THE LOW-TEMPERATURE 3-DIMENSIONAL STRUCTURES OF THE 2ND-STAGE CESIUM GRAPHITIDE

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.