1. Large Negative Thermal Expansion in Pentacene due to Steric Hindrance
- Author
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Haas, S., Batlogg, B., Besnard, C., Schiltz, M., Kloc, C., and Siegrist, T.
- Subjects
Condensed Matter - Materials Science - Abstract
The uniaxial negative thermal expansion in pentacene crystals along $a$ is a particularity in the series of the oligoacenes, and exeptionally large for a crystalline solid. Full x-ray structure analysis from 120 K to 413 K reveals that the dominant thermal motion is a libration of the rigid molecules about their long axes, modifying the intermolecular angle which describes the herringbone packing within the layers. This herringbone angle increases with temperature (by 0.3 -- 0.6$^{\circ}$ per 100 K), and causes an anisotropic rearrangement of the molecules within the layers, i.e. an expansion in the $b$ direction, and a distinct contraction along $a$. Additionally, a larger herringbone angle improves the cofacial overlap between adjacent, parallel molecules, and thus enhances the attractive van der Waals forces.
- Published
- 2007
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