A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.

Intercalation compounds of graphite

The electronic spectra of carbon nanotubes and other nanoscale systems are quantized because of their small radii. Similar quantization in the phonon spectra has been difficult to observe because of the far smaller energy scale. We probed this regime by measuring the temperature-dependent specific heat of purified single-wall nanotubes. The data show direct evidence of one-dimensional quantized phonon subbands. Above 4 kelvin, they are in excellent agreement with model calculations of individual nanotubes and differ markedly from the specific heat of two-dimensional graphene or three-dimensional graphite. Detailed modeling yields an energy of 4.3 millielectron volts for the lowest quantized phonon subband and a tube-tube (or “lattice”) Debye energy of 1.1 millielectron volts, implying a small intertube coupling in bundles.

https://science.sciencemag.org/content/sci/289/5485/1730.full.pdf

Quantized Phonon Spectrum of Single-Wall Carbon Nanotubes

The thermal properties of carbon nanotubes display a wide range of behaviors which are related both to their graphitic nature and their unique structure and size. The specific heat of individual nanotubes should be similar to that of two-dimensional graphene at high temperatures, with the effects of phonon quantization becoming apparent at lower temperatures. Inter-tube coupling in SWNT ropes, and interlayer coupling in MWNTs, should cause their low-temperature specific heat to resemble that of three-dimensional graphite. Experimental data on SWNTs show relatively weak inter-tube coupling, and are in good agreement with theoretical models. The specific heat of MWNTs has not been examined theoretically in detail. Experimental results on MWNTs show a temperature dependent specific heat which is consistent with weak inter-layer coupling, although different measurements show slightly different temperature dependences. The thermal conductivity of both SWNTs and MWNTs should reflect the on-tube phonon structure, regardless of tube-tube coupling. Measurements of the thermal conductivity of bulk samples show graphite-like behavior for MWNTs but quite different behavior for SWNTs, specifically a linear temperature dependence at low T which is consistent with one-dimensional phonons. The room-temperature thermal conductivity of highly aligned SWNT samples is over 200 W/mK, and the thermal conductivity of individual nanotubes is likely to be higher still.

Phonons and Thermal Properties of Carbon Nanotubes

The effects of applied pressure on graphite and its intercalation compounds are reviewed emphasizing the relationship between structure and transport properties. It has long been recognized that high pressure plays a crucial role in the polymorphic phase transitions of graphite, notably in the graphite-diamond transformation. More recent studies have revealed a wealth of pressure-induced phases associated with the unusual layer-stacking (‘staging’) mechanism in the intercalation compounds of graphite. The high degree of structural anisotropy associated with staging is strongly reflected in the electronic band structure and transport properties, and in the remarkable pressure dependence of the superconducting states of some of the graphite intercalation compounds. High pressure is shown to be a valuable means not only to realize new structural phases but also to improve our understanding of the fundamental behaviour of these important materials.

High pressure properties of graphite and its intercalation compounds

The first model for lattice vibrations of solids was proposed in 1907 by Einstein who applied the quantum concept to the mechanical motion of individual atoms in a crystal lattice which he assumed to be vibrating with random phases.1 He subsequently found that this model was inadequate, its major shortcoming being that it led to a thermal conductivity which disagreed with the observation on crystals both in magnitude and temperature dependence.2 This disagreement was removed by Debye3 and by Born and von Karman4 who demonstrated that in crystalline solids the atoms vibrate collectively as elastic waves.

http://science.sciencemag.org/content/sci/259/5098/1145.full.pdf

Specific Heat and Thermal Conductivity of Solid Fullerenes

The specific heat of the graphite intercalation compounds KC/sub 8/, CsC/sub 8/, and RbC/sub 8/ and their parent material, highly oriented pyrolytic graphite (HOPG), has been determined between 0.48 and 90 K. The results are analyzed in terms of electronic and lattice contributions. The electronic contribution to the specific heat of these compounds is enhanced by at least 29 times above that for HOPG. Comparisons are made to theoretical band-structure calculations. The lattice contribution shows deviations from a Debye model for T>5 K, which can be mathematically fitted by an Einstein specific-heat function. While our results for RbC/sub 8/ are unique, comparison with previous results for powder-based KC/sub 8/ and CsC/sub 8/ samples over a limited temperature range suggest that the type of parent graphite and/or details of preparation technique may influence the physical properties of these compounds.

Low-temperature specific heat of the graphite intercalation compounds K C 8 , Cs C 8 , Rb C 8 , and their parent highly oriented pyrolytic graphite

Superconductivity of the graphite intercalation compounds KHgC8 and RbHgC8: Evidence from specific heat☆

Synthesis and low temperature specific heat of the graphite intercalation compounds KHgC4 and KHgC8

Low‐temperature specific heat (Cp) and magnetic susceptibility (χ) for the graphite intercalation compounds C6.2FeCl3 (stage 1) and C17FeCl3 and C11.3NiCl2.13 (stage 2) is reported and analyzed. The FeCl3 compounds show large anomalies in Cp at 4 and 1.7 K, respectively, while the NiCl2 compound shows a peak at 20 K. For the FeCl3 compounds, χ is less than 5×10−6 emu gm−1 at all temperatures, showing only very small anomalies, while the NiCl2 compound shows a very large peak in χ at 16 K. Possible magnetic models for these layered compounds are considered, including a spin‐glass or spin‐cluster glass state.

M. Grayson Alexander

Papers

Low-temperature specific heat of the graphite intercalation compounds K C 8 , Cs C 8 , Rb C 8 , and their parent highly oriented pyrolytic graphite

Superconductivity of the graphite intercalation compounds KHgC8 and RbHgC8: Evidence from specific heat☆

Synthesis and low temperature specific heat of the graphite intercalation compounds KHgC4 and KHgC8

Heat capacity and magnetic studies of graphite intercalated with FeCl3 and NiCl+2