The Structure of a STAGE-4 Iron Chloride Graphite Intercalation Compound in Thermal Equilibrium

The crystallographic structure of FeCl _3 is refined from intensities collected on a large single crystal. The refinement of the crystallographic parameters reveals that the Cl atoms are displaced within the Cl plane towards a line connecting the two Fe atoms. There is also an anisotropy in the amplitudes of thermal vibration for the Fe atoms, with the amplitude perpendicular to the plane larger than within the plane. The structure of the stage-4 FeCl_3 graphite intercalation compound (GIC) is examined in thermal equilibrium. In situ the FeCl_3 forms a two dimensional liquid between the layers of graphite. The stacking of sets of ordered graphite layers (ABAB) that bound the intercalant layer is nearly random. The results are described by a model in which 60 percent of the stacking sequences are an A-A sequence of sets with a broad lateral distribution about the ordered position, and the remaining 40 percent of the sets are stacked with complete translational randomness. In thermal equilibrium the FeCl_3 intercalant is a 2D liquid in the form of Fe _2Cl_6 dimers arranged in a plane so that there are two layers of Fe atoms between the two Cl layers. By slightly distorting the dimers and arranging them to form the structure described above, the high-q (intra-molecular) diffraction data could be fit. We also find that the molecules are packed in the layers with a strong angular correlation. The first sharp diffraction peak could not be fit by a Percus-Yevick type packing model because of the non-negligible intermolecular correlations.