X-Ray Scattering from Sodium-Doped Polyacetylene: Incommensurate-Commensurate and Order-Disorder Transformations

X-ray scattering carried out in situ during electrochemical doping indicates an unusual sequence of structural transitions. On doping to {≈}6-7 mole%, lightly doped regions coexist with a discommensurate-domain channel structure. As the Na concentration increases, the density of domain walls decreases. When the discommensuration density falls to zero, the Na channels form a commensurate √3×√3 superlattice with respect to the (average) triangular lattice of (CH)ₓ chains. At still higher concentrations, the Na lattice becomes unstable and transforms into a disordered "fluid" phase within the ordered polyacetylene structure.