Indomethacin Polymorph δ Revealed to be Two Plastically Bendable Crystal Forms by 3D Electron Diffraction: Correcting a 47‐Year‐Old Misunderstanding

Indomethacin is a clinically classical non-steroidal anti-inflammatory drug that has been marketed since 1965. The third polymorph, Form δ, was discovered by both melt and solution crystallization in 1974. δ-indomethacin cannot be cultivated as large single crystals suitable for X-ray crystallography and, therefore, its crystal structure not yet determined. Here, we report the elucidation of 3D electron diffraction reveal truth melt-crystallized solution-crystallized are fact two polymorphs with different structures. We propose to keep polymorph δ name θ. Intriguingly, structures display plastic flexibility based on slippage mechanism, making indomethacin first have polymorphs. This discovery correction 47-year-old misunderstanding signify become powerful tool polymorphic structural studies.