Magnetic phase transition, aggregate formation, and electrical conductivity in Fe${\mathrm{Cl}}_3$-doped polyacetylene

Both ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ ions occur in Fe${\mathrm{Cl}}_3$-doped polyacetylene. At low dopant levels (<0.01 mole fraction) both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions are paramagnetic down to 4.2 K. At dopant levels >0.01 mole fraction, there is evidence from Mössbauer spectroscopy that ${\mathrm{Fe}}^{2+}$ ions associate into aggregates which are magnetically ordered below 25 K. Aggregate formation appears to correlate with high electrical conductivity.