Stage-1 phases of alkali-metal-doped conducting polymers

The stage-1 phases of both Rb-doped trans-polyacetylene (PA) and Cs-doped poly(p-phenylene vinylene) (PPV) have been investigated by in situ x-ray diffraction, using highly oriented vapor-doped samples. While the relative guest-host sizes are nearly equivalent, the resulting equatorial structures, as determined by structure-factor calculations, exhibit pronounced differences in the location and orientation of the polymer chains. PPV samples are dominated by uniform chain-axis rotations within a base-centered orthorhombic lattice of the Cs⁺ ions while PA undergoes displacive translations within a centered tetragonal lattice of the Rb⁺ ions. Significant differences in the dopant-ion organization within the quasi-one-dimensional alkali-metal ion channels are also found. These temperature insensitive and polymer specific variations are indicative of local guest-host interactions which stabilize the respective structures. Further comparisons suggest that in PA systematic intermediate-length-scale deviations from planarity by the polymer chains are probable. As a result, the electronic excitations may couple to polymeric torsional degrees of freedom.