Carbons as a route to synthetic metals

Abstract

From its crystal structure graphite must be considered as a solid organic compound containing macroaromatic molecules and delocalised electrons. A hypothesis that intercalation crystal compounds of graphite are analogous in quantum chemistry with charge transfer compounds formed between smaller aromatic systems, and electron donor or electron acceptor molecules, has been abundantly verified experimentally. A great diversity of intercalation compounds of graphite has been prepared in polycrystalline compact or powdered form. All show much higher electrical conductivity than the parent graphite, with p or n carriers predominant according to the electron acceptor or electron donor character of the intercalate. Graphite exhibits extremely high anisotropy in its crystal structure and its physical properties. Much of this anisotropy persists in synthetic metals prepared from graphite by intercalation. This makes it essential to start with well oriented graphite in large pieces, to evaluate intrinsic electronic properties in the principal crystal directions with precision. Progressive advances are described in high temperature chemical engineering of graphite, whose final outcome has been the successful production of large pieces of well oriented material. Some of these novel procedures are also of interest, e.g. in the chemical engineering production of diamond from graphite. Using such well oriented materials, experimental study of intercalation (charge transfer) compounds has become much more precise. Some challenging problems have been revealed concerning the anisotropy of electronic properties. In general, conduction of electricity in the basal plane directions follows patterns familiar with other metals. But at right angles, unusual behaviour is apparent particularly at low temperatures.