X-ray study of the structures of polyacetylene and alkali-metal doped polyacetylene derivatives

As a prototype example of conducting polymers, polyacetylene and doped polyacetylene derivatives are the simplest and the most widely studied of their class. A detailed knowledge of their structures and the interactions that stabilize them is very important but still lacking. In this work we studied the structural features of the polyacetylene system by using the x-ray and neutron diffraction techniques. For pristine polyacetylene, our low and high temperature results showed large thermal expansion coefficients along with an increase of the setting angle with temperature, and a large anisotropy between the a and b axes. These results showed that the underlying interchain rectangular lattice of polyacetylene was approaching triangular with increasing temperature. By comparison with results from other polymers and oligomers, we concluded that this triangularity at high temperatures is a direct consequence of 2D close-packing of projected polymer chains rounded by thermally-activated rotations. Behavior complementary to that at low temperature was demonstrated in the high pressure data: large anisotropy between the a and b axes and deviation away from the interchain packing of the triangular limit. In addition, we found evidence that the centered rectangular cell of polyacetylene at ambient conditions was transformed into an oblique cell at certain slightly elevated pressures. Measurements on polyacetylene doped with different alkali-metals, prepared by vapor phase, electrochemical, and chemical solution methods, were also systematically undertaken. For both alkali-metal dopants and polymer chains, our intensity analyses showed a substantial difference in ordering between the chain direction and the ab-plane. Furthermore, despite the general agreement of our data with previously-reported findings, significant discrepancies were revealed through detailed intensity analyses. In particular, we found that both the tetragonal structures for larger alkali-metal doped (CH)$\sb{x}$ (such as K-(CH)$\sb{x}$) and the trigonal structures for smaller alkali-metal doped (CH)$\sb{x}$ (such as Li-(CH)$\sb{x}$) were constructed of lower symmetry local motifs, in which the polymer chains around a dopant tunnel were displaced in a pinwheel fashion from the original high-symmetry configurations. These were argued to result from some neglected interactions that could have important implications for the modeling of the electronic and transport properties