Anisotropy of Synthetic Metals

Abstract

SYNTHETIC metals can be derived from graphite by intercalation of either electron donor or electron acceptor molecules between the carbon hexagon networks. In graphite, these networks are in effect macromolecules with aromatic character. Planar sheets of carbon hexagon networks lie parallel to the crystallographic a-axis of graphite, which has hexagonal symmetry. Physical properties in directions parallel to the networks are thus “macro-aromatic” properties. Perpendicular to these sheets, in the direction of the crystallographic c-axis of graphite, physical properties involve much weaker physical interaction between the layers; probably only van der Waals dispersion forces hold neighbouring sheets together in graphite crystals. It is comparatively easy to account for the high n or p electrical conductivities observed in the direction of the a-axis (as a consequence of charge transfer to or from the graphite electron bands¹) but effects of such charge transfer on electrical properties in the direction of the c-axis of the crystals cannot be predicted as readily. Even in the parent graphite, the anisotropy ratio of electrical conductivity σ_a/σ_c is about 4 × 10³ for well aligned stress recrystallized material². It is possible that the relatively poor conduction of electricity in the c-axis direction in pure graphite involves a different kind of mobility, akin to electron hopping in crystals between molecules of poor conductors. In the synthetic metals the charged carbon hexagon networks are separated by positively charged layers of potassium atoms as in C₈K (ref. 3) or by negatively charged layers whose molecular composition is given by formulae such as C₂₄⁺ HSO₄⁻ 2H₂SO₄, C₂₄⁺ NO₃⁻ 3HNO₃, C₈Br, C₈ICl and so on. As far as conduction mechanisms in the c-axis direction are concerned, it is significant that considerable crystal swelling accompanies this intercalation; the repeat distance between successive sheets of carbon atoms increases from 3.35 Å in the parent graphite to a much larger separation, which is about 8 Å for graphite bisulphate⁴.