Role of the steric effects for the progress of alkanes intercalation in CsC24 Original Research Article

The ability of various alkanes to be intercalated into the second-stage cesium graphitide, CsC24, was investigated using the two-bulb technique. The structural changes occurring during intercalation were studied by real-time neutron diffraction. At moderate pressure, the intercalation of large molecules such as n-butane, n-pentane and n-hexane simultaneously leads to a mixture of a second-stage ternary phase and a first-stage binary phase “CsC8”. Under increased pressure a pure first-stage ternary phase is finally formed. The intercalation of cyclopentane occurs in two steps: a pure second-stage ternary phase is first observed, whereas CsC8 only appears at about half filling. For the smallest alkane, CH4, complete ternarization leads to a second-stage ternary phase together with a small amount of an enriched second-stage binary derivative. Owing to the formation of binary domains rich in alkali metal during the ternarization, the cesium density is smaller in the second-stage ternary phase than in the starting binary compound. The in-plane cesium concentration of the ternary phase strongly depends on the projected surface area of the alkane molecule. During invasion of the interlamellar space, large molecules induce a decrease of the average distance between cesium ions. Electrostatic repulsive energy between cations is minimized through expulsion of cesium in binary domains. A pleated-layer model with canted fronts is presented, in order to account both for the various phases existing within each grain, and for the structural transformations caused by the intercalation reaction.