Phase transitions in polypyrrole and polythiophene conducting polymers demonstrated by magnetic susceptibility measurements

Magnetic susceptibility measurements conducted by the superconducting quantum-interference device (SQUID) technique in the temperature range 6--300 K on electrochemically synthesized polypyrrole and polythiophene doped with tetrafluoroborate (BF₄^-) are reported. The temperature-dependent susceptibility results indicate the possibility of phase transitions occurring at about 75 K in polypyrrole and at about 130 K in polythiophene. These possible phase transitions are analyzed in terms of electronic and morphological structures of polyheterocyclics. The spin susceptibility clearly changes from a Curie to a Pauli-like behavior in polypyrrole as the temperature rises, while polythiophene only exhibits a Curie behavior. In addition, the ESR measurements performed on polypyrrole, polythiophene, and polyfuran at room temperature are also discussed. The exceptionally narrow ESR linewidth of 0.14 G in polypyrrole indicates the presence of highly mobile unpaired spins in the doped state. A significant effect of environmental conditions on magnetic behavior was also observed. The present susceptibility results are similar to those on arsenic-pentafluoride-doped cis-polyacetylenes. The SQUID measurements also indicate the possibility of the coexistence of unpaired spins, i.e., paramagnetic species and paired spins in conducting state. The temperature-dependent susceptibility demonstrates to some extent a dominant spin-coupling type of phenomenon, particularly in polypyrrole. The existence of spinless bipolarons and their contribution to the conduction mechanism are discussed in light of the present results.