Electrical Anisotropy of Synthetic Metals Based on Graphite

Near ideal graphite with its crystallites all well-alined has unusually large electrical anisotropy, with resistivity ratio ρa/ρc ≈ 4 x 10-4. As previously established, by intercalating various molecules between the carbon hexagon networks ρa is greatly decreased. In the a-axis direction charges wander easily through the aromatic bond networks, and the synthetic metals produced rival highly conducting natural metals. But perpendicular to the carbon hexagon planes, mechanisms of charge wandering have not been previously much studied. Molecular structures of the layers, the (fractional) effective charge transferred from each intercalate molecule, and the concentration of charges per unit volume of synthetic metal, vary from case to case. The anisotropy of electrical properties of synthetic metals may be expected to show a corresponding diversity of values, and has now been measured at ambient temperatures and at 77 K. Pieces of near-ideal graphite thick in the direction of the c-axis were intercalated with potassium atoms, or with the molecules Br2 or ICl, or the composite anion AICI4- or anions of the acids HNO3, H2SO4 and HSO3Cl. With (electron donor) potassium atoms, the c-axis electrical resistance is lower than in the parent graphite. With all the electron acceptor molecules studied the c-axis electrical resistance is higher at ambient temperatures. The extreme anisotropy of most of these synthetic metals is accompanied by other unusual features. It probably involves pronounced layer conductance, and may account for certain ‘remote voltaic effects’ detectable in fully intercalated compounds.