Intercalation of pentane into the stage 2 to 4 cesium graphitides: Relation between cesium density in the intercalated layers and the stage of the parent binary

Abstract

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, CsC₃₆, 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 “CsC₈”, 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.