Heat capacity of antimony pentachloride–intercalated graphite

We have measured the heat capacity of several ${\mathrm{SbCl}}_5$-intercalated highly oriented pyrolytic graphite samples between 10 K and room temperature with ac-calorimetry techniques. The data are analyzed for low-T lattice modes, molecular rotation effects, and intramolecular vibrational modes. We find evidence for rotational unbinding in some samples near 150 K and an activation process commencing at the 220-K disordering transition, while stage-2 samples prepared by immersion in liquid ${\mathrm{SbCl}}_5$ show first-order transitions at 220 K. Our measurements support the occurrence of ${\mathrm{SbCl}}_3$ dimerization suggested by recent x-ray-scattering measurements of antimony chloride graphite intercalation compounds.