Intercalation of pentane into the stage 2 to 4 cesium graphitides: Relation between cesium density in the intercalated l
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M. Goldmann and F. Beguin CRMD, UMR CNRS Universite, lb, rue de la Ferollerie, 45071 Orleans Cedex 02, France (Received 13 January 1993; accepted 23 April 1993)
Isotherms (at 300 K and 328 K) and isobars (in the range 300 to 400 K) of n-pentane intercalation in CsC24 and CsC36 were established. With CsC24, three plateaus were identified at 0.52, 0.7, and 1.0 n-pentane/24 C, whereas only two plateaus at 0.8 and 0.97 n-pentane/36 C were found with CsC36. The progress of the reaction between n-pentane and CsC24, CsC 36 , and CsC56 (stage 2 to 4) was monitored by real-time neutron diffraction. The intercalation of n-pentane in CsC24 results in the simultaneous formation of a second stage ternary and a first stage binary "CsQ", whereas, from the third stage CsC36 or the fourth stage CsC56, only pure second stage or third stage ternary compounds are formed, respectively. Owing to the formation of binary domains rich in alkali metal (CsC8) or to stage lowering produced by the ternarization, the in-plane cesium density is smaller in the ternary layer than in the starting binary. The electrostatic repulsion between the cesium ions, provoked by the sorption of n-pentane, is believed to be at the origin of the increased coverage. During the intercalation or de-intercalation processes, three-dimensional segregation occurs in each grain. A pleated layer model with canted fronts is presented. It accounts for the various phases present within each grain and for the structural transformations caused by pressure variations. At room temperature, the ternary layer seems to be disordered. The order-disorder transition appearing either by decreasing the temperature or by increasing the n-pentane pressure is correlated to a hindered motion of the intercalated molecules.
I. INTRODUCTION Many neutral molecules can be intercalated into alkali graphitides. The reaction is strongly influenced by the polarity of the molecule. In the case of polar molecules such as tetrahydrofuran (THF)1 or ammonia,2 the driving force is essentially the ion-dipole interaction, and most of the reactions are only partly reversible. On the other hand, with nonpolar species such as H 2 , CH 4 , Ar,3'4 or n-hexane,5 van der Waals interaction is predominant; the molecules are physisorbed between the graphene planes, with a small increase of the interlayer distance and at a temperature depending on their mobility (larger than 300 K for n-hexane, less than 200 K for CH 4 ). The progress of the intercalation and the nature of the various phases formed are both determined by two important characteristics of the molecules: their thickness and their polarity. These two factors influence the various energy terms involved in the reaction: the electrostatic energy between the alkali ion and the negatively charged graphene layer, the spacing energy, 2288
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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and the interaction energy between the molecule and the substrate (either ion-dipole or van der Waals). It is, however, also
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