HIGH-RESOLUTION STUDY OF CONDUCTIVITY AND CELL POTENTIAL VERSUS DOPING CONCENTRATION IN POTASSIUM-DOPED POLYACETYLENE - CORRELATION WITH STRUCTURAL TRANSITIONS

A high-resolution quasiequilibrium in situ study of parallel conductivity sigma parallel-to and open-circuit voltage V(OC) during potassium doping and dedoping of an oriented (CH)x electrode is presented. Features in dV(OC)/dy and d-sigma parallel-to/dy (y = K mole fraction) found previously at y = 0.06, 0.12, and 0.15 are confirmed and correlated with recent x-ray and ESR results and with a model of staging via intercalation channels. We also find new features during dedoping at y = 0.03 and 0.08 in d-sigma parallel-to/dy but not in dV(OC)/dy, the former coinciding with a similar ESR feature. We propose that these are signatures of subtle structural effects that do not involve major changes in lattice constants or unit-cell symmetries. We tentatively assign the y = 0.03 feature to a metastable "dilute stage-2" phase, by analogy to graphite intercalates. Similarly, three phases with the same stage-1 channel structure are inferred from the observation of three plateaus in V(OC)(y) in the range 0.10 < y < 0.17 upon dedoping. A maximum sigma parallel-to 17 500 S/cm is found near y = 0.12 during the first doping cycle. With successive cycles-sigma parallel-to (max) decreases, the maximum attainable y also decreases, and V(OC)(y) approaches the ideal behavior of an intercalation electrode exhibiting first-order phase transitions.