L. Duclaux et al., THE LOW-TEMPERATURE 3-DIMENSIONAL STRUCTURES OF THE 2ND-STAGE CESIUM GRAPHITIDE, Journal of applied crystallography, 31, 1998, pp. 67-73
The three-dimensional (3D) structure of the second-stage CsC24 graphit
ide was investigated by recording hk.l monochromatic rotating-crystal
diffractograms. At low temperature, the two-dimensional (2D) structure
is formed of an incommensurate (2.53 x 2.53 R 14.5 degrees) and a com
mensurate (2 x 2 R 0 degrees) lattice. Below 140 K, whereas the Cs inc
ommensurate layers become 3D ordered, the 2 x 2 R 0 degrees commensura
te layers are randomly distributed in the structure. The commensurate
structure, with composition CsC16 (stage two), probably equilibrates t
he lack of Cs induced by the incommensurate structure (composition CsC
26), in order to reach the global composition CsC24 of the compound. D
epending on the temperature, two kinds of stackings appear. Below 140
K, the incommensurate Cs layers are arranged in a 2D-modulated initial
stacking, transformed at a lower temperature into a rhombohedral stac
king in which Cs atoms are modulated along the c axis. Simulation of t
he selection rules allowed the determination of models of the incommen
surate layers stacking along the c axis, with translations from one Cs
layer to another layer commensurate with the graphite lattice. The in
itial stacking is described by a pseudo-two-layer model with successiv
e random translations and a short-range order. The structure appearing
at a lower temperature is described by a pseudorhombohedral stacking
with definite translations. The transition from the initial stacking w
ith a first-neighbour interaction to the pseudorhombohedral stacking w
ith a second-neighbour interaction could be a result of the change in
interactions between the Cs atoms and the graphitic host, due to the d
ominating influence of the graphite periodic potential at low temperat
ure.