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Plastic crystal

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Title: Plastic crystal  
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Subject: Cleavage (geology), Soft matter, Crystals, State of matter, Physical quantities
Collection: Crystals, Physical Quantities, Soft Matter
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Plastic crystal

A plastic crystal is a [1] and imidazolium methanesulfonate.[2]

If the internal degree of freedom freezes in a disordered way, an orientational glass is obtained.

The orientational degree of freedom may be an almost free rotation, or it may be a jump diffusion between a restricted number of possible orientations, as was shown for carbon tetrabromide.[3]

The X-ray [1] 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 [1] 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.

References

  1. ^ a b c d e f
  2. ^ a b
  3. ^ 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).
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