Graphite intercalation compounds are complex materials having a formula where the ion or 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: 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. Examples Alkali and alkaline earth derivatives One of the best studied graphite intercalation compounds, , 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. is a superconductor with a very low critical temperature Tc = 0.14 K. Heating leads to the formation of a series of decomposition products as the K atoms are eliminated: Via the intermediates (blue in color), , , ultimately the compound results. The stoichiometry is observed for M = K, Rb and Cs. For smaller ions M = , , , , , and , the limiting stoichiometry is . Calcium graphite is obtained by immersing highly oriented pyrolytic graphite in liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of belongs to the Rm 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 ((stage 1)) or those with caesium,