Intralayer crystal structure and order-disorder transformations of graphite intercalation compounds using electron diffraction techniques

We have used electron diffraction through the c-face to study the intercalate ordering of graphite-alkali metals (K, Rb, Cs) and graphite-halogens (Br₂, IBr, ICl). Order-disorder transformations for the in-plane structure of alkali metal and halogen intercalates have been observed by the vanishing of all superlattice diffraction spots as the sample temperature is raised. Variations of the electron diffraction patterns and of the order-disorder transformation temperatures with the intercalate species are reported. Compounds of various concentrations have been investigated, including the stage 1 graphite-alkali metals. Results obtained here are compared with structural information obtained by other techniques and by previous workers. The patterns for graphite-halogens are much more complicated than those for graphite-alkali metals. Of significance is the observation that the superlattice patterns are relatively insensitive to the intercalate concentration for compounds more dilute than stage 2 and are similar for lamellar and residue compounds. Additional superlattice patterns are observed in graphite-Br₂ below 194 °C and are similar in appearance to charge-density wave superlattice patterns reported for 2H-TaSe₂.