AC-Calorimetric Measurement of Low-Temperature Specific Heat of Graphite-Iodine Monochrolide Intercalation Compounds

By means of the AC-calorimetric technique, the specific heats of well-defined specimens of highly oriented pyrolytic graphite and its stages-1, -2, and -3 intercalation compounds with ICl have been investigated at temperatures down to 2.2 K. The apparatus was calibrated through a comparison of the data for pristine graphite with those in past studies. The specific heat in the range 2.2-5 K is represented, in an approximation, as a sum of electronic and lattice terms (γ T and α T 3 ) for pure graphite, and additionally of an extra term due to the dispersionless optical-mode vibration of ionized ICl species in the compounds. γ and α both increase steeply with the ICl concentration, reflecting a monotonous increase of positive holes and softening of the lattice in the c -direction respectively. A phenomenological theory, which correlates the lattice term to the elastic constants C 33 and C 44 , turns out that α tends to decrease with increasing temperature.