D. Martin. Physical Review B 77 in press] .

Although this represents far less disorder than completely free rotation, the X-ray diffraction patterns of plastic crystals are characterized by strong diffuse intensity in addition to the sharp Bragg peaks. In a powder pattern this intensity appears to resemble an amorphous background as one would expect for a liquid, but for a single crystal the diffuse contribution reveals itself to be highly structred. The Bragg peaks can be used to determine an average structure but due to the large amount of disorder this is not very insightful. It is the structre of the diffuse scattering that reflects the details of the constrained disorder in the system. Recent advances in two-dimensional detection at synchrotron beam lines facilitate the study of such patterns.

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