The metallic yellow compound CsCs belongs to the 1st stage. The formula CsCs involves the occupation, in each interlayer, of only one in four possible sites the compact structure would entail the composition CsCz (Fig. 1).
From single crystal photographs, one can determine the c parameter as three times the identity period along the e axis: Ic.
The structural determination is based on separate study of each family: 001 (Table 1, Fig. 2), hk0 and h&l (Table 2, Fig. 3) and powder diagrams (Table 3, Fig. 4). The hkO family allows to determine the symmetry of the unit cell and the parameter; the systematic extinctions of the hk[ reflections permit to determine the space group.
The stacking of the cesium atoms: three in the four possible sites involves a screw axis 62 or 64. The position of the carbon and cesium atoms takes into account all the symmetries of the cells corresponding to the space group P&22 (or P6422) (Fig. 5). The foregone extinctions of those unit cells are respected, in particular, the 201, 202, 204. , reflexions are missed (Fig. 6).
The two domains built on afly and lr+ are actually symmetric to a mirror and not superimposable. It is shown on Fig. 8 that there are two enantiomorphous structures: the stacking a/3y corresponds to an obverse screw axis for the cesium atoms.
The presence of 6 different stacking zones even in a simple crystal involves a disorder in the compound which can explain the variations of intensity of the reflexions hk0 and hkl.
The CsCs compound is described by an hexagonal unit cell belonging to the space group P6*22 (or P6422) with 24 carbon atoms in position 12 k (x = l/6, y = l/3, z = l/3) and 6 i (x = l/6, z = 0 and x = l/3, z = 0) and the cesium atoms in 3 b. The parameters are a = 4.945 f 0.01 A and c = 17.762 0.03 A. Two unit cells are mentioned, according to the direction of the screw axis. In fact, there are 6 different possible unit cells.
We tried to imagine the different possibilities of stacking for the cesium atoms between the graphite layers. From an ideal graphite plane, one places the 1st cesium layer in position (1. The second metallic layer can be placed on the & y or S sites. Let us imagine a part in fi position, an other in y and the last one on S. In the same graphite matrix, one can define three distinct domains spotted by the successions as, ny and cr& While the filling of the last cesium layer, each domain