Structural Properties of Stage-2 Alkali-Metal Graphite Intercalation Compounds: A Molecular Dynamics Study

An extensive study on low temperature solid and high temperature liquid phases of stage 2 alkali-metal graphite intercalation compounds (GICs), as examples of two dimensional screened Coulomb (repulsive) systems on strongly corrugation modulated substrates, is presented using constant energy and constant temperature (Nosé) Molecular Dynamics (MD) simulations. We present pair correlation functions, static structure factors and snapshots of microscopic structures in the liquid phase, in the freezing region and in the solid phase. Average energy, diffusion constant and the number of disclinations are monitored as a function of temperature to track down the transition (freezing) region. We also find that the number density of the topological defects (such as dislocations and disclinations) decreases rapidly at the freezing transition. In the solid phase of RbC24 we find small triangular domains of x) Rb structure with welldefined domain walls consisting of (2x2) and (2x3x7) elemental plaquettes of Rb. These nanostructures provide a clear picture of the discommensurations (domain walls) and a consistent understanding of the X-ray diffraction measurements. Our simulation studies suggest a Periodic Domain Wall model to describe the low temperature solid phase structure of the alkali metal GIC's. This model explains the dominant features of the experimental X-ray structure factor.