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Updated: Aug 7, 2026
Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses
Published on: June 7, 2018
Plastic and glassy crystal states of caffeine
Marc Descamps1, Natalia T Correia, Patrick Derollez
1Laboratoire de Dynamique et Structure des Matériaux Moléculaires, UMR CNRS 8024, Université de Lille 1, Bât. P5, 59655 Villeneuve d'Ascq Cedex, France. marc.descamps@univ-lille1.fr
Abstract:
The present paper focuses on the high temperature form I of caffeine and on its low temperature metastable form. Structural, dynamic, and kinetic information has been obtained by X-ray, dielectric, and calorimetric investigations. This study shows the following features: (1) The high temperature phase (I) of caffeine is in a state of dynamically orientationally disordered crystalline state (so-called "plastic, or rotator, phase"). (2) This high-symmetry hexagonal phase can be maintained at low temperature in a metastable situation. (3) Under deep undercooling of form I a glass transition occurs in the disordered crystalline state near room temperature. It is associated with the orientational freezing in of the molecular motions. Otherwise stated, the metastable state I enters into a nonergodic unstable state, so-called "glassy crystal" state. These findings rationalize the difficulties seen with caffeine in pharmaceutical science.
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