Crystal stability and optical properties of organic chain compounds
Department of Physics, Faculty of Science and Art,
University of Split Teslina 10, 21000 Split, Croatia
2 Department of Physics, Faculty of Science, University of Zagreb POB 162, 10001 Zagreb, Croatia
Accepted: 19 November 1998
The solution to the long-standing problem of the cohesion of organic chain compounds is proposed. We consider the tight-binding dielectric matrix with two electronic bands per chain, determine the corresponding hybridized collective modes, and show that three among them are considerably softened due to strong dipole-dipole and monopole-dipole interactions. By this we explain the unusual low-frequency optical activity of TTF-TCNQ, including the observed 10 meV anomaly. The softening of the modes also explains the cohesion of the mixed-stack lattice, the fractional charge transfer almost independent of the material, and the formation of the charged sheets in some compounds.
PACS: 61.50.Lt – Crystal binding; cohesive energy / 61.66.Hq – Organic compounds / 78.30.Jw – Organic compounds, polymers
© EDP Sciences, 1999