Graphite intercalation compound

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Space-filling model of potassium graphite KC₈.

In the area of solid state chemistry, graphite intercalation compounds are a family of materials prepared from graphite. In particular, the sheets of carbon that comprise graphite can be pried apart by the insertion (intercalation) of ions. The graphite is viewed as a host and the inserted ions as guests. The materials have the formula (guest)C_n where n ≥ 6. The insertion of the guests increases the distance between the carbon sheets. Common guests are reducing agents such as alkali metals. Strong oxidants also intercalate into graphite. Intercalation involves electron transfer into or out of the carbon sheets. So, in some sense, graphite intercalation compounds are salts. Intercalation is often reversible: the inserted ions can be removed and the sheets of carbon collapse to a graphite-like structure.

The properties of graphite intercalation compounds differ from those of the parent graphite.^[1][2]

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. An 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.

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.^[3] The resulting solid is pyrophoric.^[4] 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.^[4][5] KC₈ is a superconductor with a very low critical temperature T_c = 0.14 K.^[6] Heating KC₈ leads to the formation of a series of decomposition products as the K atoms are eliminated:^{[citation needed]}

3 KC₈ → KC₂₄ + 2 K

Via the intermediates KC₂₄ (blue in color),^[3] KC₃₆, KC₄₈, ultimately the compound KC₆₀ results.

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.^[6] Calcium graphite CaC₆ is obtained by immersing highly oriented pyrolytic graphite in liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC₆ belongs to the R3m 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.

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)).^{[clarification needed]}

In situ adsorption on free-standing graphene and intercalation in bilayer graphene of the alkali metals K, Cs, and Li was observed by means of low-energy electron microscopy.^[7]

Different from other alkali metals, the amount of Na intercalation is very small. Quantum-mechanical calculations show that this originates from a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same group of the periodic table.^[8] The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table.^[8] However, considerable Na intercalation into graphite can occur in cases when the ion is wrapped in a solvent shell through the process of co-intercalation. A complex magnesium(I) species has also been intercalated into graphite.^[9]

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 contrast 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^{[clarification needed]}

In graphite perchlorate, planar layers of carbon atoms are 794 picometers apart, separated by ClO−4 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.^[4] Reaction of graphite with [O₂]⁺[AsF₆]⁻ affords the salt [C₈]⁺[AsF₆]⁻.^[4]

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.^[1] The materials consists of layers of close-packed metal halide layers between sheets of carbon. The derivative C_~8FeCl₃ exhibits spin glass behavior.^[10] It proved to be a particularly fertile system on which to study phase transitions.^{[citation needed]} A stage n magnetic graphite intercalation compounds 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.

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.

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

Among the superconducting graphite intercalation compounds, CaC₆ exhibits the highest critical temperature T_c = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa).^[6] 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.^[11] Analysis of pure CaC₆ 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.^[11]

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.^[12] 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.^[12] 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.

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

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