Graphite intercalation compound

Space-filling model of potassium graphite KC₈ (side view)

Space-filling model of potassium graphite KC₈ (top view)

Graphite intercalation compounds (GICs) are complex materials having a formula CX_m where the ion Xⁿ⁺ or Xⁿ⁻ is inserted (intercalated) between the oppositely charged carbon layers. Typically m is much less than 1. These materials are deeply colored solids that exhibit a range of electrical and redox properties of potential applications.

Preparation and structure

These materials are prepared by treating graphite with a strong oxidant or a strong reducing agent:

C + m X → CX_m

The reaction is reversible.

The host (graphite) and the guest X interact by charge transfer. A analogous process is the basis of commercial lithium-ion batteries.

In a graphite intercalation compound not every layer is necessarily occupied by guests. In so-called stage 1 compounds, graphite layers and intercalated layers alternate and in stage 2 compounds, two graphite layers with no guest material in between alternate with an intercalated layer. The actual composition may vary and therefore these compounds are an example of non-stoichiometric compounds. It is customary to specify the composition together with the stage. The layers are pushed apart upon incorporation of the guest ions.

Examples

Alkali and alkaline earth derivatives

File:Potassium graphite.jpg

Potassium graphite [Citation needed.] under argon in a Schlenk flask. A glass-coated magnetic stir bar is also present.

One of the best studied graphite intercalation compounds, KC₈, is prepared by melting potassium over graphite powder. The potassium is absorbed into the graphite and the material changes color from black to bronze. The resulting solid is pyrophoric. The composition is explained by assuming that the potassium to potassium distance is twice the distance between hexagons in the carbon framework. The bond between anionic graphite layers and potassium cations is ionic. The electrical conductivity of the material is greater than that of α-graphite. KC₈ is a superconductor with a very low critical temperature T_c = 0.14 K. Heating KC₈ leads to the formation of a series of decomposition products as the K atoms are eliminated:

3 KC₈ → KC₂₄ + 2 K

Via the intermediates KC₂₄, KC₃₆, KC₄₈, ultimately the blue compound KC₆₀ results. [Citation needed.]

The stoichiometry MC₈ is observed for M = K, Rb and Cs. For smaller ions M = Li⁺, Sr²⁺, Ba²⁺, Eu²⁺, Yb³⁺, and Ca²⁺, the limiting stoichiometry is MC₆ (X. Calcium graphite CaC
₆ is obtained by immersing highly oriented pyrolytic graphite in liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC
₆ belongs to the R-3m space group. The graphite interlayer distance increases upon Ca intercalation from 3.35 to 4.524 Å, and the carbon-carbon distance increases from 1.42 to 1.444 Å.

Structure of CaC
₆

With barium and ammonia, the cations are solvated, giving the stoichiometry (Ba(NH₃)_2.5C_10.9(stage 1)) or those with caesium, hydrogen and potassium (CsC₈·K₂H_4/3C₈(stage 1)).

Graphite bisulfate, perchlorate, hexafluoroarsenate: oxidized carbons

The intercalation compounds graphite bisulfate and graphite perchlorate can be prepared by treating graphite with strong oxidizing agents in the presence of strong acids. In contras to the potassium and calcium graphites, the carbon layers are oxidized in this process: 48 C + 0.25 O₂ + 3 H₂SO₄ → [C₂₄]⁺[HSO₄]^−.2H₂SO₄ + 0.5 H₂O

In graphite perchlorate, planar layers of carbon atoms are 794 picometers apart, separated by ClO₄⁻ ions. Cathodic reduction of graphite perchlorate is analogous to heating KC₈, which leads to a sequential elimination of HClO₄.

Both graphite bisulfate and graphite perchlorate are better conductors as compared to graphite, as predicted by using a positive-hole mechanism. Reaction of graphite with [O₂]⁺[AsF₆]⁻ affords the salt [C₈]⁺[AsF₆]⁻.

Metal halide derivatives

A number of metal halides intercalate into graphite. The chloride derivatives have been most extensively studied. Examples include MCl₂ (M = Zn, Ni, Cu, Mn), MCl₃ (M = Al, Fe, Ga), MCl₄ (M = Zr, Pt), etc. The materials consists of layers of close-packed metal halide layers between sheets of carbon. The derivative C_~8FeCl₃ exhibits spin glass behavior. It proved to be a particularly fertile system on which to study phase transitions. A stage n magnetic GIC has n graphite layers separating successive magnetic layers. As the stage number increases the interaction between spins in successive magnetic layers becomes weaker and 2D magnetic behaviour may arise.

Halogen- and oxide-graphite compounds

Chlorine and bromine reversibly intercalate into graphite. Iodine does not. Fluorine reacts irreversibly. In the case of bromine, the following stoichiometries are known: C_nBr for n = 8, 12, 14, 16, 20, and 28.

Because it forms irreversibly, carbon monofluoride is often not classified as an intercalation compound. It has the formula (CF)_x. It is prepared by reaction of gaseous fluorine with graphitic carbon at 215–230 °C. The color is greyish, white, or yellow. The bond between the carbon and fluorine atoms is covalent. Tetracarbon monofluoride (C₄F) is prepared by treating graphite with a mixture of fluorine and hydrogen fluoride at room temperature. The compound has a blackish-blue color. Carbon monofluoride is not electrically conductive. It has been studied as a cathode material in one type of primary (non-rechargeable) lithium batteries.

Graphite oxide is an unstable yellow solid.

Properties and applications

Graphite intercalation compounds have fascinated materials scientists for many years owing to their diverse electronic and electrical properties.

Superconductivity

Among the superconducting graphite intercalation compounds, CaC
₆ exhibits the highest critical temperature T_c = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa). Superconductivity in these compounds is thought to be related to the role of an interlayer state, a free electron like band lying roughly 2 eV (0.32 aJ) above the Fermi level; superconductivity only occurs if the interlayer state is occupied. Analysis of pure CaC
₆ using a high quality ultraviolet light revealed to conduct angle-resolved photoemission spectroscopy measurements. The opening of a superconducting gap in the π* band revealed a substantial contribution to the total electron–phonon-coupling strength from the π*-interlayer interband interaction.

Reagents in chemical synthesis: KC₈

The bronze-colored material KC₈ is one of the strongest reducing agents known. It has also been used as a catalyst in polymerizations and as a coupling reagent for aryl halides to biphenyls. In one study, freshly prepared KC₈ was treated with 1-iodododecane delivering a modification (micrometre scale carbon platelets with long alkyl chains sticking out providing solubility) that is soluble in chloroform. Another potassium graphite compound, KC₂₄, has been used as a neutron monochromator. A new essential application for potassium graphite was introduced by the invention of the potassium-ion battery. Like the lithium-ion battery, the potassium-ion battery should use a carbon-based anode instead of a metallic anode. In this circumstance, the stable structure of potassium graphite is an important advantage.

See also

• Covalent superconductors
• Magnesium diboride, which uses hexagonal planar boron sheets instead of carbon
• Pyrolytic graphite

Further reading

• (187 pages), also reprinted as

External links

• Preparation of Potassium Graphite

• structure of CaC₆