Effect of charge transfer on chain dimension in t r a n s-polyacetylene

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Abstract
X‐ray diffraction measurements on all‐trans‐polyacetylene are consistent with a chain‐axis length elongation upon donor doping (+0.026 Å for lithium and +0.04 Å for potassium) and a chain‐axis length contraction upon acceptor doping (−0.010 Å for iodine), where the changes refer to the length L of a C2H2 unit (2.457 Å in the undoped polymer). These new experimentally derived results for heavily doped compositions, which ignore possible corrections for cell nonorthogonality in the lithium and iodine complexes, are similar to experimental results for graphite intercalation complexes and are consistent with theoretical predictions for doped polyacetylene. The meridional diffraction lines observed at L and L/2 for potassium‐doped polyacetylene indicate that there is no lattice symmetry element which includes a translation operation of L/2 in the chain‐axis direction. The observations are consistent with a structural model in which alkali–metal ions with an intracolumn spacing of 4.96 Å are commensurate with the polymer chains for the composition (CHM0.125)x. The likely polymer chain‐axis repeat length is 2L (i.e., C4H4) and a lattice symmetry element which includes a translation of L is expected.