In situ intercalative polymerization chemistry of FeOCl. Generation and properties of novel, highly conductive inorganic/organic polymer microlaminates

New structural forms of polypyrrole and polythiophene are prepared by a chemical method in which intercalation and simultaneous polymerization of pyrrole and 2,2'-bithiophene are brought about within the constrained van der Waals gap of a layered inorganic solid, FeOCl. The reaction of FeOCl with pyrrole and 2,2'-bithiophene yields compounds (C₄H₃N)_0.34FeOCl, I and (C₄H₂S)_0.28FeOCl, II, respectively. (C₄H₃N)_0.34FeOCl and (C₄H₂S)_0.28FeOCl are brown-black microcrystalline solids with a shiny metallic luster. The interlayer spacing in I and II (b-axis) is 13.21(1) Å and 13.31(1) Å, respectively, compared to 7.98(2) Å in pristine FeOCl. X-ray powder diffraction data for I and II are consistent with a space group change from Pmnm (in FeOCl) to Immm or I222 and doubling of the b-axis. The body-centered unit cell results from a lateral shift of the alternate FeOCl layers in the a direction of the ac plane so that the chlorine atoms lie directly on top of each other. The nature of the organic material in I and II was probed by chemical, physical and charge transport techniques, all of which indicate the presence of a high molecular weight, conductive polymer. I and II exhibit high electrical conductivities compared to other FeOCl intercalation compounds. Four-probe electrical conductivity data (in the range 4-300 K) measured on compressed pellets of the materials show thermally activated behavior with room temperature σ≈1ω^-1 cm^-1. Thermoelectric power measurements indicate predominant hole conduction with metallic behavior. In (C₄H₂S)_0.28FeOCl, as sudden rise in the Seebeck coefficient below ∼ 35 K is observed, suggesting a possible metal-semiconductor transition.