Effect of sorbed molecules on the resistivity of alkali metal–graphite intercalation compounds

Alkali metal–graphite intercalation compounds with the composition of MC24 (M=K, Rb, Cs) were prepared by heating a mixture of MC8 (saturated compound) and graphite sheet (Grafoil) at 350–450 °C. The resistivity perpendicular to the layer planes (ρc) of the resulting compounds was determined by the two-terminal method. The anisotropy factor of the resistivity, (ρc/ρa), of KC24 prepared from Grafoil was ∼130, being about 1/6–1/10 in magnitude compared with that of KC24 prepared from highly oriented pyrolytic graphite. The resistivity change during sorption of hydrogen (at 90 K), ethylene (at 194 K) and acetylene (at 194 K) was determined. The resistivity of MC24 increased with increase of the sorbed amount of H2. The magnitude of the increase was in the order KC24>RbC24>CsC24. This resistivity increase was considered to be due to the expansion along c-direction which reduces the charge-transfer interaction between the carbon layers and potassium ions, resulting in the decrease of the density of the conduction electron. The resistivity of MC24 increased extensively during sorption of C2H4 and C2H2. It was discussed in connection with the in-plane structural transition and chemical interaction between alkali metal ions and sorbed molecules.