Large Negative Thermal Expansion in Pentacene due to Steric Hindrance

Details Large Negative Thermal Expansion in Pentacene due to Steric Hindrance Large Negative Thermal Expansion in Pentacene due to Steric Hindrance

2007-07-03

arxiv.org/abs/0707.0450v1

Phys. Rev. B 76, 205203 (2007)

Physics

Condensed Matter

Materials Science

Scientific paper

10.1103/PhysRevB.76.205203

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.

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