A plastic crystal is a crystal composed of weakly interacting molecules that possess some orientational or conformational degree of freedom. The name plastic crystal refers to the mechanical softness of such phases: they resemble waxes and are easily deformed. If the internal degree of freedom is molecular rotation, the name rotor phase is also used.
If the internal degree of freedom freezes in a disordered way, an orientational glass is obtained.
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 structured. 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 structure 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.
Plastic crystals versus liquid crystals
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.
- Coupled orientational and displacive degrees of freedom in the high-temperature plastic phase of the carbon tetrabromide α-CBr4 Jacob C. W. Folmer, Ray L. Withers, T. R. Welberry, and James D. Martin. Physical Review B 77, 144205 (2008).