INTERCALANT ORDERING IN THE STAGE-1 INDIUM CHLORIDE GRAPHITE-INTERCALATION COMPOUND

The stage-1 InCl3 graphite intercalation compound (GIC) was studied by the de Haas-van Alphen (dHvA) effect in the temperature range 1.4 to 4.2 K with magnetic fields from 3 to 5 T. The observed dHvA oscillatio ns possess frequencies of f(1) = 263 T, f(2) = 458 T, f(3) = 705 T and f(4) = 878 T. They are shown to result from Fermi surface pieces whic h are constructed from the zone folding of the two-dimensional free el ectron Fermi surface. The fragmentation of the Fermi surface is shown to be a consequence of the additional periodicity of a commensurate (8 root 3 x 8 root 3)R30 degrees in-plane superlattice formed by the InC l3 molecules. This ordering of InCl3 molecules is a result of a phase transition which occurs upon cooling at approximately 260 K. This tran sition was identified by temperature dependent resistivity studies and by differential thermal analysis (DTA) of the stage-1 InCl3 GIC sampl e.