Impurity clustering in doped polyacetylene

We use an augmented Su-Schrieffer-Heeger model to study the changes that occur in polyacetylene in the low-doping regime as doping concentration is increased. The lowest-energy configurations are found to be those for which the impurities are clustered together. The clustering results from the net interaction between impurity atoms and pinned solitons. Clustering explains the doping-induced shifts observed in the infrared spectrum of iodine-doped samples. Infrared results for widely spaced impurities are in marked contrast with experiment.