Is a bent crystal still a single crystal?

The mention of the word 'crystal' invokes images of minerals, gems and rocks, all of which are inevitably solid, hard and durable entities with well‐defined smooth faces and straight edges. With the discovery, in the first half of the 20th century that many molecular crystals are soft and can be deformed similarly to rubber or plastic (L. A. K. Staveley, Annu. Rev. Phys. Chem. 1962, 13, 351‒368), this perception is evolving, and both the common apprehension and formal definition of what a crystal is may require reinterpretation in view of their extraordinary mechanical compliance. The seemingly naïve question posed in the title of this Minireview does not have a simple answer. Here, we discuss the effects on the diffraction signature of elastic and plastic deformation of molecular crystals as primary evidence of the degree of their crystallinity. In most cases, the definition of a crystal holds for both elastically and plastically deformed crystals, and unless there is significant or complete physical separation of the crystal during the deformation, they can safely be considered (deformed) single crystals with high concentration of defects.

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