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Phase Diagram Evaluations: Section II

C-Cs (Carbon-Cesium) System James Sangster

Equilibrium Diagram There is no phase diagram available for this system. It may be conjectured that it would be similar to that for the C-Li system, and so would show (a) the compound C2Cs2, (b) a liquid and (c) a eutectic of composition very close to 100 at.% Cs and temperature very close to the M.P. of Cs. The alkali metals exhibit three types of intermediate compounds with carbon. For cesium, these may be designated (1) (2) (3)

simple C-Cs compounds (C2Cs2 in this case) cesium-graphite intercalation compounds cesium-fullerene exohedral and endohedral compounds

In the chemical literature, molecular formulas for C-Cs compounds have been written with either C or Cs placed first., without much consistency. In the present instance, C will always come before Cs. C2Cs2 (cesium acetylide, cesium carbide) was prepared in two steps in liquid ammonia [1903Moi]. Acetylene was added to a solution of cesium in the solvent. The intermediate product was C2Cs2ÆC2H2, which upon heating in vacuo at 300
C was converted to C2Cs2. According to a standard inorganic chemistry text [1924Mel], it may also be prepared by the reaction of molten cesium with acetylene. The melting behavior of C2Cs2 has not been reported. Cesium-Graphite Intercalation Compounds These are also called ‘‘lamellar,’’ ‘‘layered,’’ ‘‘interstitial’’ or ‘‘insertion’’ compounds. All alkali metals form graphite intercalation compounds, but those with Li or Na are fewer and are formed less easily than those of the heavier alkali metals. The structure of graphite and metal intercalation compounds is shown schematically in Fig. 1. Graphite has a layered structure, in which the carbon atoms are arranged in parallel planes, in each of which they form a network of hexagons [1971Nov]. There is no chemical bonding between the layers and so alkali metals can be inserted rather easily between the layers. The separation of the adjacent layers increases somewhat when alkali metal atoms are inserted. The conventional method of indicating the stoichiometry of metal-graphite intercalation compounds is CxM, where x may be an integer or fractional number. The formula CxM does not represent a molecule of (x + 1) atoms; rather, it indicates merely the atom ratio C/M in a macroscopic sample. The intercalation compounds (Fig. 1) are named ‘‘first stage,’’ ‘‘second stage,’’ etc. according to the number of graphite layers separating the metal layers. The stoichiometry CxM assigned to a particular stage compound should be

regarded as nominal or average. The overall stoichiometry is established by chemical reaction and analysis, the particular stage by XRD. The preparation and properties of alkali metal intercalation compounds have been reviewed extensively [1959Hen, 1959Rud, 1960Cro, 1971Nov, 1975Ber, 1975Boe, 1976Ebe, 1977Her, 1980Sel, 1981Dre, 1981Her, 1982Bar, 1990Lag]. The common methods for the preparation of cesiumgraphite intercalation compounds are • • •

reaction with cesium vapor at low pressures in o