THEORY OF HEAT CAPACITY OF POLYCRYSTALLINE GRAPHITE AT VERY LOW TEMPERATURES - FINITE-LATTICE CONTRIBUTION

Recent measurements of the heat capacity of polycrystalline graphite at liquid-helium temperatures indicate an abnormal enhancement in the term linear in the absolute temperature with decreasing extent of graphitization. A cause for this anomaly is sought by considering the elastic vibrations of a finite lattice. It is shown that if a platelike crystallite with a thickness of about 100 vibrates in the direction perpendicular to the plane of the plate, its contribution to the linear term in the heat capacity at very low temperatures is of the same order of magnitude as the observed excess linear heat capacity. Since polycrystalline graphite is known to be composed of flakelike crystallites, it is argued that an important, and possibly the main, cause of the abnormal enhancement is this oscillation of fine crystallite plates. © 1969 The American Physical Society.