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Reversible twisting during helical hippuric acid crystal growth
Alexander G Shtukenberg1, John Freudenthal, Bart Kahr
1Crystallography Department, St. Petersburg State University, Universitetskaya emb., 7/9, 199034, St. Petersburg, Russia. shtukenberg@mail.ru
Journal of the American Chemical Society
|April 22, 2010
Summary
Macroscopic crystals of hippuric acid exhibit reversible twisting and untwisting during growth, a phenomenon driven by dynamic elastic stresses that ultimately shape crystal morphology.
Area of Science:
- Crystallography
- Materials Science
- Solid State Physics
Background:
- Crystal growth typically involves ordered addition of units, assuming fixed orientations.
- Previous models of crystal morphology have not fully accounted for dynamic, reversible shape changes during growth.
Purpose of the Study:
- To investigate and demonstrate reversible twisting and untwisting in macroscopic hippuric acid crystals during growth.
- To establish a link between observed crystalloptical properties and the dynamic growth process.
Main Methods:
- In situ observation of hippuric acid crystal growth in undercooled melts.
- Measurement of crystalloptical properties using Mueller matrix microscopy.
- Analysis of elastic stresses and their role in morphological changes.
Main Results:
- Hippuric acid crystals exhibit reversible twisting and untwisting about the axis of elongation during growth.
- The dynamic interplay between twisting and untwisting, driven by elastic stresses, determines final crystal morphology.
- Measured optical properties correlate with a helical rotation of the optical indicatrix at the growth front.
Conclusions:
- Reversible twisting and untwisting are significant factors in macroscopic crystal morphology, challenging traditional growth models.
- The findings provide a new mechanistic understanding applicable to various molecular and mineral crystals exhibiting helical structures.
- Further research is needed to explore the implications of untwisting in existing power-law relationships for helical crystal growth.
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