Role of the steric effects for the progress of alkanes intercalation in CSC24

The ability of various alkanes to be intercalated into the second-stage ces ium graphitide, CsC24, was investigated using the two-bulb technique. The s tructural changes occurring during intercalation were studied by real-time neutron diffraction. At moderate pressure, the intercalation of large molec ules such as n-butane, n-pentane and n-hexane simultaneously leads to a mix ture of a second-stage ternary phase and a first-stage binary phase "CsC8". Under increased pressure a pure first-stage ternary phase is finally forme d. The intercalation of cyclopentane occurs in two steps: a pure second-sta ge ternary phase is first observed, whereas CsC8 only appears at about half filling. For the smallest alkane, CH4, complete ternarization leads to a s econd-stage ternary phase together with a small amount of an enriched secon d-stage binary derivative. Owing to the formation of binary domains rich in alkali metal during the ternarization, the cesium density is smaller in th e second-stage ternary phase than in the starling binary compound. The in-p lane cesium concentration of the ternary phase strongly depends on the proj ected surface area of the alkane molecule. During invasion of the interlame llar space, large molecules induce a decrease of the average distance betwe en 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. (C) 1998 Elsevier Science Ltd. All rights re served.