Is a Bent Crystal Still a Single Crystal?

Area of Science:

• Materials Science
• Solid-State Physics
• Crystallography

Background:

• Traditional perception of crystals as inherently hard and rigid, like minerals and gems.
• Emerging understanding since the mid-20th century that molecular crystals can exhibit significant elastic and plastic deformation.
• Need to re-evaluate the definition and concept of a crystal in light of these discoveries.

Purpose of the Study:

• To explore the impact of elastic and plastic deformation on the diffraction signature of molecular crystals.
• To provide evidence for the degree of crystallinity in deformed molecular crystals.
• To discuss whether deformed molecular crystals still fit the formal definition of a crystal.

Main Methods:

• Analysis of diffraction signatures from elastically and plastically deformed molecular crystals.
• Examination of the relationship between deformation, defect concentration, and crystal integrity.
• Review of existing definitions of crystalline structures.

Main Results:

• Deformation effects on the diffraction signature provide primary evidence of a molecular crystal's degree of crystallinity.
• Elastically and plastically deformed crystals generally retain their single-crystal status.
• Deformed crystals are considered single crystals with a high concentration of defects, provided physical separation does not occur.

Conclusions:

• The definition of a crystal can encompass soft, deformable molecular solids.
• Diffraction analysis is key to understanding the crystallinity of deformed materials.
• Molecular crystals, even when significantly deformed, can maintain their single-crystal nature with an increased defect density.