Structure of lithium-doped polyacetylene

A trigonal structure with three polymer chains per lithium column is proposed for lithium-doped polyacetylene. A calculation of the x-ray diffraction profile, using the unit-cell dimension and the lithium concentration as the only variables, is in good agreement with the observed data. The proposed structure optimizes the electrostatic interactions by maximizing both the Li⁺-Li⁺ separations and the coordination of negatively charged carbons and positively charged lithiums. Unlike the tetragonal lattice found in polyacetylene complexed with larger alkali-metal ions (K⁺, Rb⁺, and Cs⁺), the trigonal structure permits the undoped and the doped phases to coexist without any significant lattice mismatch at the boundary between the two phases. While the chain axis expands by {~}1% upon doping with lithium, the projected area per chain remains essentially unchanged.