Electrical properties of crystal compounds of graphite - III. The role of electron donors

Abstract

Crystal compounds between potassium and well-oriented pyrolytic graphite have been prepared with a range of compositions up to saturation. Measurements have been made of changes of electrical resistance and of thermoelectric power as a function of composition in botha- andc-axis directions. Anisotropy of electrical resistance becomes smaller, and of thermoelectric power sinks to practically zero, on compound formation. Compounds between graphite and rubidium or caesium have been studied more briefly under conditions approximating to saturation. In the direction of thea-axis, the large decreases of electrical resistance observed can be interpreted on the basis that the alkali metal atoms inject electrons into the upperπ-band of graphite. This is confirmed by the observed changes of thermoelectric power. Changes resemble but do not completely mirror those observed with electron acceptor compounds; the fractional transfer of electrons appears to be less complete with the electron donors. In the direction of thec-axis, intercalation of the electron donor alkali metal atoms leads to a much more striking decrease of electrical resistance than is observed with various electron acceptor groups. To supplement results previously published, brief studies are reported on crystal compounds between graphite and aluminium chloride, and graphite and iodine monochloride. Possible band models for graphite compounds with both electron donor and acceptor atoms are discussed in the light of the experimental findings.